Specular Reflectivity Measurements Research Articles

Determination of intrinsic scattering and absorption coefficients of light diffusing materials: application of a spectral projected gradient method

We explore a novel method to obtain scattering and absorption (S&A) coefficients of inhomogeneous materials from measurements of regular transmittance and specular reflectance. We use a Spectral Projected Gradient Method (SPGM) to invert experimental spectra. The SPGM method requires an initial approximation to the S&A coefficients, which is needed to ensure convergence to an optimized solution. We discuss problems associated with multiple solutions and conditions for the obtained optimal solution to be a good approximation to the physical one. We present results for TiO2/polyvinylpyrrolidone (PVP)-water, plasmonic Au/PVP-water and ferromagnetic Fe3O4/PVP-water materials. The obtained S&A coefficients indicate that these materials are dilute suspensions of dense particle clusters in the dependent scattering regime. We argue that the SPGM gives physically realistic solutions for this class of materials. The SPGM has the advantages that it is much less expensive in computational terms and easier to implement than previously used simulated annealing methods.

Investigating the effects of geometry and bandgap on broadband antireflection in plasma-etched semiconductor nanoarrays via specular and hemispherical optical spectra

This research investigates the influence of nanoscale morphology and bandgap on the optical properties of semiconductor nanostructures, with a particular emphasis on surface reflectance and optical absorption across the UV–VIS–NIR wavelength range. Our investigation shows that plasma-etched densely packed silicon (Si) nanotip arrays (NTs) demonstrate significant anti-reflective properties, as revealed by specular and hemispherical reflectance measurements, due to their sub-wavelength spacing and tip lengths exceeding the incident optical wavelength. In contrast, sparsely arranged Si NTs have reflective behaviour that is similar to bare Si substrates. Semiconductor substrates such as gallium phosphide (GaP), Si, gallium arsenide (GaAs), and germanium (Ge), when decorated with NTs, show notable changes in their optical characteristics, with hemispherical reflectance spectra decreasing to 1–3% and absorptance increasing to 97–99 % in the entire spectral region above the bandgap. Below the bandgap, a slight reduction in reflectance and a modest rise in absorptance are observed. These findings offer critical insights for enhancing semiconductor nanostructures to optimize their performance in photonic and optoelectronic applications across various wavelength ranges.

Parameters of differential equations in four-flux and two-flux models approximated for scattering and absorption results on solar thermal collector black paints

Abstract The intrinsic and extrinsic scattering and absorption coefficients of a thin black paint substrate used as a solar thermal collector, deposited on a thick glass substrate, were obtained in an approximated single-layer method, using the new solved equations recently determined from the collimated and diffuse differential equations of the four-flux model, for the intrinsic coefficients, and considering the differential equations of the two-flux model with total light intensities, for the extrinsic coefficients. In a first step, the optical constants were determined from the measurements of regular transmittance and specular reflectance, fitting them to the collimated transmittance and reflectance solutions of the four-flux model. In a second step, the intrinsic coefficients were obtained from the diffuse transmittance and reflectance measurements, considering four forward scattering ratios (for collimated and diffuse light intensities and for forward and backward light directions) and two average crossing parameters (for forward and backward light directions). In a third step, the extrinsic coefficients were obtained, using the new solved equations recently determined considering the total differential equations as the sum of the collimated and diffuse differential equations. Due to the approximated single-layer method, a simple rule of three was applied to the intrinsic and extrinsic results, relating the extinction coefficients calculated from the optical constants to those calculated from the collimated differential equations. The same external and internal diffuse interface reflectance was observed by integrating the collimated interface reflectance up to the critical angle of total internal reflection.

