Fresnel Reflection Research Articles

МЕТОД МОНТЕ-КАРЛО ДЛЯ РАСЧЕТА ХАРАКТЕРИСТИК СВЕТОВЫХ ПОЛЕЙ В МОРСКОЙ ВОДЕ

The development of numerical methods for solving the integro-differential radiation transfer equation remains a relevant task. Among them, we can highlight the Monte Carlo method, which is in demand in various niches of modern ocean optics. The purpose of this work is a clear and concise presentation of the basics of the forward Monte Carlo method of light fields modeling in seawater, accompanied by a detailed description of its software implementation. The basics of the method are described, the procedures for choosing the type of interaction, the mean free path and the direction of photon motion are described. A simple case is considered, corresponding to an infinitely distant point source of unpolarized light, the absence of atmospheric influence, a smooth air-seawater interface, and the absence of stratification of inherent optical properties. In this case, realistic values of the absorption and scattering coefficients were used, calculated in accordance with the Case 1 model for a chlorophyll concentration of 1 μg/L, and a strongly elongated Henyey-Greenstein phase function with the parameter g = 0.95. The Fresnel reflection of light from the air-seawater interface was taken into account. The relative errors in the values of the diffuse attenuation coefficient for downward irradiance K d and the diffuse reflectance R, calculated in the spectral range of 400–700 nm using 106 photons, in comparison with the HydroLight results were 1.5 % and 0.4 %, respectively. Spectral calculation on one core of a 2017 Intel Core i5-8250U mobile processor in MATLAB takes 6 minutes. An assessment of the choice of the optimal number of photons required to obtain the desired quantities with a given accuracy was made. The implemented method is useful for becoming familiar with the basic principles used to numerically solve the radiative transfer equation in seawater using statistical methods and is used in the “Ocean Optics” course, taught by the author to 4th year students of the Department of Thermohydromechanics of the Ocean at MIPT.

Modified Fresnel equations for the case of oblique incidence on an isotropic gyrotropic medium

Modified Fresnel equations for the case of oblique incidence on an isotropic gyrotropic medium

Speed of Light in Hollow-Core Photonic Bandgap Fiber Approaching That in Vacuum.

A Fresnel mirror is introduced at a hollow-core photonic bandgap fiber end by fusion splicing a short single-mode fiber segment, to reflect the light backward to an optical frequency domain reflectometry. The backward Fresnel reflection is used as a probe light to achieve light speed measurement with a high resolution and a high signal-to-noise ratio. Thus, its group velocity is obtained with the round-trip time delay as well as the beat frequency at the reflection peak. Multiple Fresnel peaks are observed from 2180.00 Hz to 13,988.75 Hz, corresponding to fusion-spliced hollow-core fiber segments with different lengths from 0.2595 m to 1.6678 m, respectively. The speed of light in the air guidance is calculated at 2.9753 × 108 m/s, approaching that in vacuum, which is also in good agreement with 2.9672 × 108 m/s given by the numerical analysis with an uncertainty of 10-3. Our demonstration promises a key to hollow-core waveguide characterization for future wide-bandwidth and low-latency optical communication.

Developing Correction Methods by Revisiting the Concept of Effective Thickness in Attenuated Total Reflection Spectroscopy.

We propose a new way of deriving the effective thickness in attenuated total reflection (ATR) spectroscopy, initially introduced by Hansen and Harrick in 1965. While following Hansen's approach, our derivation is more straightforward and includes an intermediate approximation that more closely aligns with results derived from Fresnel's equations, particularly for organic and biological materials. Using this intermediate approximation, we present improved estimations for the effective thicknesses with s- and p-polarized light. These estimations enabled us to enhance a recently developed ATR correction scheme that relies on effective thickness. Additionally, we examined the wavelength dependence of the product of wavenumber and effective thickness, observing that it bears a resemblance to the refractive index function of the sample. This similarity increases with the angle of incidence and the refractive index of the ATR crystal. Based on this observation, we introduce a simple correction scheme using the Kramers-Kronig transformed absorbance. This correction has the potential to address spectral shifts, facilitating applications in pattern recognition and spectra identification.

