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Optics, Volume 5, Issue 1 (March 2024) – 14 articles

Cover Story (view full-size image): Flexible and scalable low-loss optical Kerr materials are of high interest for efficient nonlinear optical device applications. Polydimethylsiloxane (PDMS) is highly optically transparent. However, it does not provide a nonlinear optical response. The present study aims to investigate the feasibility of introducing optical nonlinearity in PDMS using organic solvent swelling. The incorporation of toluene caused a maximum weight change of up to 116%, resulting in a substantial nonlinear optical response, with n2 = 3.1 × 10−15 cm2/W. Our findings demonstrate that the incorporation of nonlinear organic solvents into a PDMS host matrix via swelling enables the production of a nonlinear material, which is (in contrast to liquids) applicable in solid film stacks, as well as (in contrast to crystals) cost-efficient and area scalable.  View this paper
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12 pages, 4893 KiB  
Article
Angle-Resolved Optical Characterization of a Plasmonic Triangular Array of Elliptical Holes in a Gold Layer
by Margherita Angelini, Konstantins Jefimovs, Paola Pellacani, Dimitrios Kazazis, Franco Marabelli and Francesco Floris
Optics 2024, 5(1), 195-206; https://doi.org/10.3390/opt5010014 - 21 Mar 2024
Cited by 1 | Viewed by 1101
Abstract
Plasmonic arrays are grating-like structures able to couple an incoming electromagnetic field into either localized or propagating surface plasmonic modes. A triangular array of elliptical holes in a gold layer were realized resorting to displacement Talbot lithography. Scanning electron microscopy was used to [...] Read more.
Plasmonic arrays are grating-like structures able to couple an incoming electromagnetic field into either localized or propagating surface plasmonic modes. A triangular array of elliptical holes in a gold layer were realized resorting to displacement Talbot lithography. Scanning electron microscopy was used to evaluate the geometrical features and finite time domain simulations were performed to verify the consistency of the design. The optical response was characterized by angle-resolved reflectance and transmittance measurements. The results demonstrate the good quality and uniformity of the array. Furthermore, the study on the dependence of the optical response on both the hexagonal lattice and the elliptical hole-defined symmetry properties was conducted allowing the distinction of their effects on both the localized and propagating plasmonic modes. The results indicate that the localized component of the plasmonic modes is mainly affected by the elliptical shape, while the propagating part is influenced by the hexagonal lattice symmetry. Full article
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19 pages, 4918 KiB  
Article
A Non-Destructive Study of Optical, Geometric and Luminescent Parameters of Active Optical Fibers Preforms
by Yuri Aleksandrovich Konstantinov, Artem Timofeevich Turov, Konstantin Pavlovich Latkin, D Claude and Irina Sergeevna Azanova
Optics 2024, 5(1), 176-194; https://doi.org/10.3390/opt5010013 - 20 Mar 2024
Cited by 1 | Viewed by 970
Abstract
This work is devoted to the scientific and technical aspects of individual stages of active optical fibers preforms’ optical-geometric parameters metrological control. The concept of a system presented makes it possible to carry out a study of a rare earth element distribution in [...] Read more.
This work is devoted to the scientific and technical aspects of individual stages of active optical fibers preforms’ optical-geometric parameters metrological control. The concept of a system presented makes it possible to carry out a study of a rare earth element distribution in the preform of an active optical fiber and to monitor geometric parameters, and also to study the evolution of the refractive index profile along the length of the sample at a qualitative level. As far as it is known, it is the first description of the preform optical, geometric, and luminescent properties measurement within a single automated laboratory bench. Also, the novelty of the approach lies in the fact that the study of the refractive index profile variation along the length of the preform is, for the first time, conducted using the “dry” method, that is, without immersing the sample in synthetic oil, which makes the process less labor-intensive and safer. Full article
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11 pages, 3128 KiB  
Article
Nanofabrication Process Scale-Up via Displacement Talbot Lithography of a Plasmonic Metasurface for Sensing Applications
by Paola Pellacani, Konstantins Jefimovs, Margherita Angelini, Franco Marabelli, Valentina Tolardo, Dimitrios Kazazis and Francesco Floris
Optics 2024, 5(1), 165-175; https://doi.org/10.3390/opt5010012 - 8 Mar 2024
Viewed by 1348
Abstract
The selection of an affordable method to fabricate plasmonic metasurfaces needs to guarantee complex control over both tunability and reproducibility of their spectral and morphological properties, making plasmonic metasurfaces suitable for integration into different sensing devices. Displacement Talbot lithography could be a valid [...] Read more.
