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Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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13 pages, 2735 KB  
Article
Wavelength-Dependent Features of Photoelectron Spectra from Nanotip Photoemission
by Xiao-Yuan Wu, Hao Liang, Marcelo F. Ciappina and Liang-You Peng
Photonics 2020, 7(4), 129; https://doi.org/10.3390/photonics7040129 - 11 Dec 2020
Cited by 7 | Viewed by 5111
Abstract
If a metal nanotip is irradiated with the light of a wavelength much larger than the nanotip’s radius of curvature, optical near-fields become excited. These fields are responsible for distinct strong-field electron dynamics, due to both the field enhancement and spatial localization. By [...] Read more.
If a metal nanotip is irradiated with the light of a wavelength much larger than the nanotip’s radius of curvature, optical near-fields become excited. These fields are responsible for distinct strong-field electron dynamics, due to both the field enhancement and spatial localization. By classical trajectory, Monte Carlo (CTMC) simulation, and the integration of the time-dependent Schrödinger equation (TDSE), we find that the photoelectron spectra for nanotip strong-field photoemission, irradiated by mid-infrared laser pulses, present distinctive wavelength-dependent features, especially in the mid- to high-electron energy regions, which are different from the well known ones. By extracting the electron trajectories from the CTMC simulation, we investigate these particular wavelength-dependent features. Our theoretical results contribute to understanding the photoemission and electron dynamics at nanostructures, and pave new pathways for designing high-energy nanometer-sized ultrafast electron sources. Full article
(This article belongs to the Special Issue Strong Light Fields Coupled with Plasmonic Nano-Structures)
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18 pages, 6403 KB  
Article
Scaling of Beam Collective Effects with Bunch Charge in the CompactLight Free-Electron Laser
by Simone Di Mitri, Andrea Latina, Marcus Aicheler, Avni Aksoy, David Alesini, Graeme Burt, Alejandro Castilla, Jim Clarke, Hector Mauricio Castañeda Cortés, Michele Croia, Gerardo D’Auria, Marco Diomede, David Dunning, Massimo Ferrario, Alessandro Gallo, Anna Giribono, Vitaliy Goryashko, Andrea Mostacci, Federico Nguyen, Regina Rochow, Jessica Scifo, Bruno Spataro, Neil Thompson, Cristina Vaccarezza, Alessandro Vannozzi, Xiaowei Wu and Walter Wuenschadd Show full author list remove Hide full author list
Photonics 2020, 7(4), 125; https://doi.org/10.3390/photonics7040125 - 4 Dec 2020
Cited by 5 | Viewed by 4661
Abstract
The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges [...] Read more.
The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges in the range 75–300 pC. Space-charge forces in the injector, linac transverse wakefield, and coherent synchrotron radiation in bunch compressors are all taken into account. The study confirms efficient lasing in the soft X-rays regime with pulse energies up to hundreds of microjoules at repetition rates as high as 1 kHz. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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17 pages, 11347 KB  
Article
Coherent Beam Combining Using an Internally Sensed Optical Phased Array of Frequency-Offset Phase Locked Lasers
by Lyle E. Roberts, Robert L. Ward, Craig Smith and Daniel A. Shaddock
Photonics 2020, 7(4), 118; https://doi.org/10.3390/photonics7040118 - 28 Nov 2020
Cited by 11 | Viewed by 6426
Abstract
Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly [...] Read more.
Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly dynamic phase differences between them. We present a scalable technique based on frequency-offset phase locking and digitally enhanced interferometry to clone the coherence of multiple lasers without the use of external sampling optics, which has the potential to support both coherent and spectral beam combining, and alleviates issues of voltage wrapping associated with actuating feedback control using electro-optic modulators. This technique was demonstrated experimentally using a tiled-aperture optical phased array in which the relative output phase of three free-running lasers was stabilized with an RMS output phase stability of λ/104. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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10 pages, 334 KB  
Article
Multiwavelength Frequency Modulated CW Ladar: The Effect of Refractive Index
by Mariano Barbieri, Deborah Katia Pallotti, Mario Siciliani de Cumis and Luigi Santamaria Amato
Photonics 2020, 7(4), 90; https://doi.org/10.3390/photonics7040090 - 8 Oct 2020
Cited by 3 | Viewed by 5779
Abstract
Frequency modulated continuous wave (FMCW) laser detection and ranging is a technique for absolute distance measurements with high performances in terms of resolution, non-ambiguity range, accuracy and fast detection. It is based on a simple experimental setup, thus resulting in cost restraint with [...] Read more.
Frequency modulated continuous wave (FMCW) laser detection and ranging is a technique for absolute distance measurements with high performances in terms of resolution, non-ambiguity range, accuracy and fast detection. It is based on a simple experimental setup, thus resulting in cost restraint with potential wide spread, not only limited to research institutions. The technique has been widely studied and improved both in terms of experimental setup by absolute reference or active stabilization and in terms of data analysis. Very recently a multi-wavelength approach has been exploited, demonstrating high precision and non ambiguity range. The variability of refractive index along the path was not taken into account with consequent degradation of range accuracy. In this work we developed a simple model able to take into account refractive index effect in multi-wavelength FMCW measurement. We performed a numerical simulation in different atmospheric conditions of temperature, pressure, humidity and CO2 concentration showing a net improvement of range accuracy when refractive index modeling is used. Full article
(This article belongs to the Special Issue Spectroscopy, Metrology and Quantum Technology for Space Science and Astrophysics)
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22 pages, 1693 KB  
Article
A Metamaterial-Inspired Approach to Mitigating Radio Frequency Blackout When a Plasma Forms Around a Reentry Vehicle
by Bruce A. Webb and Richard W. Ziolkowski
Photonics 2020, 7(4), 88; https://doi.org/10.3390/photonics7040088 - 6 Oct 2020
Cited by 13 | Viewed by 5578
Abstract
Radio frequency (RF) blackout and attenuation have been observed during atmospheric reentry since the advent of space exploration. The effects range from severe attenuation to complete loss of communications and can last from 90 s to 10 min depending on the vehicle’s trajectory. [...] Read more.
Radio frequency (RF) blackout and attenuation have been observed during atmospheric reentry since the advent of space exploration. The effects range from severe attenuation to complete loss of communications and can last from 90 s to 10 min depending on the vehicle’s trajectory. This paper examines a way of using a metasurface to improve the performance of communications during reentry. The technique is viable at low plasma densities and matches a split-ring resonator (SRR)-based mu-negative (MNG) sheet to the epsilon-negative (ENG) plasma region. Considering the MNG metasurface as a window to the exterior of a reentry vehicle, its matched design yields high transmission of an electromagnetic plane wave through the resulting MNG-ENG metastructure into the region beyond it. A varactor-based SRR design facilitates tuning the MNG layer to ENG layers with different plasma densities. Both simple and Huygens dipole antennas beneath a matched metastructure are then employed to demonstrate the consequent realization of significant signal transmission through it into free space beyond the exterior ENG plasma layer. Full article
(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)
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17 pages, 10058 KB  
Article
Near- and Far-Field Excitation of Topological Plasmonic Metasurfaces
by Matthew Proctor, Xiaofei Xiao, Richard V. Craster, Stefan A. Maier, Vincenzo Giannini and Paloma Arroyo Huidobro
Photonics 2020, 7(4), 81; https://doi.org/10.3390/photonics7040081 - 24 Sep 2020
Cited by 13 | Viewed by 4941
Abstract
The breathing honeycomb lattice hosts a topologically non-trivial bulk phase due to the crystalline-symmetry of the system. Pseudospin-dependent edge states, which emerge at the interface between trivial and non-trivial regions, can be used for the directional propagation of energy. Using the plasmonic metasurface [...] Read more.
The breathing honeycomb lattice hosts a topologically non-trivial bulk phase due to the crystalline-symmetry of the system. Pseudospin-dependent edge states, which emerge at the interface between trivial and non-trivial regions, can be used for the directional propagation of energy. Using the plasmonic metasurface as an example system, we probe these states in the near- and far-field using a semi-analytical model. We provide the conditions under which directionality was observed and show that it is source position dependent. By probing with circularly-polarised magnetic dipoles out of the plane, we first characterise modes along the interface in terms of the enhancement of source emissions due to the metasurface. We then excite from the far-field with non-zero orbital angular momentum beams. The position-dependent directionality holds true for all classical wave systems with a breathing honeycomb lattice. Our results show that a metasurface in combination with a chiral two-dimensional material, could be used to guide light effectively on the nanoscale. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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10 pages, 734 KB  
Article
Tunable THz Pulses Generation in Non-Equilibrium Magnetized Plasma: The Role of Plasma Kinetics
by Anna V. Bogatskaya, Nelli E. Gnezdovskaia and Alexander M. Popov
Photonics 2020, 7(4), 82; https://doi.org/10.3390/photonics7040082 - 24 Sep 2020
Cited by 2 | Viewed by 3280
Abstract
In this paper the theoretical model to consider the influence of kinetic properties of nonequilibrium two-color plasma during the THz pulses generation in the presence of static magnetic field is developed. It is shown that applying a static magnetic field on a gas [...] Read more.