Aspect sensitivity measurement of backscattering radar echoes from 205 MHz Stratosphere–Troposphere radar at a tropical coastal station

Aspect sensitivity in VHF radar refers to the extent to which the power and spectrum width of echoes vary with changes in the zenith angle, which is related to the backscattering process. The scattering of clear air signals is primarily caused by Fresnel reflection, anisotropic scattering, and isotropic scattering. The aspect sensitivity and accuracy of moment and wind estimation in clear air radars are influenced by the beam width, beam pointing angle from zenith, and atmospheric conditions. This study proposes a method to determine the sensitivity of the recently developed Stratosphere–Troposphere (ST) wind profile Radar in Cochin, India (10.04°N, 76.33°E). The radar operates at a distinct frequency of 205 MHz in the far VHF band. The experiment is conducted at an altitude of 5 to 20 km above the ground by adjusting the beam’s orientation with a resolution of 2°in both the east–west and north-south directions. The estimation of wind components is subject to uncertainty due to the varying aspect angles caused by the distinct dispersion properties in this height range. The study found that power variance is lowest between 6 and 12 km in both north-south and east–west directions, while daily fluctuations in aspect-sensitive echoes complicate wind component estimation. Correlation length (ζ) ranges from 0.5 to 15 m, indicating various air scattering processes. Notably, θs is smaller near the zenith and increases with tilt angles, exceeding 20°up to 14 km before declining at higher altitudes, indicating significant anisotropy at elevated levels. The wide range of R factor values (0.1 to 0.9) across different heights causes significant ambiguity in wind estimation. In this study, the impact of various aspect sensitivity parameters on wind estimation on days with clear air has been analyzed.

Polarized spectral bidirectional reflectance distribution function in visible band based on Cauchy’s empirical dispersion equation

The polarized bidirectional reflectance distribution function (pBRDF) can describe the changes between the Stokes vectors of incident and reflected light. The existing model can only describe the spatial distribution of the target’s polarization characteristics at a single wavelength, so further research is needed for the description of the target’s polarized spectral characteristics. In this paper, a modified three-component polarized spectral bidirectional reflectance distribution function (pSBRDF) is proposed, which combines Fresnel equation in the specular reflection component with Cauchy’s empirical dispersion equation, by introducing the wavelength variable and dispersion constants that do not change with wavelength. The degree of the linear polarization (DoLP) of two types of coated fabric samples was measured at three incident angles and 400 to 760 nm wavelength. The error of the model in describing the spectral and the spatial distribution of DoLP are controlled within 0.00743 and 0.0757 respectively, which proves the accuracy of the model. This research provides the model basis for analysis of the target’s polarization characteristics and the theoretical basis for the application of polarization detection.

Modeling photomolecular effect using generalized boundary conditions for Maxwell equations

We recently demonstrated via experiments in hydrogels and at a single air-water interface the photomolecular effect: photons directly cleaving off water molecular clusters in the visible spectrum where bulk water has negligible absorption. To model single interface experiments, here we re-derive generalized boundary conditions for Maxwell equations by assuming a transition region of the electromagnetic fields across the interface, leading naturally to the Feibelman parameters used before to describe surface photoelectric and surface plasmon effects on metals. This generalization leads to modifications of the Fresnel coefficients and an expression for the surface absorptance that can reasonably explain trends in our single-interface experimental data on the angle and polarization dependence of the beam deflection. Our work provides further support for the existence of the photomolecular effect, suggests that surface absorption should exist in many materials, and lays a foundation for assessing the impacts of such surface absorption based on the Maxwell equations.