The selection of an affordable method to fabricate plasmonic metasurfaces needs to guarantee complex control over both tunability and reproducibility of their spectral and morphological properties, making plasmonic metasurfaces suitable for integration into different sensing devices. Displacement Talbot lithography could be a valid solution thanks to the limited fabrication steps required, also providing the highly desired industrial scalability. Fabricated plasmonic metasurfaces are represented by a gold nanohole array on a glass substrate based on a triangular pattern. Scanning electron microscopy measurements have been recorded, showing the consistency of the surface features with the optimized design parameters. Reflectance and transmittance measurements have also been carried out to test the reliability and standardization of the metasurface’s optical response. Furthermore, these plasmonic metasurfaces have also been successfully tested for probing refractive index variations in a microfluidic system, paving the way for their use in sensitive, real-time, label-free, and multiplexing detection of bio-molecular events. Full article
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14 pages, 9593 KiB  
Article
Lidar-Based Spatial Large Deflection Measurement System for Wind Turbine Blades
by Yue Hu, Yutian Zhu, Aiguo Zhou and Penghui Liu
Optics 2024, 5(1), 151-164; https://doi.org/10.3390/opt5010011 - 4 Mar 2024
Cited by 1 | Viewed by 1260
Abstract
With the advancement of China’s wind power industry, research into full-scale structural testing of wind turbine blades, including static testing and fatigue testing, has shown increasing significance. Static testing measures the deflection at fixed points, using pull-wire sensors in industrial practice. However, the [...] Read more.
With the advancement of China’s wind power industry, research into full-scale structural testing of wind turbine blades, including static testing and fatigue testing, has shown increasing significance. Static testing measures the deflection at fixed points, using pull-wire sensors in industrial practice. However, the demerits of this method involve single dimension, excessive deviation, costly experiment, and complex installment. Given the advantages that lidar provides, correspondingly, high data density, precision, and convenience, we proposed a simple and efficient spatial large deflection measurement system for wind turbine blades with multi lidars. For point clouds collected from lidar scanners, registration based on point primitives and geometric primitives, dynamic radius DBSCAN clustering, spatial line clustering, and line integrals are applied to calculate the 3D coordinates of measured points on the blade. Experimentally validated, the proposed method demonstrates its effectiveness in serving as a viable alternative to the traditional pull-wire sensor measurement approach. In the minimum oscillation direction test, the measurement error is controlled within 3% compared to the theoretical value. Simultaneously, in the maximum swing direction test, the 3D coordinates of the measured point remain consistent with the changing trend observed under small deformation. These results confirm the feasibility of the system and its potentials to be generalized. Full article
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18 pages, 2636 KiB  
Review
Photoacoustic Imaging of Human Skin for Accurate Diagnosis and Treatment Guidance
by Yue Ying, Hong Zhang and Li Lin
Optics 2024, 5(1), 133-150; https://doi.org/10.3390/opt5010010 - 1 Mar 2024
Cited by 1 | Viewed by 2245
Abstract
Photoacoustic imaging (PAI) is a cutting-edge biomedical imaging modality, providing detailed anatomical and functional information about the area beneath the skin surface. Its light energy deposition is such that PAI typically provides clear images of the skin with high signal-to-noise ratios. Specifically, the [...] Read more.
Photoacoustic imaging (PAI) is a cutting-edge biomedical imaging modality, providing detailed anatomical and functional information about the area beneath the skin surface. Its light energy deposition is such that PAI typically provides clear images of the skin with high signal-to-noise ratios. Specifically, the rich optical contrast of PAI allows biological information related to lesion growth, malignancy, treatment response, and prognosis to be seen. Given its significant advantages and emerging role in imaging skin lesions, we summarize and comment on representative studies of skin PAI, such as the guidance of skin cancer biopsies and surgical excisions, and the accurate diagnosis of psoriasis. We conclude with our insights about the clinical significance of skin PAI, showing how its use to identify biological characteristics in lesion microenvironments allows early diagnosis and prognosis of disease. Full article
(This article belongs to the Special Issue Advanced Optical Imaging for Biomedicine)
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12 pages, 4561 KiB  
Article
Design of Planar Multilayer Devices for Optical Filtering Using Surrogate Model Based on Artificial Neural Network
by Davi F. Rêgo, Fabrício G. S. Silva, Rodrigo C. Gusmão and Vitaly F. Rodriguez-Esquerre
Optics 2024, 5(1), 121-132; https://doi.org/10.3390/opt5010009 - 1 Mar 2024
Viewed by 1085
Abstract
Artificial intelligence paradigms hold significant potential to advance nanophotonics. This study presents a novel approach to designing a plasmonic absorber using an artificial neural network as a surrogate model in conjunction with a genetic algorithm. The methodology involved numerical simulations of multilayered metal–dielectric [...] Read more.