In this paper the theoretical model to consider the influence of kinetic properties of nonequilibrium two-color plasma during the THz pulses generation in the presence of static magnetic field is developed. It is shown that applying a static magnetic field on a gas along the direction of propagation of an ionizing two-color laser pulse allows one to produce two-frequency emissions in THz range with tunable central frequency and bandwidth, which are strongly dependent on electron velocity distribution function (EVDF) formed in the plasma as well as relations between collisional, plasma and cyclotron frequencies. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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14 pages, 5446 KB  
Article
Structural, Optical and Electrical Characterizations of Midwave Infrared Ga-Free Type-II InAs/InAsSb Superlattice Barrier Photodetector
by U. Zavala-Moran, M. Bouschet, J. P. Perez, R. Alchaar, S. Bernhardt, I. Ribet-Mohamed, F. de Anda-Salazar and P. Christol
Photonics 2020, 7(3), 76; https://doi.org/10.3390/photonics7030076 - 18 Sep 2020
Cited by 24 | Viewed by 5812
Abstract
In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime [...] Read more.
In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime value equal to 1 µs at 80 K, carried out on dedicated structure showing photoluminescence peak position at 4.9 µm, is extracted from a time resolved photoluminescence measurement. Dark current density as low as 3.2 × 10−5 A/cm2 at 150 K is reported on the corresponding device exhibiting a 50% cut-off wavelength around 5 µm. A performance analysis through normalized spectral response and dark current density-voltage characteristics was performed to determine both the operating bias and the different dark current regimes. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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58 pages, 8781 KB  
Review
The Interband Cascade Laser
by Jerry R. Meyer, William W. Bewley, Chadwick L. Canedy, Chul Soo Kim, Mijin Kim, Charles D. Merritt and Igor Vurgaftman
Photonics 2020, 7(3), 75; https://doi.org/10.3390/photonics7030075 - 15 Sep 2020
Cited by 142 | Viewed by 16223
Abstract
We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for [...] Read more.
We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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8 pages, 2409 KB  
Letter
Demonstration of Planar Type-II Superlattice-Based Photodetectors Using Silicon Ion-Implantation
by Arash Dehzangi, Donghai Wu, Ryan McClintock, Jiakai Li, Alexander Jaud and Manijeh Razeghi
Photonics 2020, 7(3), 68; https://doi.org/10.3390/photonics7030068 - 3 Sep 2020
Cited by 14 | Viewed by 4801
Abstract
In this letter, we report the demonstration of a pBn planar mid-wavelength infrared photodetectors based on type-II InAs/InAs1−xSbx superlattices, using silicon ion-implantation to isolate the devices. At 77 K the photodetectors exhibited peak responsivity of 0.76 A/W at 3.8 µm, [...] Read more.
In this letter, we report the demonstration of a pBn planar mid-wavelength infrared photodetectors based on type-II InAs/InAs1−xSbx superlattices, using silicon ion-implantation to isolate the devices. At 77 K the photodetectors exhibited peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency, without anti-reflection coating, of 21.5% under an applied bias of +40 mV with a 100% cut-off wavelength of 4.6 µm. With a dark current density of 5.21 × 10−6 A/cm2, under +40 mV applied bias and at 77 K, the photodetector exhibited a specific detectivity of 4.95 × 1011 cm·Hz1/2/W. Full article
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9 pages, 2191 KB  
Communication
Wide-Angle Beam-Steering Using an Optical Phased Array with Non-Uniform-Width Waveguide Radiators
by Youngin Kim, Hyeonho Yoon, Jong-Bum You, Minchul Kim and Hyo-Hoon Park
Photonics 2020, 7(3), 56; https://doi.org/10.3390/photonics7030056 - 3 Aug 2020
Cited by 17 | Viewed by 5545
Abstract
We demonstrate wide-angle beam-steering using an optical phased array (OPA) with waveguide radiators designed with non-uniform widths to reduce the crosstalk between waveguides. The OPA consists of a silicon based 1 × 16 array of electro-optic phase shifters and end-fire radiators. The 16 [...] Read more.
We demonstrate wide-angle beam-steering using an optical phased array (OPA) with waveguide radiators designed with non-uniform widths to reduce the crosstalk between waveguides. The OPA consists of a silicon based 1 × 16 array of electro-optic phase shifters and end-fire radiators. The 16 radiators were configured with four different widths and a half-wavelength spacing, which can remove the higher-order diffraction patterns in free space. The waveguides showed a low crosstalk of −10.2 dB at a wavelength of 1540 nm. With phase control, the OPA achieved wide beam-steering of over ±80° with a side-lobe suppression of 7.4 dB. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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14 pages, 3258 KB  
Article
Characterization and Direct Modulation of a Multi-Section PIC Suited for Short Reach Optical Communication Systems
by Mohab N. Hammad, Aleksandra Kaszubowska-Anandarajah, M. Deseada Gutierrez Pascual, Pascal Landais, Prajwal D. Lakshmijayasimha, Gaurav Jain and Prince M. Anandarajah
Photonics 2020, 7(3), 55; https://doi.org/10.3390/photonics7030055 - 31 Jul 2020
Cited by 4 | Viewed by 5085
Abstract
A multi-section active photonic integrated circuit (PIC) is characterized in detail to gauge its suitability as a transmitter for short reach applications. The PIC is 1.5 mm long and consists of two lasers integrated in a master-slave configuration, which enables optical injection locking [...] Read more.
A multi-section active photonic integrated circuit (PIC) is characterized in detail to gauge its suitability as a transmitter for short reach applications. The PIC is 1.5 mm long and consists of two lasers integrated in a master-slave configuration, which enables optical injection locking (OIL) of the slave laser. The beneficial impact of the injection is characterized by static and dynamic measurements. The results show a reduction of the optical linewidth from 8 MHz to 2 MHz, a relative intensity noise (RIN) value as low as −154.3 dB/Hz and a 45% improvement of the slave laser modulation bandwidth from 9.5 GHz to 14 GHz. This frequency response enhancement allows the direct modulation of the slave gain section at a data rate of 10.7 Gb/s and an error-free transmission over 25 km of standard single-mode fiber (SSMF). Transmission performance of the injected case shows a 2 dB improvement in the minimum optical power required to achieve a bit error rate of 3.8×10−3 (hard decision forward error correction limit). These results demonstrate that the multi-section PIC can serve as an attractive cost-efficient transmitter in a wide variety of low-cost short-reach data communication applications. Full article
(This article belongs to the Section Optical Communication and Network)
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17 pages, 3872 KB  
Review
Nanoplasmonics in High Pressure Environment
by Grégory Barbillon
Photonics 2020, 7(3), 53; https://doi.org/10.3390/photonics7030053 - 28 Jul 2020
Cited by 7 | Viewed by 5148
Abstract
An explosion in the interest for nanoplasmonics has occurred in order to realize optical devices, biosensors, and photovoltaic devices. The plasmonic nanostructures are used for enhancing and confining the electric field. In the specific case of biosensing, this electric field confinement can induce [...] Read more.
An explosion in the interest for nanoplasmonics has occurred in order to realize optical devices, biosensors, and photovoltaic devices. The plasmonic nanostructures are used for enhancing and confining the electric field. In the specific case of biosensing, this electric field confinement can induce the enhancement of the Raman signal of different molecules, or the localized surface plasmon resonance shift after the detection of analytes on plasmonic nanostructures. A major part of studies concerning to plasmonic modes and their application to sensing of analytes is realized in ambient environment. However, over the past decade, an emerging subject of nanoplasmonics has appeared, which is nanoplasmonics in high pressure environment. In last five years (2015–2020), the latest advances in this emerging field and its application to sensing were carried out. This short review is focused on the pressure effect on localized surface plasmon resonance of gold nanosystems, the supercrystal formation of plasmonic nanoparticles stimulated by high pressure, and the detection of molecules and phase transitions with plasmonic nanostructures in high pressure environment. Full article
(This article belongs to the Special Issue Latest Advances in Nanoplasmonics and Use of New Tools for Plasmonic Characterization)
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12 pages, 586 KB  
Article
Cloaking Using the Anisotropic Multilayer Sphere
by Sidra Batool, Mehwish Nisar, Fabrizio Frezza and Fabio Mangini
Photonics 2020, 7(3), 52; https://doi.org/10.3390/photonics7030052 - 26 Jul 2020
Cited by 13 | Viewed by 4519
Abstract
We studied a Spherically Radially Anisotropic (SRA) multilayer sphere with an arbitrary number of layers. Within each layer permittivity components are different from each other in radial and tangential directions. Under the quasi-static approximation, we developed a more generalized mathematical model that can [...] Read more.
We studied a Spherically Radially Anisotropic (SRA) multilayer sphere with an arbitrary number of layers. Within each layer permittivity components are different from each other in radial and tangential directions. Under the quasi-static approximation, we developed a more generalized mathematical model that can be used to calculate polarizability of the SRA multilayer sphere with any arbitrary number of layers. Moreover, the functionality of the SRA multilayer sphere as a cloak has been investigated. It has been shown that by choosing a suitable contrast between components of the permittivity, the SRA multilayer sphere can achieve threshold required for invisibility cloaking. Full article
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11 pages, 6410 KB  
Article
Coupled Cavity Mid-IR Quantum Cascade Lasers Fabricated by Dry Etching
by Kamil Pierściński, Dorota Pierścińska, Aleksandr Kuźmicz, Grzegorz Sobczak, Maciej Bugajski, Piotr Gutowski and Krzysztof Chmielewski
Photonics 2020, 7(3), 45; https://doi.org/10.3390/photonics7030045 - 3 Jul 2020
Cited by 5 | Viewed by 4542
Abstract
In this work, two-section, coupled cavity, mid-IR quantum cascade lasers (QCLs) were characterized in terms of their tuning range and emission stability under operation towards potential application in detection systems. Devices were processed by inductively coupled plasma reactive ion etching (ICP-RIE) from InP-based [...] Read more.