Interaction of a UWB impulse with a layer of sandy soil, having a rough upper boundary

ABSTRACT In this paper, the changes in the character of time shapes, amplitudes, frequency spectra, and time delay of ultra-wideband (UWB) impulses (duration of 0.33 ns) reflected from the layer of wet sand with a rough upper boundary were investigated. It was shown that the attenuation of UWB impulse amplitude, which is reflected from the upper rough boundary of the sand layer, can be described by the model of Fresnel reflection coefficient for a smooth surface using the Gaussian correction factor (FRGC). At this root-mean-square (RMS), height of soil surface roughness was in the range of ~0–21 mm, but the determination coefficient and RMS error (RMSE) were found to be equal to 0.94 and 0.02, respectively. An experiment close to linear increase/decrease in the propagation time of impulses reflected from the upper/lower boundary of the layer, respectively, was found with increasing the RMS heights of upper boundary. It is shown that the propagation time of such impulses can be described by the modified FRGC model with an error from RMSE = 0.09 ns to RMSE = 0.15 ns. The results of these findings are of practical importance for the interpretation of remote sensing data for precise soil moisture measurements from unmanned aerial vehicle platforms using UWB impulses.

Exotic spin-Hall effect in non-Hermitian optical systems

Abstract We systematically explore the origin and evolution of the exceptional points (EP) when a light beam is scattered by a parity-time (PT)-symmetric system using a scattering matrix approach and a full-wave theory. It is demonstrated that the PT-symmetric system switches between symmetry and symmetry-breaking phases at the EPs, giving rise to singular features in the Fresnel coefficients and causing the spin-Hall effect (SHE) near the EPs to exhibit anomalous features such as significantly enhanced transverse spin-Hall shifts and additional in-plane spin-Hall shifts. This exotic SHE can be explained by the significant beam intensity distortion caused by the destructive interference between the spin-maintained normal modes and the spin-reversed abnormal modes in the scattered light. This phenomenon can further be understood in terms of vortex mode decomposition, wherein it can be interpreted as the competition and superposition of three vortex modes with topological charges of −1, 0, and 1, respectively. These findings elucidate the mechanism of the unusual SHE around the EPs and offer potential avenues for EP-based sensing and structured light manipulation.

Modeling, Calculation and Realization of Au/Nb2O5 Substrates for Surface Plasmon Resonance Sensors

Introduction. Sensor devices based on the surface plasmon resonance (SRP) phenomenon are widely used among the tools of information and diagnostic technologies. The main part of a device based on the surface plasmon resonance phenomenon is a sensor substrate, which consists of a glass plate, a film of plasma-supporting metal, usually gold, and additional layers to increase the sensitivity of the research. Depending on the thickness of the additional dielectric layers, the following types of sensors can be implemented: a conventional SPR sensor with a gold film without a dielectric; an Au/dielectric SPR sensor with a dielectric thickness less than the critical one, in which the system can maintain the plasmonic mode; a waveguide sensor on a metal sublayer, with a dielectric thickness sufficient for the emergence of waveguide modes. The Au/Nb2O5 thin-film system is promising for creating sensor substrates due to the high refractive index of Nb2O5 and its exceptional chemical and mechanical resistance. The purpose of the work. This work is devoted to computer modelling, calculation of performance characteristics and implementation of Au/Nb2O5 sensor substrates for SPR devices, including the Plasmontest series devices developed at the Institute of Cybernetics of the National Academy of Sciences of Ukraine. The behaviour of the characteristics of the SPR substrates with a plasma-supporting gold film, SPR substrates with sensitivity enhancement by an additional layer of Nb2O5 of nanometre thickness and with a waveguide layer of Nb2O5 thickness more than 150 nm are analysed. Calculations were performed for both refractive sensors on SPR and biosensors. Results. The theoretical analysis was carried out by computer modelling in Winspall and calculations in MATLAB using the Fresnel equations and the matrix method, which allowed a comprehensive analysis of the shape of the reflection curves, angular sensitivity, resolution and angular ranges for sensors with different dielectric coatings. It is shown that at a thickness of 0-10 nm, the Au/Nb2O5 structure will work as a SPR sensor with increased sensitivity, and at a thickness of 150-210 nm as a waveguide sensor on a metal sublayer. But, as calculations have shown, only the Au/Nb2O5 SPR sensor is promising for practical implementation to increase the sensitivity of biosensor measurements. The work carried out on the development of thin-film technology, which includes vacuum deposition of a thin-film structure of adhesive Nb (1–2 nm), plasma-supported Au (50 nm), Nb (3–5 nm) and thermal annealing (to oxidise the top layer of niobium), made it possible to implement Au/Nb2O5 substrates for SPR studies. Experimental studies have shown that the angular sensitivity of the Au/Nb2O5 refractometric SPR sensor is 1.5 times higher than that of a SPR sensor with a single Au film at niobium oxide layer thickness of about 6 nm. Thus, the experimentally determined increase in the angular sensitivity of the Au-Nb2O5 SPR sensor is consistent with the theoretical data. Conclusions. Calculations have shown an increase in the angular sensitivity of SPR sensors for Au/Nb2O5 sensor substrates compared to conventional Au film substrates for both refractometric (1.6 times) and biosensor (1.8 times) studies. The sensors on Au/Nb2O5 substrates for SPR studies, which we have designed and practically implemented, are cheap to manufacture, have higher sensitivity, and due to the exceptional chemical and mechanical stability of Nb2O5, can be used repeatedly in rather aggressive environments. Keywords: computer modelling, surface plasmon resonance, sensor, refractometer, biosensor.

