Guest Editors Xinbin Cheng, Sven Schröder, and Michel Lequime introduce the Special Section on Frontiers of Optical Coatings II.

Thermal radiation from high-temperature application conditions of infrared windows can cause serious interference with infrared imaging. To suppress the thermal radiation from the window, the modulation effects of the optical properties and curvature characteristics of the two surfaces on the thermal radiation distribution from the window are investigated, respectively. The film–substrate–film radiation system is modeled. And the backward emissivity in the different cases of surface optical properties is investigated by the application of zinc sulfide (ZnS) windows and Y2O3 thin films as an example. It is shown that the backward emissivity is suppressed when the outer surface reflectivity is lower and the inner surface reflectivity is higher. A new definition of spectral directional locational emissivity is proposed to characterize the nonplate window emissivity. A model of thermal radiation propagation from the spherical substrate is developed. The emissivity distribution of the window with different curvature characteristics is investigated. It is shown that the curvature radius of the two surfaces can be matched to change the spatial distribution of the emissivity to reduce the effect of thermal radiation on the imaging of the backward optical system.

We investigate niobium-silicon mixed films prepared by plasma-assisted reactive magnetron sputtering. Multilayer films composed of niobium-silicon oxide with various Nb fractions were fabricated. The Nb fractions were calculated using the Brüggemann model and measured by x-ray photoelectron spectroscopy. The morphology of the samples shows that the stress in mixed monolayers depends on the Nb fraction and annealing temperature. A high-reflectivity (HR) multilayer film was fabricated from two mixed-oxide materials with Nb fractions of 20% and 95%, which are optimal for stress self-compensation and a maximum difference in refractive index to facilitate film design. The residual stress of this mixed-oxide multilayer HR film is completely self-compensated through annealing. Although annealing this film redshifts the transmission spectra and increases the surface roughness of the film, the results of cavity ring-down tests indicate that the reflectance remains high in the wavelength band of interest (1064 nm) and spatially uniform over the substrate surface. This detailed characterization of these films shows that the mixed-oxide multilayer HR mirror with self-compensated stress would be appropriate in numerous applications.

Mo/Al multilayers have the potential to be used as reflective mirrors in telescopes for cosmic and solar observation in a large spectral range of extreme ultraviolet radiation. Two types of Mo/Al multilayers were deposited by a magnetron sputtering system for comparison: one used pure Al and one used Al doped with 1.5wt. % Si. The structural performance of these two multilayers were characterized by a X-ray diffractometer, atomic force microscope, and optical profilometer. The angular dependence of the reflectivity was measured using a laboratory-based reflectometer at 58.4 nm. Grazing incidence X-ray reflectivity analysis revealed that the Mo/Al (1.5wt. % Si) multilayers possess a significantly improved interfacial structure compared with Mo/Al (pure). Surface morphology observations of the samples indicated that the incorporation of Si smoothes the surface and reduces the surface roughness. X-ray diffraction results suggest that the Si slightly reduces the average grain size of Al and increases the average grain size of Mo, but their crystallinity is improved. Based on the improvement of structural performance, a peak reflectivity of 29.4% at 58.4 nm is achieved for Mo/Al (1.5%Si) multilayers, whereas it is only 17.4% for Mo/Al (pure).

Multilayer structures with a thin chromium (Cr) layer embedded in the high reflective silver layer and dielectric layers, such as SiO2 and Ta2O5, are proposed. Reflectivity can be easily manipulated by adjusting the thickness of the Cr and dielectric layers. To demonstrate the potential for enhancing the flatness of reflection spectra, multilayers with average reflectivity value of 45% and 32% in the range of 450 to 900 nm are constructed, respectively. The reflectivity value exhibits a change of less than ±2.5%, and the transmittance is nearly negligible for both multilayer architectures. They can be fabricated using the physical vapor deposition technique.