In this work, two-section, coupled cavity, mid-IR quantum cascade lasers (QCLs) were characterized in terms of their tuning range and emission stability under operation towards potential application in detection systems. Devices were processed by inductively coupled plasma reactive ion etching (ICP-RIE) from InP-based heterostructure, designed for emission in the 9.x micrometer range. Single mode devices were demonstrated with a better than 20 dB side mode suppression ratio (SMRS). The fabrication method resulted in improved yield, as well as high repeatability of individual devices. Continuous, mode-hop-free tuning of emission wavelength was observed across ~4.5 cm−1 for the range of temperatures of the heat sink from 15 °C to 70 °C. Using the thermal perturbation in the lasing cavity, in conjunction with controlled hopping between coupled-cavity (CC) modes, we were able to accomplish tuning over the range of up to ~20 cm−1. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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15 pages, 3875 KB  
Review
Scattering Properties of PT-Symmetric Chiral Metamaterials
by Ioannis Katsantonis, Sotiris Droulias, Costas M. Soukoulis, Eleftherios N. Economou and Maria Kafesaki
Photonics 2020, 7(2), 43; https://doi.org/10.3390/photonics7020043 - 17 Jun 2020
Cited by 13 | Viewed by 5316
Abstract
The combination of gain and loss in optical systems that respect parity–time (PT)-symmetry has pointed recently to a variety of novel optical phenomena and possibilities. Many of them can be realized by combining the PT-symmetry concepts with metamaterials. Here we investigate the case [...] Read more.
The combination of gain and loss in optical systems that respect parity–time (PT)-symmetry has pointed recently to a variety of novel optical phenomena and possibilities. Many of them can be realized by combining the PT-symmetry concepts with metamaterials. Here we investigate the case of chiral metamaterials, showing that combination of chiral metamaterials with PT-symmetric gain–loss enables a very rich variety of phenomena and functionalities. Examining a simple one-dimensional chiral PT-symmetric system, we show that, with normally incident waves, the PT-symmetric and the chirality-related characteristics can be tuned independently and superimposed almost at will. On the other hand, under oblique incidence, chirality affects all the PT-related characteristics, leading also to novel and uncommon wave propagation features, such as asymmetric transmission and asymmetric optical activity and ellipticity. All these features are highly controllable both by chirality and by the angle of incidence, making PT-symmetric chiral metamaterials valuable in a large range of polarization-control-targeting applications. Full article
(This article belongs to the Special Issue Metamaterials for Advanced Photonic and Plasmonic Applications – Selected Papers from Metamaterials’2019)
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8 pages, 1566 KB  
Letter
Free Electron Laser Measurement of Liquid Carbon Reflectivity in the Extreme Ultraviolet
by Sumana L. Raj, Shane W. Devlin, Riccardo Mincigrucci, Craig P. Schwartz, Emiliano Principi, Filippo Bencivenga, Laura Foglia, Alessandro Gessini, Alberto Simoncig, Gabor Kurdi, Claudio Masciovecchio and Richard J. Saykally
Photonics 2020, 7(2), 35; https://doi.org/10.3390/photonics7020035 - 23 May 2020
Cited by 2 | Viewed by 3463
Abstract
Ultrafast time-resolved extreme ultraviolet (EUV) reflectivity measurements of optically pumped amorphous carbon (a-C) have been performed with the FERMI free electron laser (FEL). This work extends the energy range used in previous reflectivity studies and adds polarization dependence. The EUV probe is known [...] Read more.
Ultrafast time-resolved extreme ultraviolet (EUV) reflectivity measurements of optically pumped amorphous carbon (a-C) have been performed with the FERMI free electron laser (FEL). This work extends the energy range used in previous reflectivity studies and adds polarization dependence. The EUV probe is known to be sensitive to lattice dynamics, since in this range the reflectivity is essentially unaffected by the photo-excited surface plasma. The exploitation of both s- and p-polarized EUV radiation permits variation of the penetration depth of the probe; a significant increase in the characteristic time is observed upon increasing the probing depth (1 vs. 5 ps) due to hydrodynamic expansion and consequent destruction of the excited region, implying that there is only a short window during which the probed region is in the isochoric regime. A weak wavelength dependence of the reflectivity is found, consistent with previous measurements and implying a maximum electronic temperature of 0.8 eV ± 0.4. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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10 pages, 4017 KB  
Article
Wave Front Tuning of Coupled Hyperbolic Surface Waves on Anisotropic Interfaces
by Taavi Repän, Osamu Takayama and Andrei V. Lavrinenko
Photonics 2020, 7(2), 34; https://doi.org/10.3390/photonics7020034 - 20 May 2020
Cited by 7 | Viewed by 4578
Abstract
A photonic surface wave, a propagating optical mode localized at the interface of two media, can play a significant role in controlling the flow of light at nanoscale. Among various types of such waves, surface waves with hyperbolic dispersion or simply hyperbolic surface [...] Read more.
A photonic surface wave, a propagating optical mode localized at the interface of two media, can play a significant role in controlling the flow of light at nanoscale. Among various types of such waves, surface waves with hyperbolic dispersion or simply hyperbolic surface waves supported on anisotropic metal interfaces can be exploited to effectively control the propagation of lightwaves. We used semi-analytical and numerical methods to study the nature of surface waves on several configurations of three-layers metal–dielectric–metal systems including isotropic and anisotropic cases where the metal cladding layers were assumed to have infinite thickness. We used semi-analytical and numerical approaches to study the phenomena. We showed that the propagation of surface wave can be tuned from diverging to converging in the plane of the interface by the combination of metals with different anisotropic properties. Full article
(This article belongs to the Special Issue Topological Photonics)
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7 pages, 1512 KB  
Letter
Nonreciprocal Wavefront Manipulation in Synthetically Moving Metagratings
by Younes Ra’di and Andrea Alù
Photonics 2020, 7(2), 28; https://doi.org/10.3390/photonics7020028 - 18 Apr 2020
Cited by 20 | Viewed by 5981
Abstract
We introduce a metasurface platform for nonreciprocal wave manipulation. We study metagratings composed of nonreciprocal bianisotropic particles supporting synthetic motion, which enable nonreciprocal energy transfer between tailored Floquet channels with unitary efficiency. Based on this framework, we derive the required electromagnetic polarizabilities to [...] Read more.
We introduce a metasurface platform for nonreciprocal wave manipulation. We study metagratings composed of nonreciprocal bianisotropic particles supporting synthetic motion, which enable nonreciprocal energy transfer between tailored Floquet channels with unitary efficiency. Based on this framework, we derive the required electromagnetic polarizabilities to realize a metagrating supporting space wave circulation with unitary efficiency for free-space radiation and design a microwave metagrating supporting this functionality. The proposed concept opens new research venues to control free-space radiation with high efficiency beyond the limits dictated by Lorentz reciprocity. Full article
(This article belongs to the Special Issue Metamaterials for Advanced Photonic and Plasmonic Applications – Selected Papers from Metamaterials’2019)
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8 pages, 3285 KB  
Article
Generation of over 1000 Diffraction Spots from 2D Graded Photonic Super-Crystals
by Safaa Hassan, Yan Jiang, Khadijah Alnasser, Noah Hurley, Hualiang Zhang, Usha Philipose and Yuankun Lin
Photonics 2020, 7(2), 27; https://doi.org/10.3390/photonics7020027 - 10 Apr 2020
Cited by 4 | Viewed by 3739
Abstract
For the first time, we are able to generate over 1000 diffraction spots from a graded photonic super-crystal with a unit super-cell size of 12a × 12a where a is the lattice constant and hole radii are gradually changed in dual directions. The [...] Read more.
For the first time, we are able to generate over 1000 diffraction spots from a graded photonic super-crystal with a unit super-cell size of 12a × 12a where a is the lattice constant and hole radii are gradually changed in dual directions. The diffraction pattern from the graded photonic super-crystal reveals unique diffraction properties. The first order diffractions of (±1,0) or (0,±1) disappear. Fractional diffraction orders are observed in the diffraction pattern inside a square with vertices of (1,1), (1,−1), (−1,−1) and (−1,−1). The fractional diffraction can be understood from lattices with a period of a. However, a dual-lattice model is considered in order to explain higher-order diffractions. E-field intensity simulations show a coupling and re-distribution among fractional orders of Bloch waves. There are a total of 12 × 12 spots in E-field intensity in the unit supercell corresponding to 12 × 12 fractional diffraction orders in the diffraction pattern and 12 × 12 fractional orders of momentum in the first Brillouin zone in k-space. Full article
(This article belongs to the Special Issue Advanced Optical Materials and Devices)
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11 pages, 8376 KB  
Article
Topological Protection and Control of Quantum Markovianity
by Gian Luca Giorgi, Salvatore Lorenzo and Stefano Longhi
Photonics 2020, 7(1), 18; https://doi.org/10.3390/photonics7010018 - 8 Feb 2020
Cited by 11 | Viewed by 5313
Abstract
Under the Born–Markov approximation, a qubit system, such as a two-level atom, is known to undergo a memoryless decay of quantum coherence or excitation when weakly coupled to a featureless environment. Recently, it has been shown that unavoidable disorder in the environment is [...] Read more.