Analysis of tissue-substrate adhesion by hyperspectral surface plasmon resonance microscopy.

The preparation of histology slides is a critical step in histopathology, and poor-quality histology slides with weak adhesion of tissue sections to the substrate often affect diagnostic accuracy and sometimes lead to diagnostic failure due to tissue section detachment. This issue has been ofconcern and some methods have been proposed to enhance tissue-substrate adhesion. Unfortunately, quantitative analysis of the adhesion between tissue sections and glass slides is still challenging. In this work, the adhesion of mouse brain tissue sections on gold-coated glass slides was analyzed using a laboratory-fabricated hyperspectral surface plasmon resonance microscopy (HSPRM) system that enabled single-pixel spectral SPR sensing and provided two-dimensional (2D) distribution of resonance wavelengths (RWs). The existence of the nanoscale water gap between the tissue section and the substrate was verified by fitting the RW measured in each pixel using the five-layer Fresnel reflection model. In addition, a 2D image of the tissue-substrate adhesion distance (AD) was obtained from the measured 2D distribution of RWs. The results showed that tissue-substrate AD was 20-35 nm in deionized water and 4-24 nm in saline solution. The HSPRM system used in this work has a wide wavelength range of 400-1000 nm and can perform highly sensitive and label-free detection over a large dynamic detection range with high spectral and spatial resolutions, showing significant potential applications in stain-free tissue imaging, quantitative analysis of tissue-substrate adhesion, accurate identification of tumor cells, and rapid histopathological diagnosis.

Boundary-layer structures arising in linear transport theory.

We consider boundary-layer structures that arise in connection with the transport of neutral particles (e.g., photons or neutrons) through a participating medium. Such boundary-layer structures were previously identified by the authors in certain particular cases [Phys. Rev. E 104, L032801 (2021)2470-004510.1103/PhysRevE.104.L032801]. Extending the previous work to anisotropic scattering and general Fresnel boundary conditions, this contribution presents computational algorithms which (1) resolve the aforementioned layers as well as previously unreported boundary layers associated with Fresnel boundary transmission and reflection, and (2) yield accurate simulations at fixed computational cost for transport under phase functions with arbitrarily strong anisotropy. The present paper additionally includes (3) Mathematical proofs which justify the numerical methods proposed for resolution of boundary-layer structures. The impact of the new theory on algorithmic performance is demonstrated through a series of 1D computational benchmarks that emulate typical photon- and neutron-transport applications such as, e.g., optical tomography, and nuclear reactor analysis and design. Experimental results for transmission of photons through turbid media are presented, exhibiting close agreement between simulated and experimental data. As illustrated by means of a variety of numerical results, the proposed boundary-layer-based approach tackles transport problems with unprecedented accuracy and efficiency.