Artificial intelligence paradigms hold significant potential to advance nanophotonics. This study presents a novel approach to designing a plasmonic absorber using an artificial neural network as a surrogate model in conjunction with a genetic algorithm. The methodology involved numerical simulations of multilayered metal–dielectric plasmonic structures to establish a dataset for training an artificial neural network (ANN). The results demonstrate the proficiency of the trained ANN in predicting reflectance spectra and its ability to generalize intricate relationships between desired performance and geometric configurations, with values of correlation higher than 98% in comparison with ground-truth electromagnetic simulations. Furthermore, the ANN was employed as a surrogate model in a genetic algorithm (GA) loop to achieve target optical behaviors. The proposed methodology provides a powerful means of inverse designing multilayered metal–dielectric devices tailored for visible band wavelength filtering. This research demonstrates that the integration of AI-driven approaches in nanophotonics leads to efficient and effective design strategies. Full article
(This article belongs to the Section Photonics and Optical Communications)
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20 pages, 1273 KiB  
Article
Surface Topological Plexcitons: Strong Coupling in a Bi2Se3 Topological Insulator Nanoparticle-Quantum Dot Molecule
by George Kountouris and Vassilios Yannopapas
Optics 2024, 5(1), 101-120; https://doi.org/10.3390/opt5010008 - 27 Feb 2024
Viewed by 963
Abstract
Strong coupling of quantum states with electromagnetic modes of topological matter offer an interesting platform for the exploration of new physics and applications. In this work, we report a novel hybrid mode, a surface topological plexciton, arising from strong coupling between the surface [...] Read more.
Strong coupling of quantum states with electromagnetic modes of topological matter offer an interesting platform for the exploration of new physics and applications. In this work, we report a novel hybrid mode, a surface topological plexciton, arising from strong coupling between the surface topological plasmon mode of a Bi2Se3 topological insulator nanoparticle and the exciton of a two-level quantum emitter. We study the power absorption spectrum of the system by working within the dipole and rotating-wave approximations, using a density matrix approach for the emitter, and a classical dielectric-function approach for the topological-insulator nanoparticle. We show that a Rabi-type splitting can appear in the spectrum suggesting the presence of strong coupling. Furthermore, we study the dependence of the splitting on the separation of the two nanoparticles as well as the dipole moment of the quantum emitter. These results can be useful for exploring exotic phases of matter, furthering research in topological insulator plasmonics, as well as for applications in the far-infrared and quantum computing. Full article
(This article belongs to the Section Photonics and Optical Communications)
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13 pages, 4169 KiB  
Article
Electronic Population Reconstruction from Strong-Field-Modified Absorption Spectra with a Convolutional Neural Network
by Daniel Richter, Alexander Magunia, Marc Rebholz, Christian Ott and Thomas Pfeifer
Optics 2024, 5(1), 88-100; https://doi.org/10.3390/opt5010007 - 26 Feb 2024
Viewed by 1185
Abstract
We simulate ultrafast electronic transitions in an atom and corresponding absorption line changes with a numerical, few-level model, similar to previous work. In addition, a convolutional neural network (CNN) is employed for the first time to predict electronic state populations based on the [...] Read more.
We simulate ultrafast electronic transitions in an atom and corresponding absorption line changes with a numerical, few-level model, similar to previous work. In addition, a convolutional neural network (CNN) is employed for the first time to predict electronic state populations based on the simulated modifications of the absorption lines. We utilize a two-level and four-level system, as well as a variety of laser-pulse peak intensities and detunings, to account for different common scenarios of light–matter interaction. As a first step towards the use of CNNs for experimental absorption data in the future, we apply two different noise levels to the simulated input absorption data. Full article
(This article belongs to the Special Issue Ultrafast Light-Matter Interaction)
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12 pages, 4523 KiB  
Article
Dual-Band Image Fusion Approach Using Regional Weight Analysis Combined with a Multi-Level Smoothing Filter
by Jia Yi, Huilin Jiang, Xiaoyong Wang and Yong Tan
Optics 2024, 5(1), 76-87; https://doi.org/10.3390/opt5010006 - 21 Feb 2024
Viewed by 1042
Abstract
Image fusion is an effective and efficient way to express the feature information of an infrared image and abundant detailed information of a visible image in a single fused image. However, obtaining a fused result with good visual effect, while preserving and inheriting [...] Read more.