Under the Born–Markov approximation, a qubit system, such as a two-level atom, is known to undergo a memoryless decay of quantum coherence or excitation when weakly coupled to a featureless environment. Recently, it has been shown that unavoidable disorder in the environment is responsible for non-Markovian effects and information backflow from the environment into the system owing to Anderson localization. This turns disorder into a resource for enhancing non-Markovianity in the system–environment dynamics, which could be of relevance in cavity quantum electrodynamics. Here we consider the decoherence dynamics of a qubit weakly coupled to a two-dimensional bath with a nontrivial topological phase, such as a two-level atom embedded in a two-dimensional coupled-cavity array with a synthetic gauge field realizing a quantum-Hall bath, and show that Markovianity is protected against moderate disorder owing to the robustness of chiral edge modes in the quantum-Hall bath. Interestingly, switching off the gauge field, i.e., flipping the bath into a topological trivial phase, allows one to re-introduce non-Markovian effects. Such a result indicates that changing the topological phase of a bath by a tunable synthetic gauge field can be harnessed to control non-Markovian effects and quantum information backflow in a qubit-environment system. Full article
(This article belongs to the Special Issue Topological Photonics)
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17 pages, 5514 KB  
Review
Ultrahigh-Resolution Optical Vector Analyzers
by Oleg Morozov, Ilnur Nureev, Airat Sakhabutdinov, Artem Kuznetsov, Gennady Morozov, German Il’in, Samvel Papazyan, Alexander Ivanov and Roman Ponomarev
Photonics 2020, 7(1), 14; https://doi.org/10.3390/photonics7010014 - 20 Jan 2020
Cited by 35 | Viewed by 7135
Abstract
The optical vector analyzer is a device used to measure the magnitude, phase responses, and other parameters of optical devices. There have been increasingly higher demands placed on optical vector analyzers during the development of optical technologies, which are satisfied by the creation [...] Read more.
The optical vector analyzer is a device used to measure the magnitude, phase responses, and other parameters of optical devices. There have been increasingly higher demands placed on optical vector analyzers during the development of optical technologies, which are satisfied by the creation of new devices and their operating principles. For further development in this area, it is necessary to generalize the experience gained during the development of optical vector analyzers. Thus, in this report, we provide an overview of all the basic types of approaches used for the realization of optical vector analyzers, including the advanced ones with the best performances. The principles of their working, as well as their associated advantages, disadvantages, and existing solutions to the identified problems, are examined in detail. The presented approaches could be of value and interest to those working in the field of laser dynamics and optical devices, as we propose one use of the optical vector analyzer as being the characterization of Fano resonance structures. Full article
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10 pages, 1373 KB  
Article
Equivalent Circuit Model of High-Performance VCSELs
by Marwan Bou Sanayeh, Wissam Hamad and Werner Hofmann
Photonics 2020, 7(1), 13; https://doi.org/10.3390/photonics7010013 - 18 Jan 2020
Cited by 8 | Viewed by 7660
Abstract
In this work, a general equivalent circuit model based on the carrier reservoir splitting approach in high-performance multi-mode vertical-cavity surface-emitting lasers (VCSELs) is presented. This model accurately describes the intrinsic dynamic behavior of these VCSELs for the case where the lasing modes do [...] Read more.
In this work, a general equivalent circuit model based on the carrier reservoir splitting approach in high-performance multi-mode vertical-cavity surface-emitting lasers (VCSELs) is presented. This model accurately describes the intrinsic dynamic behavior of these VCSELs for the case where the lasing modes do not share a common carrier reservoir. Moreover, this circuit model is derived from advanced multi-mode rate equations that take into account the effect of spatial hole-burning, gain compression, and inhomogeneity in the carrier distribution between the lasing mode ensembles. The validity of the model is confirmed through simulation of the intrinsic modulation response of these lasers. Full article
(This article belongs to the Special Issue Semiconductor Laser Dynamics: Fundamentals and Applications)
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8 pages, 2790 KB  
Article
RF Injection of THz QCL Combs at 80 K Emitting over 700 GHz Spectral Bandwidth
by Andres Forrer, Lorenzo Bosco, Mattias Beck, Jérôme Faist and Giacomo Scalari
Photonics 2020, 7(1), 9; https://doi.org/10.3390/photonics7010009 - 16 Jan 2020
Cited by 15 | Viewed by 5808
Abstract
We report about RF injection locking of an homogeneous THz quantum cascade laser operating at 3 THz central frequency. The extremely diagonal nature of the optical transition, combined with low-loss copper-based double-metal waveguides, allow CW operation up to 105 K and CW power [...] Read more.
We report about RF injection locking of an homogeneous THz quantum cascade laser operating at 3 THz central frequency. The extremely diagonal nature of the optical transition, combined with low-loss copper-based double-metal waveguides, allow CW operation up to 105 K and CW power in excess of 5.6 mW measured at 80 K. Terahertz emission spanning up to 600 GHz, together with a narrow beatnote, indicate comb operation at 80 K, and strong RF injection clearly modifies the laser spectrum up to 700 GHz spectral bandwidth making these devices ideal candidates for an on-chip dual comb spectrometer. Full article
(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)
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33 pages, 11871 KB  
Review
Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits
by Klaus-J. Boller, Albert van Rees, Youwen Fan, Jesse Mak, Rob E. M. Lammerink, Cornelis A. A. Franken, Peter J. M. van der Slot, David A. I. Marpaung, Carsten Fallnich, Jörn P. Epping, Ruud M. Oldenbeuving, Dimitri Geskus, Ronald Dekker, Ilka Visscher, Robert Grootjans, Chris G. H. Roeloffzen, Marcel Hoekman, Edwin J. Klein, Arne Leinse and René G. Heideman
Photonics 2020, 7(1), 4; https://doi.org/10.3390/photonics7010004 - 21 Dec 2019
Cited by 124 | Viewed by 23151
Abstract
Hybrid integrated semiconductor laser sources offering extremely narrow spectral linewidth, as well as compatibility for embedding into integrated photonic circuits, are of high importance for a wide range of applications. We present an overview on our recently developed hybrid-integrated diode lasers with feedback [...] Read more.
Hybrid integrated semiconductor laser sources offering extremely narrow spectral linewidth, as well as compatibility for embedding into integrated photonic circuits, are of high importance for a wide range of applications. We present an overview on our recently developed hybrid-integrated diode lasers with feedback from low-loss silicon nitride (Si 3 N 4 in SiO 2 ) circuits, to provide sub-100-Hz-level intrinsic linewidths, up to 120 nm spectral coverage around a 1.55 μ m wavelength, and an output power above 100 mW. We show dual-wavelength operation, dual-gain operation, laser frequency comb generation, and present work towards realizing a visible-light hybrid integrated diode laser. Full article
(This article belongs to the Special Issue Semiconductor Laser Dynamics: Fundamentals and Applications)
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14 pages, 791 KB  
Article
Synchronization of Mutually Delay-Coupled Quantum Cascade Lasers with Distinct Pump Strengths
by Thomas Erneux and Daan Lenstra
Photonics 2019, 6(4), 125; https://doi.org/10.3390/photonics6040125 - 10 Dec 2019
Cited by 15 | Viewed by 3520
Abstract
The rate equations for two delay-coupled quantum cascade lasers are investigated analytically in the limit of weak coupling and small frequency detuning. We mathematically derive two coupled Adler delay differential equations for the phases of the two electrical fields and show that these [...] Read more.
The rate equations for two delay-coupled quantum cascade lasers are investigated analytically in the limit of weak coupling and small frequency detuning. We mathematically derive two coupled Adler delay differential equations for the phases of the two electrical fields and show that these equations are no longer valid if the ratio of the two pump parameters is below a critical power of the coupling constant. We analyze this particular case and derive new equations for a single optically injected laser where the delay is no longer present in the arguments of the dependent variables. Our analysis is motivated by the observations of Bogris et al. (IEEE J. Sel. Top. Quant. El. 23, 1500107 (2017)), who found better sensing performance using two coupled quantum cascade lasers when one laser was operating close to the threshold. Full article
(This article belongs to the Special Issue Semiconductor Laser Dynamics: Fundamentals and Applications)
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11 pages, 943 KB  
Article
Task-Independent Computational Abilities of Semiconductor Lasers with Delayed Optical Feedback for Reservoir Computing
by Krishan Harkhoe and Guy Van der Sande
Photonics 2019, 6(4), 124; https://doi.org/10.3390/photonics6040124 - 2 Dec 2019
Cited by 34 | Viewed by 4770
Abstract
Reservoir computing has rekindled neuromorphic computing in photonics. One of the simplest technological implementations of reservoir computing consists of a semiconductor laser with delayed optical feedback. In this delay-based scheme, virtual nodes are distributed in time with a certain node distance and form [...] Read more.
Reservoir computing has rekindled neuromorphic computing in photonics. One of the simplest technological implementations of reservoir computing consists of a semiconductor laser with delayed optical feedback. In this delay-based scheme, virtual nodes are distributed in time with a certain node distance and form a time-multiplexed network. The information processing performance of a semiconductor laser-based reservoir computing (RC) system is usually analysed by way of testing the laser-based reservoir computer on specific benchmark tasks. In this work, we will illustrate the optimal performance of the system on a chaotic time-series prediction benchmark. However, the goal is to analyse the reservoir’s performance in a task-independent way. This is done by calculating the computational capacity, a measure for the total number of independent calculations that the system can handle. We focus on the dependence of the computational capacity on the specifics of the masking procedure. We find that the computational capacity depends strongly on the virtual node distance with an optimal node spacing of 30 ps. In addition, we show that the computational capacity can be further increased by allowing for a well chosen mismatch between delay and input data sample time. Full article
(This article belongs to the Special Issue Semiconductor Laser Dynamics: Fundamentals and Applications)
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10 pages, 2320 KB  
Article
Biofunctionalized Polyelectrolyte Microcapsules Encoded with Fluorescent Semiconductor Nanocrystals for Highly Specific Targeting and Imaging of Cancer Cells
by Galina Nifontova, Daria Kalenichenko, Maria Baryshnikova, Fernanda Ramos Gomes, Frauke Alves, Alexander Karaulov, Igor Nabiev and Alyona Sukhanova
Photonics 2019, 6(4), 117; https://doi.org/10.3390/photonics6040117 - 8 Nov 2019
Cited by 9 | Viewed by 4633
Abstract
Fluorescent semiconductor nanocrystals or quantum dots (QDs) are characterized by unique optical properties, including a high photostability, wide absorption spectrum, and narrow, symmetric fluorescence spectrum. This makes them attractive fluorescent nanolabels for the optical encoding of microcarriers intended for targeted drug delivery, diagnosis, [...] Read more.
