Optics

Phase transitions are an intriguing yet poorly understood aspect of transition-metal-based materials; these phase transitions can result in changes to the refractive index, absorption coefficient, and other optical properties of the materials. Transition-metal-based materials exist in a variety of crystalline phases and also have metallic, semi-metallic, and semi-conducting characteristics. In this review, we demonstrate that alloyed W- and Mo-based dichalcogenides enable phase transitions in structures, with phase transition temperatures that are tunable across a wide range using various alloy models and modern DFT-based calculations. We also analyze the tuning the optical bandgap of the metal oxide nanoparticles through doping of the transition metal in a manner that is suitable for optical switching and thermal imaging. After the introduction and a brief illustration of the structures and their exceptional properties, we discuss synthetic methodologies and their application as part of important strategies toward the enhanced performance of transition-metal-based dichalcogenides and oxides. In the end, our conclusion highlights the prospects of 2D materials as phase transition materials due to their advantages in terms of scalability and adaptability. Full article

Image-guided liver biopsies can improve their success rate when combined with the optical detection of Indocyanine Green (ICG) fluorescence accumulated in tumors. Previous works used a camera coupled to a thin borescope to capture and quantify images from fluorescence emission during procedures; however, light-scattering prevented the formation of sharp images, and the time response for weakly fluorescent tumors was very low. Instead, replacing the camera with a photodiode detector shows an improved temporal resolution in a more compact and lighter device. This work presents the new design in a comparative study between both detection technologies, including an assessment of the temporal response and sensitivity to the presence of background fluorescence. Full article

Nuisance alarm rate (NAR) is one of the key performance parameters in a phase-sensitive optical time domain reflectometry (φ-OTDR)-based fence intrusion detection system. Typically, the vibrations caused by ambient environmental conditions, such as heavy rain, strong wind, and passing vehicles, easily result in many nuisance alarms. Significant research efforts have been undertaken to suppress the NAR. In this paper, we propose to utilize short segments of the sensing fiber as reference sensors for significant reduction in the NAR in φ-OTDR for the first time, to the best of our knowledge. According to our field trial results, the proposed approach can reduce the NAR by more than 90%. The proposed approach is very simple, practical, and cost-effective, which can be easily integrated with the existing methods of reducing NAR and act as an additional level of decision-making algorithm for triggering alarms. Full article

(1) Background: Applications using touch screens are increasingly deployed in medical facilities, as well as in public areas. When touching the display with fingers, potentially pathogenic microorganisms such as methicillin-resistant Staphylococcus aureus (MRSA) can be transmitted. An automated process to decontaminate the device in between users would be highly useful. (2) Methods: Thin glass plates were superficially contaminated with the non-pathogenic Staphylococcus carnosus in a controlled manner. Subsequently, UVA radiation of 400 or 380 nm was laterally coupled into the glass plate, which acted as a light guide. Contact agar plates recorded the change in the staphylococci concentration over time. Additionally, the UVA radiation emitted by the glass plates was measured and the potential risk to humans assessed. (3) Results: Staphylococci concentration decreased as a result of UVA radiation for both wavelengths. At 400 nm, it took about 7.5 h and at 380 nm about 1 h until a reduction of 90% was reached. To meet higher disinfection requirements, disproportionately longer irradiation times were necessary. The potential UVA irradiation of humans in front of the glass pane was about 35 µW/cm² or less and posed no risk to humans. (4) Conclusions: Side-coupled UVA radiation is in principle capable of safely automatically disinfecting microorganisms on touch screens. However, the required irradiation times are still in the hour range, so that a rapid disinfection within a minute or less is not yet possible with the presented setup. However, higher UVA intensities might reduce the current disinfection durations. Full article