Radiative cooling is a passive cooling strategy that can radiate heat to outer space through the 8 to 13 μm waveband (atmospheric window) and is now widely used for buildings, wearable fabric, solar cells, and electronic devices. Daytime radiative cooling requires both high reflection in the solar spectrum and high absorption/emission in the 8 to 13 μm range. Previous multilayered structures were optimized by changing the thickness ratio of the layers, but the optical properties of multilayer thin films, such as absorptivity, transmissivity, and reflectivity, are determined by complex factors. In this work, several initial multilayer structures were selected and then the thickness of each layer was globally optimized; the theoretically smallest thickness with the best absorption performance was achieved in the 8 to 13 μm range, which significantly improved the cooling performance and reduced costs. We developed an optimized SiO2-Ta2O5 alternating multilayer photonic radiative cooling thin film and fabricated it using ion source assisted electron beam evaporation with an average emissivity of 0.876 within the 8 to 13 μm range and an average reflectivity of 0.963 in the 0.3 to 2.5 μm waveband; it achieved an average temperature reduction of 20.1°C lower than the uncoated substrate and 3.2°C lower than the ambient temperature under direct sunlight with an average solar power of 859.3 W/m2.

Mirror stability is crucial for their applications, especially in the far ultraviolet (FUV) where Al mirrors protected with fluoride are the preferred choice. This study evaluates the effect of storage conditions on the performance of Al/LiF/MgF2 mirrors and compares them with previous investigations of Al/LiF/eMgF2 mirrors. This study focuses on FUV reflectivity, structural characteristics, and the long-term stability of mirrors stored in various environments. Coatings were deposited on the super-polished silicon wafers using thermal evaporation. Three sets of mirrors were stored in the environment with 20% relative humidity (RH), 40% RH, and 80% RH, respectively. Additionally, three other sets were stored in vacuum, nitrogen (N2), and oxygen (O2), respectively, in sealed bags kept in a 40% RH environment. Mirrors stored in a 20% RH environment demonstrated the best performance, exhibiting the smallest decline in reflectivity and a relatively stable surface morphology. Conversely, mirrors stored in 80% RH experienced a significant reduction in reflectivity, accompanied by the most pronounced deterioration in surface morphology. It was concluded that the chemical changes and high roughness of the films contributed to the decrease in the reflectivity.

Advantages of high-refractive index (HRI) glass wafers for augmented reality wearables (ARWs) include high transparency, low birefringence and total thickness variation, high durability and optical quality, high mechanical stability, stiffness, and geometrical stability. ARWs enable users to see computer-generated images overlaid on top of normal view. ARWs give the user an enhanced experience and are hands-free. Waveguide technology uses a combination of diffractive or reflective gratings and HRI glass wafers to transfer or project images from the light source (projector) to the user’s eye through total internal reflection (TIR). With all these advantages, glass is truly one of the world’s most transformative materials. To make the waveguide-based devices a reality, Corning has developed a series of HRI glass wafers with a diameter of up to Φ300 mm. Corning augmented reality solution (ARS) provides high-throughput metrology expertise, fully automated laser cutting technology, and the capability to capture the total thickness variation over the entire glass wafers using a frequency stepping interferometer. In this paper, we first conducted optical characterization of uncoated high-refractive index glass wafers. Corning^® Tropel^® Flatmaster^® MSP-300 was used to characterize the wafer profile properties of manufactured high-refractive index glass wafers. A variable angle spectroscopic ellipsometer (VASE) J.A. Woollam M-2000 was applied to evaluate the refractive index homogeneity of the high-refractive index glass wafers. A Metricon 2010/M system was used to measure the losses of optical waveguide at a wavelength of 409 nm, 448 nm, and 519 nm, respectively. A refractive index homogeneity of 99.976% and an optical thickness homogeneity of 99.979% were realized. Finally, an anti-reflective coating over a broad visible spectral range was added on top of a glass wafer for surface reflectance suppression. SiO2 and Nb2O5 were selected as low- and high-refractive index coating materials. The anti-reflective coating leads to a visible reflectance below 0.5% with a desired neutral aesthetic appearance.