Fluorescent semiconductor nanocrystals or quantum dots (QDs) are characterized by unique optical properties, including a high photostability, wide absorption spectrum, and narrow, symmetric fluorescence spectrum. This makes them attractive fluorescent nanolabels for the optical encoding of microcarriers intended for targeted drug delivery, diagnosis, and imaging of transport processes on the body, cellular, and subcellular levels. Incorporation of QDs into carriers in the form of polyelectrolyte microcapsules through layer-by-layer adsorption of oppositely charged polyelectrolyte polymers yields microcapsules with a bright fluorescence signal and adaptable size, structure, and surface characteristics without using organic solvents. The easily modifiable surface of the microcapsules allows for its subsequent functionalization with capture molecules, such as antibodies, which ensures specific and selective interaction with cells, including tumor cells, with the use of the bioconjugation technique developed here. We obtained stable water-soluble nanolabels based on QDs whose surface was modified with polyethylene glycol (PEG) derivatives and determined their colloidal and optical characteristics. The obtained nanocrystals were used to encode polyelectrolyte microcapsules optically. The microcapsule surface was modified with humanized monoclonal antibodies (Abs) recognizing a cancer marker, epidermal growth factor receptor (EGFR). The possibility of effective, specific, and selective delivery of the microcapsules to tumor cells expressing EGFR has been demonstrated. The results show that the QD-encoded polyelectrolyte microcapsules functionalized with monoclonal Abs against EGFR can be used for targeted imaging and diagnosis. Full article
(This article belongs to the Special Issue Advances on Applications of Optics and Photonics – Selected Papers from the 4th International Conference on Applications of Optics and Photonics, AOP2019)
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16 pages, 1034 KB  
Article
Pupil Function in Pseudophakia: Proximal Miosis Behavior and Optical Influence
by Elsa Fonseca, Paulo Fiadeiro, Renato Gomes, Angel Sanchez Trancon, António Baptista and Pedro Serra
Photonics 2019, 6(4), 114; https://doi.org/10.3390/photonics6040114 - 6 Nov 2019
Cited by 5 | Viewed by 7285
Abstract
The pseudophakic eye lacks the ability to produce a refractive change in response to object proximity. Thus, individual anatomical features such as the pupil size play an important role in achieving functional vision levels. In this work, the range of pupil sizes at [...] Read more.
The pseudophakic eye lacks the ability to produce a refractive change in response to object proximity. Thus, individual anatomical features such as the pupil size play an important role in achieving functional vision levels. In this work, the range of pupil sizes at varying object distance was measured in pseudophakic participants. Furthermore, the impact of the measured values on eye optical quality was investigated using a computer simulation model. A binocular eye-tracker was used to measure the participants’ pupil sizes at six object distances, ranging from 0.33 m (i.e., vergence of 3.00 D) to 3.00 m (i.e., vergence of 0.33 D), while observing a Maltese cross with a constant angular size of 1 ∘ . In total, 58 pseudophakic participants were enrolled in this study (age mean ± standard deviation: 70.5 ± 11.3 years). The effects of object distance and age on pupil size variation were investigated using linear mixed effects regression models. Age was found to have a small contribution to individual variability. The mean infinite distance pupil size (intercept) was 4.45 ( 95 % CI: 2.74, 6.17) mm and the mean proximal miosis (slope) was − 0.23 ( 95 % CI: −0.53, 0.08) mm/D. The visual acuity (VA) estimation for a distant object ranged from − 0.1 logMAR (smallest pupil) to 0.04 logMAR (largest pupil) and the near VA ( 0.33 m) when mean proximal miosis was considered ranged from 0.28 logMAR (smallest pupil) to 0.42 logMAR (largest pupil). When mean distance pupil was considered, proximal miosis individual variability produced a variation of 0.04 logMAR for the near object and negligible variation for the distant object. These results support the importance of distance pupil size measurement for the prediction of visual performance in pseudophakia, while suggesting that proximal miosis has a negligible impact in VA variability. Full article
(This article belongs to the Special Issue Advances on Applications of Optics and Photonics – Selected Papers from the 4th International Conference on Applications of Optics and Photonics, AOP2019)
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14 pages, 5918 KB  
Article
A Comparison between off and On-Chip Injection Locking in a Photonic Integrated Circuit
by Alison H. Perrott, Ludovic Caro, Mohamad Dernaika and Frank H. Peters
Photonics 2019, 6(4), 103; https://doi.org/10.3390/photonics6040103 - 1 Oct 2019
Cited by 9 | Viewed by 4304
Abstract
The mutual and injection locking characteristics of two integrated lasers are compared, both on and off-chip. In this study, two integrated single facet slotted Fabry–Pérot lasers are utilised to develop the measurement technique used to examine the different operational regimes arising from optically [...] Read more.
The mutual and injection locking characteristics of two integrated lasers are compared, both on and off-chip. In this study, two integrated single facet slotted Fabry–Pérot lasers are utilised to develop the measurement technique used to examine the different operational regimes arising from optically locking a semiconductor diode laser. The technique employed used an optical spectrum analyser (OSA), an electrical spectrum analyser (ESA) and a high speed oscilloscope (HSO). The wavelengths of the lasers are measured on the OSA and the selected optical mode for locking is identified. The region of injection locking and various other regions of dynamical behaviour between the lasers are observed on the ESA. The time trace information of the system is obtained from the HSO and performing the FFT (Fast Fourier Transform) of the time traces returns the power spectra. Using these tools, the similarities and differences between off-chip injection locking with an isolator, and on-chip mutual locking are examined. Full article
(This article belongs to the Special Issue Semiconductor Laser Dynamics: Fundamentals and Applications)
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13 pages, 7165 KB  
Article
Flowline Optical Simulation to Refractive/Reflective 3D Systems: Optical Path Length Correction
by Angel García-Botella, Lun Jiang and Roland Winston
Photonics 2019, 6(4), 101; https://doi.org/10.3390/photonics6040101 - 28 Sep 2019
Cited by 6 | Viewed by 3819
Abstract
Nonimaging optics is focused on the study of techniques to design optical systems for the purpose of energy transfer instead of image forming. The flowline optical design method, based on the definition of the geometrical flux vector J, is one of these [...] Read more.
Nonimaging optics is focused on the study of techniques to design optical systems for the purpose of energy transfer instead of image forming. The flowline optical design method, based on the definition of the geometrical flux vector J, is one of these techniques. The main advantage of the flowline method is its capability to visualize and estimate how radiant energy is transferred by the optical systems using the concepts of vector field theory, such as field line or flux tube, which overcomes traditional raytrace methods. The main objective this paper is to extend the flowline method to analyze and design real 3D concentration and illumination systems by the development of new simulation techniques. In this paper, analyzed real 3D refractive and reflective systems using the flowline vector potential method. A new constant term of optical path length is introduced, similar and comparable to the gauge invariant, which produces a correction to enable the agreement between raytrace- and flowline-based computations. This new optical simulation methodology provides traditional raytrace results, such as irradiance maps, but opens new perspectives to obtaining higher precision with lower computation time. It can also provide new information for the vector field maps of 3D refractive/reflective systems. Full article
(This article belongs to the Special Issue Nonimaging Optics in Solar Energy)
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11 pages, 1049 KB  
Article
Effects of Performance and Task Duration on Mental Workload during Working Memory Task
by Kosar Khaksari, Emma Condy, John B. Millerhagen, Afrouz A. Anderson, Hadis Dashtestani and Amir H. Gandjbakhche
Photonics 2019, 6(3), 94; https://doi.org/10.3390/photonics6030094 - 28 Aug 2019
Cited by 21 | Viewed by 6554
Abstract
N-back is a working memory (WM) task to study mental workload on the prefrontal cortex (PFC). We assume that the subject’s performance and changes in mental workload over time depends on the length of the experiment. The performance of the participant can change [...] Read more.
N-back is a working memory (WM) task to study mental workload on the prefrontal cortex (PFC). We assume that the subject’s performance and changes in mental workload over time depends on the length of the experiment. The performance of the participant can change positively due to the participant’s learning process or negatively because of objective mental fatigue and/or sleepiness. In this pilot study, we examined the PFC activation of 23 healthy subjects while they performed an N-back task with two different levels of task difficulty (2-, and 3-back). The hemodynamic responses were analyzed along with the behavioral data (correct answers). A comparison was done between the hemodynamic activation and behavioral data between the two different task levels and between the beginning and end of the 3-back task. Our results show that there is a significant difference between the two task levels, which is due to the difference in task complication. In addition, a significant difference was seen between the beginning and end of the 3-back task in both behavioral data and hemodynamics due to the subject’s learning process throughout the experiment. Full article
(This article belongs to the Special Issue Neurophotonics – Optics for the Brain)
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18 pages, 7938 KB  
Article
Experimental Investigation on the Performances of Innovative PV Vertical Structures
by Gianluca Acciari, Gabriele Adamo, Guido Ala, Alessandro Busacca, Massimo Caruso, Graziella Giglia, Antonino Imburgia, Patrizia Livreri, Rosario Miceli, Antonino Parisi, Filippo Pellitteri, Riccardo Pernice, Pietro Romano, Giuseppe Schettino and Fabio Viola
Photonics 2019, 6(3), 86; https://doi.org/10.3390/photonics6030086 - 31 Jul 2019
Cited by 24 | Viewed by 4683
Abstract
The sustainable development of our planet is considerably related to a relevant reduction of CO2 global emissions, with building consumption contributing more than 40%. In this scenario, new technological conceptions, such as building-integrated photovoltaic technology, emerged in order to satisfy the requirements [...] Read more.