Image fusion is an effective and efficient way to express the feature information of an infrared image and abundant detailed information of a visible image in a single fused image. However, obtaining a fused result with good visual effect, while preserving and inheriting those characteristic details, seems a challenging problem. In this paper, by combining a multi-level smoothing filter and regional weight analysis, a dual-band image fusion approach is proposed. Firstly, a series of dual-band image layers with different details are obtained using smoothing results. With different parameters in a bilateral filter, different smoothed results are achieved at different levels. Secondly, regional weight maps are generated for each image layer, and then we fuse the dual-band image layers with their corresponding regional weight map. Finally, by imposing proper weights, those fused image layers are synthetized. Through comparison with seven excellent fusion methods, both subjective and objective evaluations for the experimental results indicate that the proposed approach can produce the best fused image, which has the best visual effect with good contrast, and those small details are preserved and highlighted, too. In particular, for the image pairs with a size of 640 × 480, the algorithm could provide a good visual effect result within 2.86 s, and the result has almost the best objective metrics. Full article
(This article belongs to the Section Engineering Optics)
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10 pages, 2496 KiB  
Article
Introducing Optical Nonlinearity in PDMS Using Organic Solvent Swelling
by Sudhakara Reddy Bongu, Maximilian Buchmüller, Daniel Neumaier and Patrick Görrn
Optics 2024, 5(1), 66-75; https://doi.org/10.3390/opt5010005 - 15 Feb 2024
Cited by 1 | Viewed by 1278
Abstract
The feasibility of introducing optical nonlinearity in poly-dimethyl siloxane (PDMS) using organic solvent swelling was investigated. The third-order nonlinear refraction and absorption properties of the individual materials, as well as the PDMS/solvent compounds after swelling were characterized. The well-established Z-scan technique served as [...] Read more.
The feasibility of introducing optical nonlinearity in poly-dimethyl siloxane (PDMS) using organic solvent swelling was investigated. The third-order nonlinear refraction and absorption properties of the individual materials, as well as the PDMS/solvent compounds after swelling were characterized. The well-established Z-scan technique served as characterization method for the nonlinear properties under picosecond pulsed laser excitation at a 532 nm wavelength. These experiments included investigations on the organic solvents nitrobenzene, 2,6-lutidine, and toluene, which showed inherent optical nonlinearity. We showed that nitrobenzene, one of the most well-known nonlinear optical materials, has proven suboptimal in this context due to its limited swelling effect in PDMS and comparatively high (non)linear absorption, resulting in undesirable thermal effects and potential photo-induced damage in the composite material. Toluene and 2,6-lutidine not only exhibited lower absorption compared to nitrobenzene but also show a more pronounced swelling effect in PDMS. The incorporation of toluene caused a weight change of up to 116% of PDMS, resulting in substantial nonlinear optical effects, reflected in the nonlinear refractive index of the PDMS/toluene composite n2=3.1×1015 cm2/W. Full article
(This article belongs to the Section Nonlinear Optics)
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10 pages, 1069 KiB  
Article
Simple Estimate of the Impact of M2 and Strehl Ratio on the Effective Focusable Spot Size
by Samuel Arba-Mosquera, Pascal Naubereit and Simas Sobutas
Optics 2024, 5(1), 56-65; https://doi.org/10.3390/opt5010004 - 29 Jan 2024
Viewed by 1207
Abstract
A simple method (first-order approximation) to determine the impact of M2 and the Strehl Ratio on the effective focusable spot size avoiding complex propagations of the beam wavefront is proposed. The model is based upon previous models and the definition of M [...] Read more.
A simple method (first-order approximation) to determine the impact of M2 and the Strehl Ratio on the effective focusable spot size avoiding complex propagations of the beam wavefront is proposed. The model is based upon previous models and the definition of M2 and the Strehl Ratio in a simple manner. This work provides qualitative and quantitative estimates for the interplay of M2 and the Strehl Ratio on the effectively focusable spot size. Full article
(This article belongs to the Special Issue Advanced Optical Imaging for Biomedicine)
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12 pages, 4498 KiB  
Article
An Anti-Noise-Designed Residual Phase Unwrapping Neural Network for Digital Speckle Pattern Interferometry
by Biao Wang, Xiaoling Cao, Meiling Lan, Chang Wu and Yonghong Wang
Optics 2024, 5(1), 44-55; https://doi.org/10.3390/opt5010003 - 19 Jan 2024
Viewed by 1125
Abstract
DSPI (Digital Speckle Pattern Interferometry) is a non-destructive optical measurement technique that obtains phase information of an object through phase unwrapping. Traditional phase unwrapping algorithms depend on the quality of the images, which demands preprocessing such as filtering and denoising. Moreover, the unwrapping [...] Read more.