Perovskite manganese oxide-based superposed multilayer film with high emittance tunability for smart radiation devices

Smart radiation devices (SRDs) with the ability to switch emittance according to the radiator surface’s temperature are significant for spacecraft thermal control on exploration missions with drastic changes in thermal environments. However, it is quite urgent to improve the performance of the intelligent thermal control films in SRDs due to the limitations such as low emittance tunability and inappropriate phase transition temperature. Consequently, a film with self-adaptive infrared emittance based on anti-reflective design is proposed in this paper. The main structure is a superposed multilayer film based on a doped perovskite manganese oxide La0.825Sr0.175MnO3 (LSMO) and BaF2 stacks. The emittances of the designed multilayer film are 0.18 and 0.83 at the temperatures of 100 K and 295 K, and therefore the emittance tunability achieves 0.65. Compared with single-layer LSMO, the emittance tunability has increased 25 % approximately. The enhancement in emittance tunability can be explained by the fact that multilayer films with staggered low and high refractive index dielectric layers may cause destructive interference and suppress the Fresnel reflections at high temperatures. In addition, the thickness of dielectric part is carefully designed to avoid the formation of high emissivity Fabry-Perot (FP) resonance when LSMO switches to metallic state at low temperatures. This perovskite manganese oxide-based multilayer film exhibits remarkable intelligent thermal control properties. It provides a significant opportunity to improve the performance of SRDs and a promising potential for maintaining the optimal operating temperature of the spacecraft.

THz Metamaterial Sensitivity Enhancement by Reduction of Substrate's Fabry-Pérot Oscillations Using Back Plates as an Optical De-Coupler.

This work presents a new method for the enhancement of sensitivity in Terahertz (THz) spectroscopy on metamaterial (MM) in terms of its resonance frequency shift (ΔF), by attaching the dielectric back plate to the MM's silicon (Si) wafer. The dielectric back plates are designed to minimize the Fresnel reflections at the backside of the substrate, identical to a broadband antireflective (AR) plate tailored for THz. Utilizing broadband AR technology, we demonstrate the concept of decoupling MM resonance from the substrate's Fabry-Pérot (FP) oscillations. This is done by effectively coupling the THz light out of the high-permittivity substrate, resulting in the improved quality factor of the MM resonance and overall plasmonic enhancement on the metasurface. The back plate acts as a surface plasmonic enhancer to the THz MM by increasing the field intensity on the front metasurface, leading to enhancement of dielectric response (MM's ΔF). This makes the MM resonance ultrasensitive to the minor changes of particle size/concentration under test spread on the metasurface, contributing to enhanced resonance ΔF. The plate also makes the Si substrate optically lossless, enabling the full effect of MM resonance shift and increasing the resonance ΔF by 8-fold compared with MM's fabricated on conventional Si substrates. This research is backed-up with system-level CST simulations and experimental THz impedance spectroscopy of the MM. This method and chip structure is CMOS compatible having potential applications for any resonant MM fabricated on a substrate aimed to maximize dielectric sensitivity for biosensing and nanoparticle THz spectroscopy.

Flexible Epitaxial Lift‐Off InGaP/GaAs/InGaAs Triple‐Junction Solar Cells Integrated with Micro/Nanostructured Polymer Film

Epitaxial lift‐off (ELO) InGaP/GaAs/InGaAs inverted metamorphic triple‐junction solar cells are encapsulated with a micro/nanostructured polydimethylsiloxane (PDMS) film. The microprism array (MPA) is realized on the PDMS film to redirect the light incident on the metal grid line to the active area. Subwavelength structures (SWSs) are also introduced onto the PDMS film to suppress the Fresnel optical reflection loss. Triangular and hemicylindrical shapes are considered for the MPA. The optical responses of the two MPAs are calculated by using ray‐tracing methods. The triangular MPA performs better than the hemicylindrical MPA in terms of light‐redirection efficiency. It is confirmed that 82.0% of the light incident on the metal grid can be harvested by the effect of the triangular MPA and the Fresnel optical reflection loss is reduced effectively by the SWSs. These effects contribute to photocurrent enhancement. The short‐circuit current density and power conversion efficiency of the flexible ELO triple‐junction solar cells integrated with the micro/nanostructured PDMS film improve by 7.0% and 7.1%, respectively, compared with those of the solar cells without the PDMS film. By using the flexible PDMS film for light management, the flexibility of the ELO solar cells is preserved.