The sustainable development of our planet is considerably related to a relevant reduction of CO2 global emissions, with building consumption contributing more than 40%. In this scenario, new technological conceptions, such as building-integrated photovoltaic technology, emerged in order to satisfy the requirements of sustainability imposed by the European Union. Therefore, the aim of this work is to provide a technical and economical comparison of the performances of different vertical-mounted innovative photovoltaic systems, potentially integrated on a building instead of on traditional windows or glass walls. The proposed investigation was carried out by means of experimental tests on three different next-generation vertical structures. The related results are described and discussed, highlighting the advantages and the drawbacks of the proposed technologies. Full article
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15 pages, 2580 KB  
Article
Multi-Distance Frequency-Domain Optical Measurements of Coherent Cerebral Hemodynamics
by Giles Blaney, Angelo Sassaroli, Thao Pham, Nishanth Krishnamurthy and Sergio Fantini
Photonics 2019, 6(3), 83; https://doi.org/10.3390/photonics6030083 - 26 Jul 2019
Cited by 20 | Viewed by 4857
Abstract
We report non-invasive, bilateral optical measurements on the forehead of five healthy human subjects, of 0.1 Hz oscillatory hemodynamics elicited either by cyclic inflation of pneumatic thigh cuffs, or by paced breathing. Optical intensity and the phase of photon-density waves were collected with [...] Read more.
We report non-invasive, bilateral optical measurements on the forehead of five healthy human subjects, of 0.1 Hz oscillatory hemodynamics elicited either by cyclic inflation of pneumatic thigh cuffs, or by paced breathing. Optical intensity and the phase of photon-density waves were collected with frequency-domain near-infrared spectroscopy at seven source-detector distances (11–40 mm). Coherent hemodynamic oscillations are represented by phasors of oxyhemoglobin (O) and deoxyhemoglobin (D) concentrations, and by the vector D/O that represents the amplitude ratio and phase difference of D and O. We found that, on an average, the amplitude ratio (|D/O|) and the phase difference (∠(D/O)) obtained with single-distance intensity at 11–40 mm increase from 0.1° and −330° to 0.2° and −200°, respectively. Single-distance phase and the intensity slope featured a weaker dependence on source-detector separation, and yielded |D/O| and ∠(D/O) values of about 0.5 and −200°, respectively, at distances greater than 20 mm. The key findings are: (1) Single-distance phase and intensity slope are sensitive to deeper tissue compared to single-distance intensity; (2) deeper tissue hemodynamic oscillations, which more closely represent the brain, feature D and O phasors that are consistent with a greater relative flow-to-volume contributions in brain tissue compared to extracerebral, superficial tissue. Full article
(This article belongs to the Special Issue Neurophotonics – Optics for the Brain)
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9 pages, 1391 KB  
Letter
A Robust Method for Adjustment of Laser Speckle Contrast Imaging during Transcranial Mouse Brain Visualization
by Vyacheslav Kalchenko, Anton Sdobnov, Igor Meglinski, Yuri Kuznetsov, Guillaume Molodij and Alon Harmelin
Photonics 2019, 6(3), 80; https://doi.org/10.3390/photonics6030080 - 13 Jul 2019
Cited by 35 | Viewed by 11233
Abstract
Laser speckle imaging (LSI) is a well-known and useful approach for the non-invasive visualization of flows and microcirculation localized in turbid scattering media, including biological tissues (such as brain vasculature, skin capillaries etc.). Despite an extensive use of LSI for brain imaging, the [...] Read more.
Laser speckle imaging (LSI) is a well-known and useful approach for the non-invasive visualization of flows and microcirculation localized in turbid scattering media, including biological tissues (such as brain vasculature, skin capillaries etc.). Despite an extensive use of LSI for brain imaging, the LSI technique has several critical limitations. One of them is associated with inability to resolve a functionality of vessels. This limitation also leads to the systematic error in the quantitative interpretation of values of speckle contrast obtained for different vessel types, such as sagittal sinus, arteries, and veins. Here, utilizing a combined use of LSI and fluorescent intravital microscopy (FIM), we present a simple and robust method to overcome the limitations mentioned above for the LSI approach. The proposed technique provides more relevant, abundant, and valuable information regarding perfusion rate ration between different types of vessels that makes this method highly useful for in vivo brain surgical operations. Full article
(This article belongs to the Special Issue Biomedical Photonics Advances)
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22 pages, 1073 KB  
Review
Photobiomodulation for Alzheimer’s Disease: Has the Light Dawned?
by Michael R. Hamblin
Photonics 2019, 6(3), 77; https://doi.org/10.3390/photonics6030077 - 4 Jul 2019
Cited by 92 | Viewed by 28620
Abstract
Next to cancer, Alzheimer’s disease (AD) and dementia is probably the most worrying health problem facing the Western world today. A large number of clinical trials have failed to show any benefit of the tested drugs in stabilizing or reversing the steady decline [...] Read more.
Next to cancer, Alzheimer’s disease (AD) and dementia is probably the most worrying health problem facing the Western world today. A large number of clinical trials have failed to show any benefit of the tested drugs in stabilizing or reversing the steady decline in cognitive function that is suffered by dementia patients. Although the pathological features of AD consisting of beta-amyloid plaques and tau tangles are well established, considerable debate exists concerning the genetic or lifestyle factors that predispose individuals to developing dementia. Photobiomodulation (PBM) describes the therapeutic use of red or near-infrared light to stimulate healing, relieve pain and inflammation, and prevent tissue from dying. In recent years PBM has been applied for a diverse range of brain disorders, frequently applied in a non-invasive manner by shining light on the head (transcranial PBM). The present review discusses the mechanisms of action of tPBM in the brain, and summarizes studies that have used tPBM to treat animal models of AD. The results of a limited number of clinical trials that have used tPBM to treat patients with AD and dementia are discussed. Full article
(This article belongs to the Special Issue Neurophotonics – Optics for the Brain)
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13 pages, 11248 KB  
Article
Long-Range, High-Resolution Camera Optical Design for Assisted and Autonomous Driving
by Furkan E. Sahin
Photonics 2019, 6(2), 73; https://doi.org/10.3390/photonics6020073 - 25 Jun 2019
Cited by 29 | Viewed by 32345
Abstract
High-quality cameras are fundamental sensors in assisted and autonomous driving. In particular, long-range forward-facing cameras can provide vital information about the road ahead, including detection and recognition of objects and early hazard warning. These automotive cameras should provide high-resolution images consistently under extreme [...] Read more.
High-quality cameras are fundamental sensors in assisted and autonomous driving. In particular, long-range forward-facing cameras can provide vital information about the road ahead, including detection and recognition of objects and early hazard warning. These automotive cameras should provide high-resolution images consistently under extreme operating conditions of the car for robust operation. This paper aims to introduce the design of fixed-focus, passively athermalized lenses for next-generation automotive cameras. After introducing an overview of essential and desirable features of automotive cameras and state-of-the-art, based on these features, two different camera designs that can achieve traffic sign recognition at 200 m distance are presented. These lenses are designed from scratch, with a unique design approach that starts with a graphical lens material selection tool and arrives at an optimized design with optical design software. Optical system analyses are performed to evaluate the lens designs. The lenses are shown to accomplish high contrast from − 40 °C to 100 °C and allow for a 4 × increase in resolution of automotive cameras. Full article
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15 pages, 4951 KB  
Article
Optoacoustic Calcium Imaging of Deep Brain Activity in an Intracardially Perfused Mouse Brain Model
by Oleksiy Degtyaruk, Benedict Mc Larney, Xosé Luís Deán-Ben, Shy Shoham and Daniel Razansky
Photonics 2019, 6(2), 67; https://doi.org/10.3390/photonics6020067 - 12 Jun 2019
Cited by 10 | Viewed by 6635
Abstract
One main limitation of established neuroimaging methods is the inability to directly visualize large-scale neural dynamics in whole mammalian brains at subsecond speeds. Optoacoustic imaging has advanced in recent years to provide unique advantages for real-time deep-tissue observations, which have been exploited for [...] Read more.
One main limitation of established neuroimaging methods is the inability to directly visualize large-scale neural dynamics in whole mammalian brains at subsecond speeds. Optoacoustic imaging has advanced in recent years to provide unique advantages for real-time deep-tissue observations, which have been exploited for three-dimensional imaging of both cerebral hemodynamic parameters and direct calcium activity in rodents. Due to a lack of suitable calcium indicators excitable in the near-infrared window, optoacoustic imaging of neuronal activity at deep-seated areas of the mammalian brain has been impeded by the strong absorption of blood in the visible range of the light spectrum. To overcome this, we have developed and validated an intracardially perfused mouse brain preparation labelled with genetically encoded calcium indicator GCaMP6f that closely resembles in vivo conditions. By overcoming the limitations of hemoglobin-based light absorption, this new technique was used to observe stimulus-evoked calcium dynamics in the brain at penetration depths and spatio-temporal resolution scales not attainable with existing neuroimaging techniques. Full article
(This article belongs to the Special Issue Neurophotonics – Optics for the Brain)
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9 pages, 1026 KB  
Article
High Concentration Photovoltaics (HCPV) with Diffractive Secondary Optical Elements
by Furkan E. Sahin and Musa Yılmaz
Photonics 2019, 6(2), 68; https://doi.org/10.3390/photonics6020068 - 12 Jun 2019
Cited by 18 | Viewed by 16908
Abstract
Multi-junction solar cells can be economically viable for terrestrial applications when operated under concentrated illuminations. The optimal design of concentrator optics in high concentration photovoltaics (HCPV) systems is crucial for achieving high energy conversion. At a high geometric concentration, chromatic aberration of the [...] Read more.