Optical constants and thickness gradients for light intensities in glass substrate layers and for complex electric fields in indium tin oxide (ITO) transparent conductor thin film layer, using Bode and Nyquist wavelength-dependent diagrams

Abstract The optical constants of a thin film layer of transparent conductive indium tin oxide (ITO), deposited on a thick glass substrate (G), were obtained for an ITO-G sample using a multilayer matrix method by fitting the regular transmittance and specular reflectance measurements to the collimated equations of the four-flux model, once the optical constants of the glass substrate layer had previously been obtained from a G sample of a single glass substrate layer. The accuracy of the results was validated using a feedback system called the three-extinction matching requirement, that is, obtaining the same extinction coefficients from the optical constants and from the forward and backward collimated differential equations of the four-flux model. To calculate the extinction coefficients of the glass from optical constants, the compression of the wavelength of the incident light in the glass substrate and in the thin ITO film layer was demonstrated, with a thickness less than the wavelength of incident light. The thickness gradients of the forward and backward collimated light intensities, on the glass substrate of the ITOG sample, were compared with those obtained for the single-layer G sample. Then, the thickness gradients for the complex forward and backward electric fields of the ITO thin film layer were obtained and plotted using spectral magnitude and phase Bode plots and new Nyquist plots, i.e. imaginary versus real parts, dependent on wavelength.

Interference Lithography‐Based Fabrication of 3D Metallic Mesostructures on Reflective Substrates using Electrodeposition‐Compatible Anti‐Reflection Coatings for Power Electronics Cooling

AbstractA nanostructured copper oxide (nCO) coating which can be electrochemically reduced to copper metal is demonstrated as an anti‐reflection coating, enabling interference lithography of three‐dimensionally structured templates on a surface compatible with subsequent electrodeposition steps. The nCO presents a black needle‐like structure which effectively absorbs the incident radiation during interference lithography. Specular and diffused reflectivity measurements confirm nCO has near‐zero reflectivity from at least UV (350 nm) to near IR (700 nm) wavelengths. A particularly important aspect of the nCO is its ability to be reduced to copper metal, enabling electrodeposition inside porous templates fabricated on the nCO. It is demonstrated electrodeposition of copper within 3D templates defined by interference lithography and proximity field nano‐patterning processes, forming mesostructured metals which enhance two‐phase cooling. The resultant 5 µm thick structures exhibited up to 3 times the critical heat flux and 2 times heat transfer coefficient of bare silicon. The structures are optimized via computational tools including Finite Difference Time Domain (FDTD) and COMSOL Multiphysics. The use of the approach demonstrated here can potentially find application in many areas given the broad importance of mesostructured metals for energy, biomedical, and mechanical applications.

Modelling structural colour from helicoidal multi-layer thin films with natural disorder.

A coupled mode theory based on Takagi-Taupin equations describing electromagnetic scattering from distorted periodic arrays is applied to the problem of light scattering from beetles. We extend the method to include perturbations in the permittivity tensor to helicoidal arrays seen in many species of scarab beetle and optically anisotropic layered materials more generally. This extension permits analysis of typical dislocations arising from the biological assembly process and the presence of other structures in the elytra. We show that by extracting structural information from transmission electron microscopy data, including characteristic disorder parameters, good agreement with spectral specular and non-specular reflectance measurements is obtained.

Enhanced Diffuse Reflectance of Photons in Molybdenum Back Electrode Films Achieved via Surface Texturing for Thin Film Solar Cells

Molybdenum (Mo) thin film is widely used as the bottom electrode in substrate‐configured thin film solar cells due to its high conductivity and reflectivity. Mo grown onto microtextured substrates increases the diffuse reflection of incident photons via scattering/multiple reflections on the corner/edges, in addition to the contribution from increased surface area. Herein, bilayer Mo is grown using direct current magnetron sputtering onto plain and textured substrates. To texture the glass substrate surface, a periodic array of cross‐linked photoresist structures (SU‐8 2000.5 & SU‐8 2005) is introduced via UV‐lithography process. Characterizations are done using X‐ray diffractometer, optical microscope, scanning electron microscope, and optical profilometer. Angle‐dependent specular reflectance, diffuse reflectance, and electrical measurements done on Mo grown on plain and textured substrates lead to a comprehensive understanding of the advantages of growing Mo films on textured substrates. The electrical resistivity of Mo grown on plain and textured substrates is of the same order, measuring 3.09 × 10−5 Ω cm, 3.25 × 10−5 Ω cm, and 3.87 × 10−5 Ω cm, respectively. A high‐diffuse reflectance of 62.7% is obtained for SU‐8 2005 textured Mo films. Investigations using COMSOL simulations support the optical reflectance analysis.