Optimizing Spacecraft Re-entry Communication: Plasma Electron Depletion Through Controlled Particulate Injection and Fresnel Coefficients Analysis

Optimizing Spacecraft Re-entry Communication: Plasma Electron Depletion Through Controlled Particulate Injection and Fresnel Coefficients Analysis

Sea surface Fresnel reflections difference driven de-glint algorithm for airborne optical images.

This paper presents a glint correction algorithm for high spatial resolution optical remote sensing imagery captured by the ER-2 Airborne Visual Infrared Imaging Spectrometer (AVIRIS). The algorithm employs linear and differential techniques to mitigate sun glint and sky glint effects, encompassing statistical glint reflections resulting from variations in imaging angles within strips and inter-strip variations due to Fresnel reflectance disparities. It aims to diminish Fresnel reflectance diversity on water surfaces and mitigate the distortions induced by glint reflectance during spectral and ocean color inversion. A comparative analysis of spectral and ocean color information in AVIRIS images before and after correction reveals enhanced accuracy following the glint correction. By systematically addressing multiple glint reflections and their ramifications, this method offers a valuable framework for correcting water surface glint in diverse high spatial resolution optical imagery.

Method for remote measurement of specific conductivity and moisture of subsurface soil horizons

The aim of the research is to develop a radar method for determining the physical and chemical parameters of subsurface soil horizons, providing rapid determination of moisture and specific conductivity in the area of the plant root system. The proposed method is based on a set of Fresnel equations which describe reflection of electromagnetic waves from the interface between dielectric media in vertical and horizontal polarization of the probing signal. For the practical implementation of the method, it is proposed to use two unmanned aerial vehicles that form a bistatic radar system which irradiates the Earth's surface obliquely in order to create the Brewster's effect and increase the fraction of the radio signal reflected from subsurface horizons. The percentage of moisture and the specific conductivity of soil are calculated from the measured values of the imaginary part of the complex permittivity. The required accuracy of moisture and conductivity measurements is achieved by two-step calibration of the measuring device. The values of the moisture content and specific conductivity of soil obtained by radar at a frequency of 469 MHz are in good agreement with the results of measuring these parameters using the soil moisture meter TDR 150 Spectrum Technologies, Inc.

Layer Dependence of Complex Refractive Index in CrSBr.

CrSBr is a recently discovered two-dimensional anti-ferromagnet. It has attracted much attention due to its superior properties for potential optoelectronic and spintronic applications. However, its complex refractive index with layer dependence has not been systematically studied yet. In this work, we studied the room-temperature complex refractive indices of thin CrSBr flakes of different thicknesses in the visible light range. Using micro-reflectance spectroscopy, we measured the optical contrast of thin CrSBr flakes with respect to different substrates. The complex refractive index was extracted by modeling the optical contrast with the Fresnel equations. We extracted the band gap values of CrSBr in the few-layer limit. We determined the band gaps for monolayer, bilayer, and trilayer CrSBr to be 1.88 eV, 1.81 eV, and 1.77 eV, respectively. As a comparison, the band gap for multilayer CrSBr is outside our measured range, that is, below 1.55 eV. Our results suggest that the band gap of CrSBr decreases as thickness increases.

Plasma-Etched Black GaAs Nanoarrays with Gradient Refractive Index Profile for Broadband, Omnidirectional, and Polarization-Independent Antireflection.

Black GaAs nanotip arrays (NTs) with 3300 nm lengths were fabricated via self-masked plasma etching. We show, both experimentally and numerically, that these NTs, with three gradient refractive index layers, effectively suppress Fresnel reflections at the air-GaAs interface over a broad range of wavelengths. These NTs exhibit exceptional UV-Vis light absorption (up to 99%) and maintain high NIR absorption (33-60%) compared to bare GaAs. Moreover, possessing a graded layer with a low refractive index (n = 1.01 to 1.12), they achieve angular and polarization-independent antireflection properties exceeding 80° at 632.8 nm, aligning with perfect antireflective coating theory predictions. This approach is anticipated to enhance the performance of optoelectronic devices across a wide range of applications.