Multi-junction solar cells can be economically viable for terrestrial applications when operated under concentrated illuminations. The optimal design of concentrator optics in high concentration photovoltaics (HCPV) systems is crucial for achieving high energy conversion. At a high geometric concentration, chromatic aberration of the primary lens can restrict the optical efficiency and acceptance angle. In order to correct chromatic aberration, multi-material, multi-element refractive elements, hybrid refractive/diffractive elements, or multi-element refractive and diffractive systems can be designed. In this paper, the effect of introducing a diffractive surface in the optical path is analyzed. An example two-stage refractive and diffractive optical system is shown to have an optical efficiency of up to 0.87, and an acceptance angle of up to ±0.55° with a 1600× geometric concentration ratio, which is a significant improvement compared to a single-stage concentrator system with a single material. This optical design can be mass-produced with conventional fabrication methods, thus providing a low-cost alternative to other approaches, and the design approach can be generalized to many other solar concentrator systems with different cell sizes and geometric concentration ratios. Full article
(This article belongs to the Special Issue Nonimaging Optics in Solar Energy)
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17 pages, 4723 KB  
Review
Photonic and Iontronic Sensing in GaInAsP Semiconductor Photonic Crystal Nanolasers
by Toshihiko Baba
Photonics 2019, 6(2), 65; https://doi.org/10.3390/photonics6020065 - 10 Jun 2019
Cited by 12 | Viewed by 5743
Abstract
The GaInAsP semiconductor photonic crystal nanolaser operates at room temperature by photopumping and emits near-infrared light at a wavelength longer than 1.3 μm. Immersion of the nanolaser in a solution causes its laser characteristics to change. Observation of this phenomenon makes it possible [...] Read more.
The GaInAsP semiconductor photonic crystal nanolaser operates at room temperature by photopumping and emits near-infrared light at a wavelength longer than 1.3 μm. Immersion of the nanolaser in a solution causes its laser characteristics to change. Observation of this phenomenon makes it possible to perform biosensing without a fluorescent label or a chromogenic substrate. The most common phenomenon between many photonic sensors is that the resonance wavelength reflects the refractive index of attached media; an index change of 2.5 × 10−4 in the surrounding liquid can be measured through an emission wavelength shift without stabilization. This effect is applicable to detecting environmental toxins and cell behaviors. The laser emission intensity also reflects the electric charge of surface ions. The intensity varies when an electrolyte or a negatively charged deoxyribonucleic acid (DNA), which is positively or negatively charged in water, is accumulated on the surface. This effect allows us to detect the antigen-antibody reaction of a biomarker protein from only the emission intensity without any kind of spectroscopy. In detecting a small amount of DNA or protein, a wavelength shift also appears from its concentration that is 2–3 orders of magnitude lower than those of the conventional chemical methods, such as the enzyme-linked immuno-solvent assay. It is unlikely that this wavelength behavior at such low concentrations is due to the refractive index of the biomolecules. It is observed that the electric charge of surface ions is induced by various means, including plasma exposure and an electrochemical circuit shifting the wavelength. This suggests that the superhigh sensitivity is also due to the effect of charged ions. Thus, we call this device an iontronic photonic sensor. This paper focuses on such a novel sensing scheme of nanolaser sensor, as an example of resonator-based photonic sensors, in addition to the conventional refractive index sensing. Full article
(This article belongs to the Special Issue Photonic Crystal Laser and Related Optical Devices)
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12 pages, 8488 KB  
Article
Propagation of Cylindrical Vector Laser Beams in Turbid Tissue-Like Scattering Media
by Alexander Doronin, Nicolás Vera, Juan P. Staforelli, Pablo Coelho and Igor Meglinski
Photonics 2019, 6(2), 56; https://doi.org/10.3390/photonics6020056 - 24 May 2019
Cited by 33 | Viewed by 7239
Abstract
We explore the propagation of the cylindrical vector beams (CVB) in turbid tissue-like scattering medium in comparison with the conventional Gaussian laser beam. The study of propagation of CVB and Gaussian laser beams in the medium is performed utilizing the unified electric field [...] Read more.
We explore the propagation of the cylindrical vector beams (CVB) in turbid tissue-like scattering medium in comparison with the conventional Gaussian laser beam. The study of propagation of CVB and Gaussian laser beams in the medium is performed utilizing the unified electric field Monte Carlo model. The implemented Monte Carlo model is a part of a generalized on-line computational tool and utilizes parallel computing, executed on the NVIDIA Graphics Processing Units (GPUs) supporting Compute Unified Device Architecture (CUDA). Using extensive computational studies, we demonstrate that after propagation through the turbid tissue-like scattering medium, the degree of fringe contrast for CVB becomes at least twice higher in comparison to the conventional linearly polarized Gaussian beam. The results of simulations agree with the results of experimental studies. Both experimental and theoretical results suggest that there is a high potential of the application of CVB in the diagnosis of biological tissues. Full article
(This article belongs to the Special Issue Biomedical Photonics Advances)
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7 pages, 2606 KB  
Letter
The Role of Electron Transfer in the Nonlinear Response of Ge2Sb2Te5-Mediated Plasmonic Dimers
by Burak Gerislioglu and Arash Ahmadivand
Photonics 2019, 6(2), 52; https://doi.org/10.3390/photonics6020052 - 16 May 2019
Cited by 13 | Viewed by 3574
Abstract
Here, we study the possibility of exquisitely selective harmonic generation based on the concept of charge transfer plasmons (CTPs) in bridged nanoparticle assemblies. By choosing plasmonic dimer nanoantenna, as a fundamental member of the nanocluster family, and bridging the capacitive gap space between [...] Read more.
Here, we study the possibility of exquisitely selective harmonic generation based on the concept of charge transfer plasmons (CTPs) in bridged nanoparticle assemblies. By choosing plasmonic dimer nanoantenna, as a fundamental member of the nanocluster family, and bridging the capacitive gap space between the proximal nanoparticles with an optothermally controllable substance, we judiciously showed that variations in the generation of third harmonic light in the visible regime can be possible by considering distinct states of the functional bridge. To this end, the conductive connection between the nanoparticles is mediated with Ge2Sb2Te5 (GST) with inherently opposite optical and electrical properties below (dielectric, amorphous state) and above 477 °C (conductive, crystalline state). This helped to actively control the transition of charges across the bridge and thereby control the excitation of CTP resonances and provide a switching feature between dipolar and CTP modes. This versatile approach also allowed for production of the intended harmonic signal at different wavelengths depending on the conductivity of the interparticle nanojunction. Full article
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24 pages, 2713 KB  
Review
Multifunctional Smart Optical Fibers: Materials, Fabrication, and Sensing Applications
by Zhengyong Liu, Zhi Feng Zhang, Hwa-Yaw Tam and Xiaoming Tao
Photonics 2019, 6(2), 48; https://doi.org/10.3390/photonics6020048 - 6 May 2019
Cited by 63 | Viewed by 11413
Abstract
This paper presents a review of the development of optical fibers made of multiple materials, particularly including silica glass, soft glass, polymers, hydrogels, biomaterials, Polydimethylsiloxane (PDMS), and Polyperfluoro-Butenylvinyleth (CYTOP). The properties of the materials are discussed according to their various applications. Typical fabrication [...] Read more.
This paper presents a review of the development of optical fibers made of multiple materials, particularly including silica glass, soft glass, polymers, hydrogels, biomaterials, Polydimethylsiloxane (PDMS), and Polyperfluoro-Butenylvinyleth (CYTOP). The properties of the materials are discussed according to their various applications. Typical fabrication techniques for specialty optical fibers based on these materials are introduced, which are mainly focused on extrusion, drilling, and stacking methods depending on the materials’ thermal properties. Microstructures render multiple functions of optical fibers and bring more flexibility in fiber design and device fabrication. In particular, micro-structured optical fibers made from different types of materials are reviewed. The sensing capability of optical fibers enables smart monitoring. Widely used techniques to develop fiber sensors, i.e., fiber Bragg grating and interferometry, are discussed in terms of sensing principles and fabrication methods. Lastly, sensing applications in oil/gas, optofluidics, and particularly healthcare monitoring using specialty optical fibers are demonstrated. In comparison with conventional silica-glass single-mode fiber, state-of-the-art specialty optical fibers provide promising prospects in sensing applications due to flexible choices in materials and microstructures. Full article
(This article belongs to the Special Issue Polymer Optical Fibre)
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7 pages, 2622 KB  
Article
Exploiting the Nonlinear Dynamics of Optically Injected Semiconductor Lasers for Optical Sensing
by Maria S. Torre and Cristina Masoller
Photonics 2019, 6(2), 45; https://doi.org/10.3390/photonics6020045 - 24 Apr 2019
Cited by 4 | Viewed by 4224
Abstract
Optically injected semiconductor lasers are known to display a rich variety of dynamic behaviours, including the emission of excitable pulses, and of rare giant pulses (often referred to as optical rogue waves). Here, we use a well-known rate equation model to explore the [...] Read more.