Измерения спектральной зависимости коэффициента отражения комплектного алюминиевого зеркала спектрофотометра Shimadzu UV-2700

In this paper, we present the results of experimental investigation of the spectral dependence of the aluminum mirror reflection coefficient in the operating range of 185…900 nm. The aluminum mirror is a part of the Specular Reflectance Measurement Attachment to the Shimadzu UV-2700 spectrophotometer. We used experimental data obtained on the same spectro-photometer for a standard incidence angle of 5° for the reflection coefficient of a fused silicon dioxide plate as a reference sample. This is because the spectral characteristics of the optical parameters of fused silicon dioxide determined by the Sellmeier formular are well known. We used a standard quartz glass plate with a thickness of 2,2 mm and a spectrophotometer slit of 5 nm in our experiment. In this case it is possible to neglect interference effects due to multiple reflection. We found from our experimental data the spectrum of the absorption coefficient required for determining the calculated spectral dependence of its reflection coefficient in the spectrophotometer operating range. On the basis of this absorption spectrum the absolute values of the reflection coefficient of the quartz plate were calculated. This calculation was made for the normal incidence approximation and taking into account multiple reflections and dispersion of the refractive coefficient of fused quartz in accordance with the Sellmeier formula. The comparison of the spectra of calculated absolute and relative values of the reflection coefficient of the quartz plate measured on a spectrophotometer with experimental data for the reflectance of aluminum mirror made it possible to obtain the spectral dependence for the aluminum mirror for the entire operating range from 185 to 900 nm. It was found that spectral dependence of the reflection coefficient of a complete aluminum mirror was nonmonotonic. In case under consideration the deviations of its reflection coefficient from an ideal unit value do not exceed 10 % only in the spectral range from 350 to 670 nm. Thus, it is necessary to take into account the obtained spectral dependence of the reflection coefficient of the complete aluminum mirror for quantitative measurements in the entire operative range of the Shimadzu UV-2700 spectrophotometer from 185 to 900 nm.

Pearl‐Like Sheen in Soft Capsules: An Unusual Optical Effect that is Reversibly Induced by Temperature

AbstractA pearl‐like sheen (i.e., pearlescence) is seen in many natural materials like nacre and in some commercial paints and cosmetics. This phenomenon is attributed to the interaction of light with plate‐like particles in the material. Here, for the first time, pearlescence is demonstrated in soft millimeter‐scale capsules that contain no plate‐like particles. The capsules have a thin (~500 µm) outer shell of N‐isopropylacrylamide (NIPA) hydrogel, which has a lower critical solution temperature (LCST) of 32 °C. When a transparent NIPA‐shelled capsule is heated above this LCST, it turns pearlescent. The effect is reversible, with the transparent state being recovered upon cooling. This is the first example of reversible pearlescence in any solid. Specular reflectance measurements show that the pearlescence of the capsules is comparable to that of natural pearls. Pearlescence is not observed in NIPA hydrogels; it arises only in NIPA‐shelled capsules, and that too only when the shell is thin. Above its LCST, the NIPA shell shrinks and gets stretched, and nanoscale NIPA‐rich domains arise within this shell, which induce the pearlescence. This study sheds fresh insight into the nature of pearlescence, on how it can be tuned, and on how this property can be introduced into various soft materials.

MROX 2.0: a software tool to explore quantum heterostructures by combining X-ray reflectivity and diffraction

New software for the fitting of XRR measurements is developed. It is exemplified on 1 to 10 Si/Ge periods grown on Si template samples and on (001) substrates and one complex ZnO/Zn1−xMgxO superlattice grown on a m-ZnO substrate.

Model to subtract contributions of scattered radiation from measured direct transmittance and specular reflectance by light diffusing materials.