Advanced Raman spectroscopy (RS) systems have gained new interest in the field of medicine as an emerging tool for in vivo tissue discrimination. The coupling of RS with artificial intelligence (AI) algorithms has given a boost to RS to analyze spectral data in real time with high specificity and sensitivity. However, limitations are still encountered due to the large amount of clinical data which are required for the pre-training process of AI algorithms. In this study, human healthy and cancerous colon specimens were surgically resected from different sites of the ascending colon and analyzed by RS. Two transfer learning models, the one-dimensional convolutional neural network (1D-CNN) and the 1D–ResNet transfer learning (1D-ResNet) network, were developed and evaluated using a Raman open database for the pre-training process which consisted of spectra of pathogen bacteria. According to the results, both models achieved high accuracy of 88% for healthy/cancerous tissue discrimination by overcoming the limitation of the collection of a large number of spectra for the pre-training process. This gives a boost to RS as an adjuvant tool for real-time biopsy and surgery guidance. Full article

One-dimensional photonic crystals have been used in sensing applications for decades, due to their ability to induce highly reflective photonic bandgaps. In this study, one-dimensional photonic crystals with alternating low- and high-density layers were fabricated from a single photosensitive polymer (IP-Dip) by two-photon polymerization. The photonic crystals were modified to include a central defect layer with different elastic properties compared to the surrounding layers, for the first time. It was observed that the defect mode resonance can be controlled by compressive force. Very good agreement was found between the experimentally measured spectra and the model data. The mechanical properties of the flexure design used in the defect layer were calculated. The calculated spring constant is of similar magnitude to those reported for microsprings fabricated on this scale using two-photon polymerization. The results of this study demonstrate the successful control of a defect resonance in one-dimensional photonic crystals fabricated by two-photon polymerization by mechanical stimuli, for the first time. Such a structure could have applications in fields, such as micro-robotics, and in micro-opto–electro–mechanical systems (MOEMSs), where optical sensing of mechanical fluctuations is desired. Full article

Data augmentation using generative adversarial networks (GANs) is vital in the creation of new instances that include imaging modality tasks for improved deep learning classification. In this study, conditional generative adversarial networks (cGANs) were used on a dataset of OCT (Optical Coherence Tomography)-acquired images of coronary atrial plaques for synthetic data creation for the first time, and further validated using deep learning architecture. A new OCT images dataset of 51 patients marked by three professionals was created and programmed. We used cGANs to synthetically populate the coronary aerial plaques dataset by factors of 5×, 10×, 50× and 100× from a limited original dataset to enhance its volume and diversification. The loss functions for the generator and the discriminator were set up to generate perfect aliases. The augmented OCT dataset was then used in the training phase of the leading AlexNet architecture. We used cGANs to create synthetic images and envisaged the impact of the ratio of real data to synthetic data on classification accuracy. We illustrated through experiments that augmenting real images with synthetic images by a factor of 50× during training helped improve the test accuracy of the classification architecture for label prediction by 15.8%. Further, we performed training time assessments against a number of iterations to identify optimum time efficiency. Automated plaques detection was found to be in conformity with clinical results using our proposed class conditioning GAN architecture. Full article

Measurements of the cathodoluminescence (CL) and the degree of polarization (DOP) of (CL) from the facet of a GaAs substrate and in the vicinity of a SiN stripe are reported and analyzed. The deformation induced by the SiN stripe is estimated by fitting the measured DOP to 3D finite element method (FEM) simulations. The deformation is found to be more complex than an initial condition of biaxial stress in the SiN. A ratio of fit coefficients suggests that the dependence of DOP on strain is described by equations presented in Appl. Opt. 59, 5506–5520 (2020). These equations give a DOP that is either proportional to a weighted difference of the principal components of strain in the measurement plane, or proportional to the shear strain in the measurement plane, depending on the chosen orientation of the measurement axes. Full article

