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This study examines the optical properties of thin Cu (Ag)-layered structures covered with protective layers based on graphene, titanium (TiO2), or aluminium (Al2O3) oxides. The objective is to investigate the impact of these coatings on the optical behaviors of underlying metallic layers, specifically in the spectral range of excitation of surface plasmon resonances. Combining the methods of spectroreflectometry and spectroellipsometry was used to analyze the optical characteristics of the hybrid metal-oxidegraphene films. The study shows that graphene, due to its exceptional electrical conductivity and unique optoelectronic properties, significantly modifies the optical behavior of investigated structures. It includes notable changes in refractive and absorption indices, and optical conductivity indicating potential for enhancing light-matter interactions in plasmonic-graphene layered structures with the aim to apply as biosensor. It is important that addition of TiO2 and Al2O3 layers has also strong effects on the optical properties, which are relevant to their respective applications in the fields of optoelectronics and microelectronics. Employing the effective medium approximation and the Tauc-Lorentz model promotes deeper understanding the interplay between interband and intraband electronic transitions at the nanoscale level. It was revealed that the layer thickness of constituted materials and their individual dielectric functions together with addition of a graphene monolayer commit the significance for altering the optical properties of hybrid layered structures. The obtained results are important for the fields of plasmonics and nanotechnology, providing insights for designing sensors and devices with improved optical characteristics. [ABSTRACT FROM AUTHOR]

Deep learning (DL) has emerged as a promising tool for photonic inverse design. Nevertheless, despite the initial success in retrieving spectra of modest complexity with nearly instantaneous readout, DL-assisted design methods often underperform in accuracy compared with advanced optimization techniques and have not proven competitive in handling spectra of practical usefulness. Here, we introduce a tandem optimization model that combines a mixture density network (MDN) and a fully connected (FC) network to inversely design practical thin-film high reflectors. The multimodal nature of the MDN gives access to infinite candidate designs described by probability distributions, which are iteratively sampled and evaluated by the FC network to allow for rapid optimization. We show that the proposed model can retrieve the reflectance spectra of 20-layer thin-film structures. More interestingly, it reproduces with high precision the periodic structures of high reflectors derived from physical principles, even though no such information is included in the training data. Improved designs with extended high-reflectance zones are also demonstrated. Our approach combines the high-efficiency advantage of DL with the optimization-enabled performance improvement, enabling efficient and on-demand inverse design for practical applications.

This paper summarizes our results and findings made throughout the past years on the way towards a one millimeter thin, flexible, and scalable foil camera. The sensor part of the camera consists of multiple thin-film luminescent concentrator (LC) layers, each of which is sensitive to a different band of the light spectrum. A special optical micro-structure cut into the edges of the LC layers multiplexes the transported light signal into a variant of the Radon transform of the image focused on the LC surface. For increasing the camera's depth of field beyond the sensor surface, various thin-film imaging layers, such as optical Söller collimators realized by means of X-ray lithography on a PMMA wafer, have been investigated. The flexibility and scalability of our thin-film camera has the potential to lead to new human–computer interfaces that are unconstrained in shape and sensing-distance. Applications such as contact-less sensing, smart skin sensors for autonomous robots and industrial machines, and environment sensing for vehicles are imaginable. [ABSTRACT FROM AUTHOR]

Photoluminescence spectroscopy is a simple and powerful characterization technique to determine material properties and dynamic effects in metal–halide perovskite optoelectronic systems. However, self‐absorption and thin film cavity effects, amplified due to their high refractive indices, can have a significant impact on spectral lineshapes, affecting the inferences drawn from such characterization. In this work, key variables extrinsic to the perovskite material influencing the photoluminescence spectrum of perovskite thin films are identified and an optical model to account for these factors is proposed. The model is used to extract the intrinsic spectrum of a film using complex refractive indices and film thickness as input variables. Lastly, the use of the model is extended to multilayer systems to demonstrate its applicability to complex device‐relevant stacks. [ABSTRACT FROM AUTHOR]

Chiral liquid crystals (CLCs) self-assemble into a helical structure and can efficiently reflect circularly polarized light with corresponding handedness. Utilizing a curved glass substrate and polymerization of photoaligned CLCs, the operation of focusing and diffraction of incident light can be performed efficiently by a single component. When focusing and diffraction in a planar CLC cell are combined between two glass plates, the imaging suffers from astigmatism in the resulting spectrum. In this work, we demonstrate the operation of a spectrometer with low astigmatism using a polymerized CLC layer on a curved substrate. Two samples are fabricated, and the resulting components are operating in the wavelength range of 500-650 nm. Numerical optical modeling is used to minimize transverse aberrations and obtain a highly linear mapping on a camera sensor. In this way, it is demonstrated that a single reflective thin-film optical CLC component with a thickness of only a few micrometers can be used to realize a compact and efficient spectrometer.