Optically injected semiconductor lasers are known to display a rich variety of dynamic behaviours, including the emission of excitable pulses, and of rare giant pulses (often referred to as optical rogue waves). Here, we use a well-known rate equation model to explore the combined effect of excitability and extreme pulse emission, for the detection of variations in the strength of the injected field. We find parameter regions where the laser always responds to a perturbation by emitting an optical pulse whose amplitude is above a pre-defined detection threshold. We characterize the sensing capability of the laser in terms of the amplitude and the duration of the perturbation. Full article
(This article belongs to the Special Issue Semiconductor Laser Dynamics: Fundamentals and Applications)
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18 pages, 6508 KB  
Review
Latest Achievements in Polymer Optical Fiber Gratings: Fabrication and Applications
by Rui Min, Beatriz Ortega and Carlos Marques
Photonics 2019, 6(2), 36; https://doi.org/10.3390/photonics6020036 - 29 Mar 2019
Cited by 46 | Viewed by 6660
Abstract
Grating devices in polymer optical fibers (POFs) have attracted huge interest for many potential applications in recent years. This paper presents the state of the art regarding the fabrication of different types of POF gratings, such as uniform, phase-shifted, tilted, chirped, and long [...] Read more.
Grating devices in polymer optical fibers (POFs) have attracted huge interest for many potential applications in recent years. This paper presents the state of the art regarding the fabrication of different types of POF gratings, such as uniform, phase-shifted, tilted, chirped, and long period gratings, and explores potential application scenarios, such as biosensing and optical communications. Full article
(This article belongs to the Special Issue Polymer Optical Fibre)
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12 pages, 5224 KB  
Article
Verification of Non-thermal Effects of 0.3–0.6 THz-Waves on Human Cultured Cells
by Noriko Yaekashiwa, Hisa Yoshida, Sato Otsuki, Shin’ichiro Hayashi and Kodo Kawase
Photonics 2019, 6(1), 33; https://doi.org/10.3390/photonics6010033 - 25 Mar 2019
Cited by 11 | Viewed by 5308
Abstract
Recent progress has been made in the development of terahertz (THz) waves for practical applications. Few studies that have assessed the biological effects of THz waves have been reported, and the data currently available regarding the safety of THz waves is inadequate. In [...] Read more.
Recent progress has been made in the development of terahertz (THz) waves for practical applications. Few studies that have assessed the biological effects of THz waves have been reported, and the data currently available regarding the safety of THz waves is inadequate. In this study, the effect of THz wave exposure on two cultured cells was assessed using a widely tunable THz source with a 0.3–0.6 THz frequency range, which can be used and increased in one GHz increments. The THz waves applied to the cultured cells were weak enough such that any thermal effects could be disregarded. The influence of THz wave exposure on both the proliferative and metabolic activities of these cells was investigated, as well as the extent of the thermal stress placed on the cells. In this work, no measurable effect on the proliferative or metabolic activities of either cell type was observed following the exposure to THz waves. No differences in the quantity of cDNA related to heat shock protein 70 was detected in either the sham or exposure group. As such, no differences in cellular activity between cells exposed to THz waves and those not exposed were observed. Full article
(This article belongs to the Special Issue Terahertz Photonics)
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8 pages, 1886 KB  
Article
Long Wavelength (λ > 17 µm) Distributed Feedback Quantum Cascade Lasers Operating in a Continuous Wave at Room Temperature
by Hoang Nguyen Van, Zeineb Loghmari, Hadrien Philip, Michael Bahriz, Alexei N. Baranov and Roland Teissier
Photonics 2019, 6(1), 31; https://doi.org/10.3390/photonics6010031 - 21 Mar 2019
Cited by 26 | Viewed by 6932
Abstract
The extension of the available spectral range covered by quantum cascade lasers (QCL) would allow one to address new molecular spectroscopy applications, in particular in the long wavelength domain of the mid-infrared. We report in this paper the realization of distributed feedback (DFB) [...] Read more.
The extension of the available spectral range covered by quantum cascade lasers (QCL) would allow one to address new molecular spectroscopy applications, in particular in the long wavelength domain of the mid-infrared. We report in this paper the realization of distributed feedback (DFB) QCLs, made of InAs and AlSb, that demonstrated a continuous wave (CW) and a single mode emission at a wavelength of 17.7 µm, with output powers in the mW range. This is the longest wavelength for DFB QCLs, and for any QCLs or semiconductor lasers in general, operating in a CW at room temperature. Full article
(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)
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11 pages, 2574 KB  
Article
The Design of Optical Circuit-Analog Absorbers through Electrically Small Nanoparticles
by Alessio Monti, Andrea Alù, Alessandro Toscano and Filiberto Bilotti
Photonics 2019, 6(1), 26; https://doi.org/10.3390/photonics6010026 - 6 Mar 2019
Cited by 11 | Viewed by 4895
Abstract
In the last few years, the perfect absorption of light has become an important research topic due to its dramatic impact in photovoltaics, photodetectors, color filters and thermal emitters. While broadband optical absorption is relatively easy to achieve using bulky devices, today there [...] Read more.
In the last few years, the perfect absorption of light has become an important research topic due to its dramatic impact in photovoltaics, photodetectors, color filters and thermal emitters. While broadband optical absorption is relatively easy to achieve using bulky devices, today there is a strong need and interest in achieving the same effects by employing nanometric structures that are compatible with modern nanophotonic components. In this paper, we propose a general procedure to design broadband nanometer-scale absorbers working in the optical spectrum. The proposed devices, which can be considered an extension to optics of microwave circuit-analog absorbers, consist of several layers containing arrays of elongated nanoparticles, whose dimensions are engineered to control both the absorption level and the operational bandwidth. By combining a surface-impedance homogenization and an equivalent transmission-line formalism, we define a general analytical procedure that can be employed to achieve a final working design. As a relevant example, we show that the proposed approach allows designing an optical absorber exhibiting a 20% fractional bandwidth on a thickness of λ/4 at the central frequency of operation. Full-wave results confirming the effectiveness of the analytical findings, as well as some considerations about the experimental realization of the proposed devices are provided. Full article
(This article belongs to the Special Issue Metamaterials for Advanced Photonic and Plasmonic Applications – Selected Papers from Metamaterials’2018)
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13 pages, 1097 KB  
Article
International System of Units (SI) Traceable Noise-Equivalent Power and Responsivity Characterization of Continuous Wave ErAs:InGaAs Photoconductive Terahertz Detectors
by Anuar de Jesus Fernandez Olvera, Axel Roggenbuck, Katja Dutzi, Nico Vieweg, Hong Lu, Arthur C. Gossard and Sascha Preu
Photonics 2019, 6(1), 15; https://doi.org/10.3390/photonics6010015 - 13 Feb 2019
Cited by 22 | Viewed by 5918
Abstract
A theoretical model for the responsivity and noise-equivalent power (NEP) of photoconductive antennas (PCAs) as coherent, homodyne THz detectors is presented. The model is validated by comparison to experimental values obtained for two ErAs:InGaAs PCAs. The responsivity and NEP were obtained from the [...] Read more.
A theoretical model for the responsivity and noise-equivalent power (NEP) of photoconductive antennas (PCAs) as coherent, homodyne THz detectors is presented. The model is validated by comparison to experimental values obtained for two ErAs:InGaAs PCAs. The responsivity and NEP were obtained from the measured rectified current, the current noise floor in the PCAs, and the incoming THz power for the same conditions. Since the THz power measurements are performed with a pyroelectric detector calibrated by the National Metrology Institute of Germany (PTB), the experimentally obtained values are directly traceable to the International System of Units (SI) for the described conditions. The agreement between the presented model and the experimental results is excellent using only one fitting parameter. A very low NEP of 1.8 fW/Hz at 188.8 GHz is obtained at room temperature. Full article
(This article belongs to the Special Issue Terahertz Photonics)
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12 pages, 6513 KB  
Article
A Bio-Compatible Fiber Optic pH Sensor Based on a Thin Core Interferometric Technique
by Magnus Engholm, Krister Hammarling, Henrik Andersson, Mats Sandberg and Hans-Erik Nilsson
Photonics 2019, 6(1), 11; https://doi.org/10.3390/photonics6010011 - 30 Jan 2019
Cited by 15 | Viewed by 5600
Abstract
There is an increasing demand for compact, reliable and versatile sensor concepts for pH-level monitoring within several industrial, chemical as well as bio-medical applications. Many pH sensors concepts have been proposed, however, there is still a need for improved sensor solutions with respect [...] Read more.
There is an increasing demand for compact, reliable and versatile sensor concepts for pH-level monitoring within several industrial, chemical as well as bio-medical applications. Many pH sensors concepts have been proposed, however, there is still a need for improved sensor solutions with respect to reliability, durability and miniaturization but also for multiparameter sensing. Here we present a conceptual verification, which includes theoretical simulations as well as experimental evaluation of a fiber optic pH-sensor based on a bio-compatible pH sensitive material not previously used in this context. The fiber optic sensor is based on a Mach-Zehnder interferometric technique, where the pH sensitive material is coated on a short, typically 20-25 mm thin core fiber spliced between two standard single mode fibers. The working principle of the sensor is simulated by using COMSOL Multiphysics. The simulations are used as a guideline for the construction of the sensors that have been experimentally evaluated in different liquids with pH ranging from 1.95 to 11.89. The results are promising, showing the potential for the development of bio-compatible fiber optic pH sensor with short response time, high sensitivity and broad measurement range. The developed sensor concept can find future use in many medical- or bio-chemical applications as well as in environmental monitoring of large areas. Challenges encountered during the sensor development due to variation in the design parameters are discussed. Full article
(This article belongs to the Special Issue Advanced Optical Materials and Devices)
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