In the last half-decade, the extended shortwave infrared (eSWIR) atmospheric band has become a focus of investigation for its potential to provide better object discrimination at range than the visible, as well as the near, shortwave, midwave, and longwave infrared bands, particularly in degraded visual environments such as smoke, dust, and smog. However, any detection band is only as useful as the best available detector, and thus, an investigation into the design of detectors for use in the eSWIR band is necessary before standards are established and applications put into practice. We examine the relationship between detector parameters and targeting performance in the eSWIR band for both passive and active detection. The effects of pixel pitch, dark current, read noise, frame rate, quantum efficiency, and well depth are examined and ranked in importance to an eSWIR system’s performance.

A three-dimensional holographic image computation method based on angular spectrum layering is proposed that further optimizes the selection of hybrid constraint factors and rationalizes the allocation of spatial resources for multi-layer plane image reconstruction. By employing separately independent angular spectrum propagation processes, potential aliasing reconstruction errors during the image propagation process are circumvented. Through numerical simulations and optical experiments, our method was compared with the hybrid constraint iterative angular spectrum sequence algorithm in the reconstruction involving eight plane images, validating the effectiveness and feasibility of our approach.

Hyperspectral imaging systems are widely used for their convenient way of capturing 3D information. The coded aperture snapshot spectral imaging (CASSI) systems and diffractive optical element (DOE)-based hyperspectral imaging systems are two types. CASSI systems can deal with the underdetermined inverse problem from a single-shot acquisition and further improve the quality of reconstructed results by utilizing multiple measurements; however, the systems based on DOEs cannot change the point spread function (PSF) encodings after DOEs are designed. Inspired by the tunable Moiré lens, we propose a multishot DOE-based hyperspectral imaging system that utilizes the mutual rotation between two DOEs to realize the change of the PSF encodings and enables multiple measurements. Our system can snapshot similar to fixed DOE-based systems. Meanwhile it requires only rotation and achieves the variable PSF encodings without moving along the horizontal and vertical axes. Moreover, our DOEs have a more regular and controllable dispersion. With the spectral reconstruction algorithm, the simulation results show that increasing measurements improves performance. We suggest a scheme to improve spectral accuracy by changing DOE encodings for multiple measurements.

A unique method for removing backgrounds from inline holograms using a numerically scaled illuminating beam intensity is proposed. To eliminate differences between the intensities of the illuminating beam and recorded holograms, the scaling employs a contrast function, which is calculated in the spatial frequency domain. Since it is free from manual intensity ratio adjustment, the proposed method is applicable to Gabor’s setup. The experimental results show that the proposed nonuniform background elimination method can retrieve the interference fringes better than conventional methods, improving the quality of the reconstructed images.

Absolute phase retrieval is an important part of 3D measurement. The conventional phase-unwrapping algorithm requires the projection of extra patterns to retrieve the absolute phase, which reduces the measurement speed. Based on the theory of phase domain modulation, the phase is pre-modulated in the form of a specific code embedded into the phase shift method to reduce the number of projected images for improving the speed. To improve the number of codewords and for the accurate determination of the fringe order, the coding strategy of segmented quantization is adopted at the time of coding. When decoding, the original wrapped phase is obtained by solving the wrapped phase map based on the wrapped phase map with the corresponding fringe order for absolute phase retrieval. The experimental results reveal that this method requires only one set of phase-shift modes to realize absolute phase retrieval. This feature improves the measurement speed and simultaneously maintains an accuracy that is consistent with that of the phase-coding method.

Efficiently shaping femtosecond, transverse Gaussian laser beams to flat-top beams with flat wavefronts is critical for large-scale material processing and manufacturing. Existing beam shaping devices fall short either in final beam homogeneity or efficiency. We present an approach that uses refractive optics to perform the majority of the beam shaping and then uses a fine-tune device (spatial light modulator) to refine the intensity profile. For the beam that we selected, circularly asymmetric with intensity fluctuations, our method achieved a uniformity of 0.055 within 90% of the beam area at 92% efficiency. The optimization involved an iterative beam shaping process that converged to optimum within 10 iterations.