An electromagnetic (EM) wave is a form of continuous energy, of which both the frequency and the amplitude are parts, as shown in a recent report. All the facts relating to the photoelectric effect are explained by the new modified EM wave concept. Since the photon concept is not able to explain the intensity effect and the ejection direction clearly, it cannot be used to explain nonlinear optical phenomena clearly. The current understanding of the interaction process between orbital electrons and light may not be realistic. In this work, the electron transition process is explained with the new modified EM wave concept. The orbital electrons of a material rotate circularly by the sinusoidal fields of the EM waves. In this way, the electrons absorb light energy as rotational kinetic energy. During the first rotational cycle, the electrons with large enough radii face different potential barriers in neighboring orbits. Consequently, the electrons’ speed is obstructed, and the electrons move behind their natural places (phase); in other words, the electrons cannot follow the required phase of EM waves. Thus, sufficient energetic electrons are scattered from their orbit. The high-intensity EM waves reach the inner orbits of the targeted atom and transit electrons from different orbits. The light can regenerate through processes with different frequencies. The frequency of the regenerated light can be higher than that of primary light, depending on the energy (frequency and amplitude) of the primary light. The results of previous reports match the prediction of the new concept of EM waves. The new wave concept may be able to explain all photonic behaviors of light clearly. Full article

The generation of high-order harmonics in solid crystals has received considerable attention recently. Using a driver laser with 0.8 µm wavelength and 28 fs ultrashort pulses, we present experimental results, accompanied with theoretical considerations, suggesting that the actual sources of the harmonics are nanometer-sized localized and transient electronic states on the surface of the materials when the laser intensity is in the non-perturbative regime. Adaptation of the bond model of the harmonic generation into the non-perturbative regime and including the quantum features of the process provide a localized excitation approach that correctly describes the measured polarization dependence of the harmonic signal, reflecting the microscopic surface structure and symmetries of the examined materials. Full article

In the design of photonic integrated circuits (PICs), the optical connections of the PIC surface, along with the electronic components of the chips, are significant issues. One of the optoelectronics components that utilizes these surface connections are electro-reflective modulators, consisting of an optical section and an electronic section. In this paper, a novel scheme of two-dimensional photonic crystals (PhCs) is presented for the optical and reflective sections of this device. This design is two-dimensional; thus, it has less volume than the current bulky structures. The finite element method is utilized to simulate and optimize the scheme of PhCs and gold layer parameters. Furthermore, optimization of design parameters is accomplished through the Nelder–Mead method. Moreover, the modeling and simulation of the proposed hybrid PhCs has been investigated according to the structural parameters with tolerance. These tolerances, related to the nanorods’ radius and lattice constants, are considered to justify and vindicate the fabrication technology limitations and conditions. In the “on” state of the modulator, the light transmission ratio is 98% for a 903 nm wavelength with a 45° angle of deflection and incident light, nd the bandwidth is 20 nm. For an 897 nm wavelength with a 41° angle, the transmission ratio is 95%, and the bandwidth is 7 nm. Full article

The thermal blooming effect, platform jitter, and other effects of laser beam propagation cause serious deviation, which will have a negative impact on laser tracking systems. It is important to ensure that the laser beam does not deviate. Based on a fast steering mirror, a CMOS camera, and a Flex RIO system, a laser beam jitter control system is designed and implemented. The error is detected by camera and compensated for by the fast steering mirror (FSM), which is controlled by the Flex RIO device. By using LabVIEW and MATLAB, a closed loop model is realized. The results show that the system response is highly stabilized and has a short rise time, providing a reference for engineering applications. Full article

In this study, a scatterometry-based monitoring system designed for tracking the quality and reproducibility of laser-textured surfaces in industrial environments was validated in off-line and real-time modes. To this end, a stainless steel plate was structured by direct laser interference patterning (DLIP) following a set of conditions with artificial patterning errors. Namely, fluctuations of the DLIP process parameters such as laser fluence, spatial period, and focus position are introduced, and also, two patterning strategies are implemented, whereby pulses are deliberately not fired at both deterministic and random positions. The detection limits of the system were determined by recording the intensities of the zero, first, and second diffraction order using a charge-coupled device (CCD) camera. As supported by topographical measurements, the system can accurately calculate spatial periods with a resolution of at least 100 nm. In addition, focus shifts of 70 µm from the optimum focus position can be detected, and missing patterned lines with a minimum width of 28 µm can be identified. The validation of this compact characterization unit represents a step forward for its implementation as an in-line monitoring tool for industrial laser-based micropatterning. Full article