DSPI (Digital Speckle Pattern Interferometry) is a non-destructive optical measurement technique that obtains phase information of an object through phase unwrapping. Traditional phase unwrapping algorithms depend on the quality of the images, which demands preprocessing such as filtering and denoising. Moreover, the unwrapping time is highly influenced by the size of the images. In this study, we proposed a new deep learning-based phase unwrapping algorithm combining the residual network and U-Net network. Additionally, we incorporated an improved SSIM function as the loss function based on camera characteristics. The experimental results demonstrated that the proposed method achieved higher quality in highly noisy phase unwrapping maps compared to traditional algorithms, with SSIM values consistently above 0.98. In addition, we applied image stitching to the network to process maps of various sizes and the unwrapping time remained around 1 s even for larger images. In conclusion, our proposed network is able to achieve efficient and accurate phase unwrapping. Full article
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33 pages, 3133 KiB  
Article
Degree of Polarization of Cathodoluminescence from a (100) GaAs Substrate with SiN Stripes
by Daniel T. Cassidy, Philippe Pagnod-Rossiaux and Merwan Mokhtari
Optics 2024, 5(1), 11-43; https://doi.org/10.3390/opt5010002 - 17 Jan 2024
Viewed by 1129
Abstract
Notes on fits of analytic estimations, 2D finite element method (FEM), and 3D FEM simulations to measurements of the cathodoluminescence (CL) and to the degree of polarization (DOP) of the CL from the top surface of a (100) GaAs substrate with [...] Read more.
Notes on fits of analytic estimations, 2D finite element method (FEM), and 3D FEM simulations to measurements of the cathodoluminescence (CL) and to the degree of polarization (DOP) of the CL from the top surface of a (100) GaAs substrate with a 6.22 μm wide SiN stripe are presented. Three interesting features are found in the DOP of CL data. Presumably these features are noticeable owing to the spatial resolution of the CL measurement system. Comparisons of both strain and spatial resolutions obtained by CL and photoluminescence (PL) systems are presented. The width of the central feature in the measured DOP is less than the width of the SiN, as measured from the CL. This suggests horizontal cracks or de-laminations into each side of the SiN of about 0.7 μm. In addition, it appears that deformed regions of widths of ≈1.5 μm and adjacent to the SiN must exist to explain some of the features. Full article
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10 pages, 4990 KiB  
Article
Hybrid Approach for Multiscale and Multimodal Time-Resolved Diagnosis of Ultrafast Processes in Materials via Tailored Synchronization of Laser and X-ray Sources at MHz Repetition Rates
by Nikita Marchenkov, Evgenii Mareev, Anton Kulikov, Fedor Pilyak, Eduard Ibragimov, Yuri Pisarevskii and Fedor Potemkin
Optics 2024, 5(1), 1-10; https://doi.org/10.3390/opt5010001 - 16 Jan 2024
Cited by 2 | Viewed by 1078
Abstract
The synchronization of laser and X-ray sources is essential for time-resolved measurements in the study of ultrafast processes, including photo-induced piezo-effects, shock wave generation, and phase transitions. On the one hand, optical diagnostics (by synchronization of two laser sources) provides information about changes [...] Read more.
The synchronization of laser and X-ray sources is essential for time-resolved measurements in the study of ultrafast processes, including photo-induced piezo-effects, shock wave generation, and phase transitions. On the one hand, optical diagnostics (by synchronization of two laser sources) provides information about changes in vibration frequencies, shock wave dynamics, and linear and nonlinear refractive index behavior. On the other hand, optical pump–X-ray probe diagnostics provide an opportunity to directly reveal lattice dynamics. To integrate two approaches into a unified whole, one needs to create a robust method for the synchronization of two systems with different repetition rates up to the MHz range. In this paper, we propose a universal approach utilizing a field-programmable gate array (FPGA) to achieve precise synchronization between different MHz sources such as various lasers and synchrotron X-ray sources. This synchronization method offers numerous advantages, such as high flexibility, fast response, and low jitter. Experimental results demonstrate the successful synchronization of two different MHz systems with a temporal resolution of 250 ps. This enables ultrafast measurements with a sub-nanosecond resolution, facilitating the uncovering of complex dynamics in ultrafast processes. Full article
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