To observe the polarimetric properties of an uncharacterized infrared transmitting material (IRTM) under various mechanical forces, a Mueller matrix (MM) imaging experiment was augmented with a force apparatus. Principal stress fields were computed from both finite element and closed-form models and spatially aligned with images of birefringence. The slope of the linear relationship between birefringence and principal stress difference is the stress optic coefficient. We discussed the advantages of MM polarimetry for stress optic coefficient measurements. First, no assumptions about the sample’s optical properties are necessary. Second, experimental deviations from the intended in-plane stress field can be identified. Third, independent pixels, over a small but appreciable range of stress values, can be selected to quantify experimental variation and improve statistical significance. To validate our experimental procedures, an N-BK7 sample was characterized at room temperature and compared with the industry-accepted value of 2.77 TPa−1±3% at 589.3 nm. To our knowledge, this is the first report on the stress optic coefficient of N-BK7 in the infrared, which was observed as 2.764±0.1526 TPa−1. The IRTM stress optic coefficient was observed to be 1.948±0.1197 TPa−1. Experimental sources of uncertainty are discussed and quantified.

A silicon-on-insulator substrate hosts a broadband wavelength filter featuring perturbed Bragg gratings. This perturbation is induced by strategically placing waveguides near the gratings. The use of smaller grating corrugation widths leads to a larger coupling coefficient, thereby significantly improving the grating reflectivity. Theoretical analysis of this compact device reveals a simplified circuit design, which is further corroborated by finite difference time domain simulation results. The achieved broadband filter boasts a bandwidth of approximately 40 nm, coupled with an impressive extinction ratio of around 39 dB, all achieved with a modest grating length of only 20 μm. The rate of change of the Bragg wavelength concerning the width of the perturbed waveguide is approximately 0.0625. It is noteworthy that this achievement is attained by maintaining fixed waveguide and grating parameters while selectively altering the width of the perturbed waveguide.

We propose a design leveraging photonic crystal semiconductor optical amplifiers (PhC-SOA), harnessing its unique capacity to manipulate and govern light at the nanoscale. Optimization of critical parameters to enhance performance is pursued through the utilization of the particle swarm optimization (PSO) algorithm. Initially, a multiplexer was constructed using logic gates, achieving a peak output power of 1.94e−11 mW. A PSO algorithm was employed to find the optimal PhC-SOA parameters such as length of active layer, input energy, simulation steps, full width at half maximum (FWHM), carrier recovery time, linewidth enhancement factor, saturation energy, and linear gain to maximize performance. The peak output power obtained from the PSO-optimized multiplexer is 2.9e−11 mW. This paper demonstrates the performance of PhC-SOA-based all-optical multiplexers and the effectiveness of PSO in optimizing the performance of all-optical multiplexers and contributes to the advancement of optical communication systems for high-speed applications.

Laser activation annealing using of an Mg-doped GaN four-point probe small mesa device using a 193-nm ArF excimer laser is investigated. Fabricated mesa device has 2-μm-high small mesa structures with In/Au contacts formed using standard semiconductor device process. In our setup, the ArF excimer laser was directed onto the sample using a series of mirrors and was focused through an aspheric lens to the sample, which is mounted on to a movable stage. The mesa device and the laser were aligned while being viewed on the computer monitor screen using a UV CCD camera. This method has several merits: (1) the four-point probe measurement allows accurate resistivity measurement of the GaN layer independently of the contact resistance, (2) utilizing a small area mesa also as the alignment marker during laser annealing ensures that the irradiated area precisely corresponds to the resistivity measurement area, and (3) multiple irradiation-measurement cycles are possible with a single mesa device, which avoids wafer-scale variations. The dependence of the resistivity to the laser fluence and irradiation time were investigated by multiple-irradiation measurement cycles to a single mesa device in combination with atomic force microscopy analysis for surface morphology characterization. Results reveal that laser annealing at 530 mJ/cm2 for 1500 s (at 150 Hz repetition rate) decreases the resistivity from 11.5 (before irradiation) to 5.6 Ω·cm (after irradiation). This value is similar to resistivity achieved by rapid thermal annealing (RTA) at 800°C for 120 s, suggesting successful Mg-doped GaN activation. Results also show stronger dependence on the irradiation time (temporal dependence) than the laser fluence, which may imply that activation mechanism for the laser annealing has a thermal contribution. This is the first report demonstrating ArF laser annealing that achieves a similar degree of activation with conventional RTA using resistivity measurements.