The current generation is witnessing a huge interest in optical waveguides due to their salient features: they are of low cost, immune to electromagnetic interference, easy to multiplex, have a compact size, etc. These features of optical fibers make them a useful tool for various sensing applications including in medicine, automotives, biotechnology, food quality control, aerospace, physical and chemical monitoring. Among all the reported applications, optical waveguides have been widely exploited to measure the physical and chemical variations in the surrounding environment. Optical fiber-based temperature sensors have played a crucial role in this decade to detect high fever and tackle COVID-19-like pandemics. Recognizing the major developments in the field of optical fibers, this article provides recent progress in temperature sensors utilizing several sensing configurations including conventional fiber, photonic crystal fiber, and Bragg grating fibers. Additionally, this article also highlights the advantages, limitations, and future possibilities in this area. Full article

The COVID pandemic is forcing the renewal of scientific conferences, offering opportunities to introduce technological and inclusive developments. Our analysis focuses on the implementation of inclusive practices for female and early-career researchers in a virtual scientific conference. This organization approach was applied in the XIII Spanish Optical Meeting (RNO2021), which was also characterized by avatars interacting in an online metaverse. The effectiveness of inclusive policies and novel technological tools was evaluated using the participation data and a post-conference survey. Our study reveals the high impact of inclusive actions and a strong interest in the scientific community to explore conference advances. Full article

To obtain a continuous range of clear vision for pseudophakic eyes, a design of intraocular lens (IOL) with extended depth of focus (EDoF) was proposed. The IOL was optimized with a multi-configuration approach based on a pseudophakic eye model and the optical performances of the designed IOL were analyzed. The modulation transfer function (MTF) values remain above 0.2 at 50 lp/mm for object distance ranging from 0.35 m to infinity in both photopic vision and mesopic vision over a field of 4°. The optical performances remain stable when the pupil diameter changes from 2.25 mm to 5 mm. Besides, the presented theoretical analyses show the designed IOL has good optical performances for polychromatic light and corneal asphericity. The above shows that the IOL exhibits an excellent ability for pseudophakic eyes to see the object in a continuous range of distance. Full article

Photodissolution is a process that is well known for its ability to cause inclusion of silver into the matrix of a chalcogenide layer, changing its optical properties. In this paper, using e-beam deposition, we developed Ag (74 nm)/As₂S₃ (355 nm) bilayers and characterized the photodissolution kinetics when exposed to actinic radiation. We showed that local complete silver photodissolution at the micron scale can be achieved. Based on this result, we then developed amplitude-based computer-generated holograms using direct laser writing. CW lasers with beam shaping and short pulse lasers with beam scanning were both implemented. Elements with 8.5 µm and <1 µm spatial resolution and close to theoretical intensity distribution, respectively, were successfully demonstrated. Full article

Optical isolation with high-quality, large-aperture polarizers is commonly used in high-power laser facilities to suppress retro-reflected pulses. However, it is hard to manufacture these polarizers. We propose an approach of optical isolation with two plasma-electrode Pockels cells instead of large-aperture polarizers. In this approach, Nd:glass slabs placed at the Brewster′s angle are used as polarizers. The analysis results and the application performances in the Xingguang (Star Light) XG-III PW beamline indicate that this approach can supply good protection to optical components in laser facilities. Full article

This study examines the characteristics of the electromagnetic waves that propagate through an unbounded space filled with a homogeneous isotropic chiral medium. The resulting characters are compared to those of the electromagnetic waves propagating through an achiral free space. To this goal, we form energy conservation laws for key bilinear parameters in a chiral case. Due to a nonzero medium chirality, conservation laws turn out to contain extra terms that are linked to the spin–orbit coupling, which is absent for an achiral case. In this way, we identified where the neat hierarchy exhibited by the achiral case among the key bilinear parameters is destroyed by a medium chirality. As an example, we took a plane wave for the chiral case to evaluate those bilinear parameters. Resultantly, the conservation laws for a chiral case are found to reveal inconsistencies among several bilinear parameters that constitute the conservation laws, thereby prompting us to establish partial remedies for formulating proper wave-propagation problems. Therefore, adequate applications of boundary conditions are found to be necessary after examining typical problems available from the literature. Full article