Accurate determination of the photoluminescence (PL) spectrum of molecules is crucial in many areas including molecular diagnostics, genome analysis, and organic electronics. However, interpretation of experimental spectra from thin films can be complicated by phenomena extrinsic to the site of photon emission such as optical interference and self‐absorption. In this study, the alteration of fluorescence spectral lineshape is modeled from a thin film due to extrinsic factors as function of film thickness, incident angle, bandgap, and Stokes shift. The method is experimentally verified for thin (<400 nm) layers of molecular semiconductors on quartz substrates. Finally, a chart is provided for estimating the magnitude of the error due to extrinsic factors in the bandshape of fluorescence spectra from thin films. This demonstrates that the 0‐0/0‐1 intensity ratio, an indicator of the nature of excitonic coupling, is especially sensitive to possibly unconsidered influence from extrinsic effects. [ABSTRACT FROM AUTHOR]

Numerous effective medium models have been proposed for the effective optical properties nanoporous media. However, validations of these models against experimental data are often contradictory and inconclusive. This issue was numerically investigated by solving the two-dimensional Maxwell's equations in non-absorbing nanoporous thin-films with various morphologies. It was found that below a certain critical film thickness, the effective index of refraction depends on the porosity and on the pore size, shape and spatial distribution. For thick enough films the effective index of refraction depends solely on porosity and on the indices of refraction of the two phases. The numerical results agree very well with a recent model obtained by applying the Volume Averaging Theory to the Maxwell's equations. However, commonly used models systematically and sometimes significantly underpredict the numerical results.

We demonstrate a concept and fabrication of lithography-free layered metal-SiO2 thin-film structures which have reduced reflectivity (black appearance), to as low as 0.9%, with 4.9% broadband reflectance (8.9% for soda lime) in the 500−1400 nm range. The multi-layered (four layers) thin-film metamaterial is designed so that optical impedance matching produces minimal reflectance and transmittance within the visible and infra-red (IR) spectral region for a range of incident angles. The structure has enhanced absorbance and is easily tuned for reduced minimal transmission and reflection. This approach should allow for novel anti-reflection surfaces by impedance matching to be realized.

The low absorptivity of monolayer or few-layer graphene is one of the key limitations for high performance. To improve the graphene absorption, highly nanostructured or metallic systems have been usually employed, which however rely on the advanced nanofabrication with high cost or strong metallic parasitic absorption. In this study, via thin-film optics, the photonic surface waves based on purely dielectric planar system are proposed to realize perfect optical absorption by monolayer graphene with the thickness of ~ 0.34 nm. The Bloch surface wave (BSW) is found to be excited efficiently from 7-layer dielectric system by carefully addressing the admittance matching conditions, electric/magnetic field confinement, and band structure. Coupling with the strongly localized BSW field, a monolayer graphene shows an absorption ~ 100% at the designed infrared band (1310 nm, which can be tuned readily). With detailedly addressing the excitation condition of BSW, it is found that the perfect absorber can also be realized based on the more generalized surface waves from aperiodic structure. It is believed that the thin-film and purely dielectric surface wave system provides two-dimensional devices a promising opportunity for low-cost and high-performance applications. [ABSTRACT FROM AUTHOR]

This paper summarizes our progress toward a first fully transparent, flexible, and scalable thin-film image sensor and reviews lensless imaging approaches. Combining sensing and imaging in multiple layers has the potential to enable entire thin-film camera systems that make the capturing of images, light fields, and depth information possible. [ABSTRACT FROM AUTHOR]

This paper summarizes our progress towards a first fully transparent, flexible, scalable, and disposable thin-film image sensor. In contrast to conventional image sensors, it does not capture pixels in image space on the sensor surface, but makes integral measurements in Radon space along the sensor׳s edges. Image reconstruction is achieved by inverse Radon transform. By stacking multiple layers, it enables a variety of information, such as color, dynamic range, spatial resolution, and defocus, to be sampled simultaneously. Lensless multi-focal imaging allows reconstructing an entire focal stack after only one recording. The focal stack can then be applied to estimate depth from defocus. Measuring and classifying directly in Radon space yields robust and high classification rates. Dimensionality reduction results in task-optimized classification sensors that record a minimal number of samples. This enables simple devices with low power consumption and fast read-out times. [ABSTRACT FROM AUTHOR]