An external cavity diode laser (ECDL) in Littrow configuration with narrowband emission at 448 nm is presented. This wavelength coincides with a strong absorption cross-section line of the NO2 gas molecule in the blue spectral region. The ECDL is based on a commercially available multi-longitudinal modes GaN Fabry-Perot laser diode. Longitudinal mode selection is performed using a reflective holographic grating. The Littrow angle is fixed at 43.2 deg relative to the laser diode axis and the diffraction grating is at 3 cm from its output facet. Maximum tuning range of 3.8 nm is achieved with a linewidth of about 0.01 nm at the rated current. The lowest linewidth is obtained with the grating lines oriented parallel to the plane of the p-n junction of the laser diode. The slope efficiency of the ECDL is 0.35 (mW/mA), which is 30% lower than that of the laser diode. The maximum output power achieved by the ECDL is about 40 mW, which is about 75% of that of the laser diode. Maximum power efficiency is also obtained with the polarization direction of the laser diode parallel to the grating grooves. An excellent power stability is achieved over 2 h of continuous operation, with <1% fluctuation.

We modeled and investigated the silicon microring modulator (SMM) based on a PIN junction. The proposed model’s results are in good agreement with the experimental results. The effects of various parameters were considered in an optimized model to decrease power consumption as well as increase the bandwidth and extinction ratio of the SMM. The power consumption parameter plays a key role in the modulator design. To decrease this parameter, modulator dimensions were optimized using a genetic algorithm. This was done by considering the effect of the quality factor on the gradient change of the SMM transmission profile. In comparison with experimental results, the power consumption in the presented optimized model decreased dramatically from 9.7 to 0.042 μW. In addition, the extinction ratio and bandwidth increased by 12.5% and 50%, respectively.

We used modulating retro-reflector (MRR), hybrid frequency-phase-keying (FPK) modulation, and Reed-Solomon (RS) code technology to improve the performance of the terrestrial-satellite laser communication link. A system using FPK modulation and RS codes based on the Fisher-Snedecor (F) distribution atmospheric turbulence model is proposed for the MRR-assisted terrestrial-high-altitude-platform (HAP)-satellite laser communication, in which MRR is installed on the HAP. The laser beam without signal is transmitted to HAP through the forward path. The MRR loads the optical signal from the terrestrial-HAP link onto the optical carrier, which is then reflected back to the satellite terminal along the backward path. The combined effects of light intensity scintillation of atmospheric turbulence, beam wander of uplink, atmospheric attenuation, pointing error, and HAP random jitter on the terrestrial-HAP (equipped with MRR)-satellite (T-HMRR-S) laser communication link are considered. The expression of the BER of RS codes and FPK modulation (RS-FPK) system under weak atmospheric turbulence is derived and compared with terrestrial-HAP-satellite (T-H-S) system with different modulation and without MRR. At the same time, the effects of modulation coding parameters, zenith angle, the field-of-view angle, and the coverage area of the MRR on the performance of the T-HMRR-S system are simulated and analyzed. The simulation results show that the T-HMRR-S system using RS-FPK technology has better performance. These works provide a theoretical basis for the research of modulation coding technology of T-HMRR-S link systems.