Thermal and optical engineering applications of electromagnetic wave scattering from rough surfaces include temperature measurement, radiation heating process, etc. Most of the surfaces have random roughness and are often with coating material different from the substrate. However, the understanding of radiative properties of coated rough surfaces is not well addressed at this point. This paper presented a novel hybrid partial coherence and geometry optics (HPCGO) model to improve the generic geometry optics (GO) prediction by incorporating a previously developed partial coherence reflectance equation. In this way, HPCGO expands the applicable region of GO model and largely reduces the computation time of integrating different wavelength results in the regular hybrid model that considers coherence effect only. In this study, the HPCGO model is first compared with the more rigorous Maxwell equations solvers, the finite-difference time-domain (FDTD) method, and integral equation (IE) method. Then, the HPCGO model is applied to study the coherent effect of directional-hemispherical reflectance from coated rough surfaces. It is found the roughness of coated rough surface can cause partially coherent or noncoherent scattered light even if the incident light source is coherent. It also shows the reflected electromagnetic wave's coherence effect reduces with increased coating thickness and surface roughness, besides the previously recognized incident wave-number bandwidth. The effect of reduce coherence in scattered wave is quantified. Finally a regime map, even limited in the roughness and coating thickness dimensionless parameter ranges, provides the region of validity of the HPCGO model. [ABSTRACT FROM AUTHOR]

Abstract: In search of next-generation solar cells, silicon nanowire arrays have attracted great attention since they are cost-effective and may absorb more light compared to current thin-film silicon solar cells. Theoretical studies using finite-difference time-domain and transfer matrix methods have been performed to investigate the optical properties of silicon nanowire (SiNW) arrays. However, these methods are computationally intensive and require periodic conditions, which may not be satisfied with most fabricated samples. In the present study, an effective medium analysis considering the anisotropic nature of vertically aligned SiNWs is performed to study their optical properties in the wavelength range from 310 nm to 1100 nm, which is of the most importance for solar photovoltaic cells. The effective dielectric functions of the SiNW layer for both ordinary and extraordinary waves are obtained from the Bruggeman approximation. Thin-film optics formulae incorporating the anisotropic wave propagation in uniaxial media are employed to calculate the reflectance and absorptance of the SiNWs on silicon substrates for different polarizations. The effect of geometric parameters such as filling ratio and wire length is investigated. In addition to modeling the directional radiative properties at various angles of incidence, the hemispherical properties are also calculated to understand the light absorption and to facilitate the optimal design of high-performance SiNW solar cells. [Copyright &y& Elsevier]

We present a versatile, simple, and rapid numerical technique based on thin-film optics for analyzing distributed Bragg reflectors composed of any material system. This technique is an alternative to the commonly used coupled-mode theories or matrix methods. Simulation results agree with those obtained using conventional matrix methods, and with the measured reflectivity of an MBE-grown III–V DBR. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 21: 11–15, 1999. [ABSTRACT FROM AUTHOR]

Abstract: The transfer matrix method was used to calculate the transmission of electromagnetic waves through alternate layers of thin silver films separated by air. It was found that the plasma frequency is a clear divide between good and poor transmission. Equally the calculations show that for frequencies above the plasma frequency the transmission coefficient shows a Bragg-like behavior with oscillations that manifest coherent reflections on the entrance side of the layer at favored wavelengths. [Copyright &y& Elsevier]

Fabrication and characterization of a guided-mode resonance-based polarizer is presented. This polarizer is made by electron-beam patterning a single layer of amorphous silicon on a glass substrate. The fabricated device has high and low transmittance for transverse-electric and transverse-magnetic polarizations, respectively, over a ~ 40-nm wavelength range for normally incident light. Its experimental extinction ratio is ~ 97:1 at a central wavelength of 1510 nm. [ABSTRACT FROM PUBLISHER]

We demonstrate a concept and fabrication of lithography-free layered metal-SiO2 thin-film structures which have reduced reflectivity (black appearance), to as low as 0.9%, with 4.9% broadband reflectance (8.9% for soda lime) in the 500–1400 nm range. The multi-layered (four layers) thin-film metamaterial is designed so that optical impedance matching produces minimal reflectance and transmittance within the visible and infra-red (IR) spectral region for a range of incident angles. The structure has enhanced absorbance and is easily tuned for reduced minimal transmission and reflection. This approach should allow for novel anti-reflection surfaces by impedance matching to be realized. [ABSTRACT FROM AUTHOR]