The formalism of the Lorenz-Mie (LM) theory is used to estimate the contributions of radiation being scattered into the detector acceptance angle of spectrophotometers (with or without an integrating sphere) to the apparent direct transmittance and specular reflectance measurements. These contributions are subtracted from the measurements before doing the inversion to obtain the spectral variation of the scattering and absorption (S&A) coefficients of optically particulate liquid samples, decoupling the contribution to extinction from these two mechanisms. The method is applied to transmittance spectra of Pickering emulsions of dodecane oil (DO) and DO micelles with saturated dissolved Nile red dye, stabilized with anatase (T i O 2) in water. What we believe to be novel expressions are given to estimate the forward and backward average path-length parameters of propagating diffuse radiation from its equivalent quantities defined within the formalism of the LM theory and to relate these single particle average path-length parameters with the forward scattering ratio, asymmetry parameter, and higher-order coefficients in the expansion of the LM phase function. This novel approach makes it possible to avoid the underestimation of the S&A coefficients, which arises when the correction is not made by subtracting that contribution due to scattered radiation that is reaching the detectors from the directional reflectance and transmittance measurements.

The effect of transverse wavefront width on specular neutron reflection.

In the analysis of neutron scattering measurements of condensed matter structure, it normally suffices to treat the incident and scattered neutron beams as if composed of incoherent distributions of plane waves with wavevectors of different magnitudes and directions that are taken to define an instrumental resolution. However, despite the wide-ranging applicability of this conventional treatment, there are cases, such as specular neutron reflectometry, in which the structural length scales of the scattering object require that the wavefunction of an individual neutron in the beam be described by a spatially localized packet - in particular with respect to the transverse extent of its wavefronts (i.e. normal to the packet's mean direction of propagation). It is shown in the present work that neutron diffraction patterns observed for periodic transmission phase gratings, as well as specular reflection measurements from patterned thin films with repeat units of the order of micrometres, can be accurately described by associating an individual neutron with a wave packet and treating a beam as a collection of independent packets. In these cases, accurate analysis requires that the transverse spatial extent of a neutron packet wavefront be accounted for in addition to the angular divergence of the beam that is characterized by a distribution of packet mean wavevector directions. It is shown how a measure of the effective transverse spatial extent of the neutron packet - over which its wavefronts are of sufficient uniformity to produce coherent scattering - can be determined by employing reference diffraction gratings and patterned thin films of known structure and composition.

Temperature-controlled spectrophotometry: a simultaneous analysis of phase transition, thermal degradation and optical properties of semi-transparent composites from 20 °C to 450 °C.

So far, optical and effective radiative properties of polymer matrix based composites were investigated at temperatures well below their degradation temperature. At the same time, polymers exhibit temperature dependent physical properties and may undergo structural changes as their temperature raises. In this work, we employ the "Temperature-Controlled Spectrophotometry", a new method enabling to identify simultaneously phase transitions, thermal degradation and radiative properties of semi-transparent composites over a large temperature range. The method consists of measuring simultaneously the normal-normal and the normal-hemispherical transmittances and reflectances of the sample subjected to a laser irradiation with tuneable wavelength while the temperature is rised from room temperature up to 450 °C by means of a CO2 laser. Physical changes of the sample are identified from the temperature variation of normal-normal transmittance and specular reflectance measurements. Most of the results here are presented at a specific wavelength of 1070 nm but the proposed method is suitable over the semi-transparency spectral domain of the material by changing the wavelength of the probe laser. An inverse method for parameter identification based on normal-hemispherical measurements is employed to determine the transport effective radiative properties of the sample, namely the transport extinction coefficient and the transport scattering albedo from room temperature to 325 °C.

Depth-resolved magnetization profile of MgO/CoFeB/W perpendicular half magnetic tunnel junctions

In this work, we used the soft X-ray resonant magnetic reflectivity to study the depth-resolved out-of-plane (oop) magnetization profile of a CoFeB/MgO sample with W/Ta cap layer after annealing at 400°C. It is a powerful technique to probe buried magnetic interfaces of ultra-thin films by combining the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism. It allowed us to resolve the oop magnetization within a 1.36 nm thick CoFeB layer by the measurement of angle-dependent specular reflectivity at large scattering angles (up to 80°). We determined a graded magnetic distribution for both Fe and Co with a 20% increase at the interface with MgO, decreasing slightly over a thickness of 0.7 nm from MgO before it rapidly decreases to 50% at the interface with W. After applying a non-saturating magnetic field in the plane of the sample, we also quantified a similar magnetization profile with an inclined moment configuration. This indicates that the magnetization gradient is a robust property of the CoFeB layer in the studied sample.

Effect of gas composition during Pt sputtering on structural and magnetic properties of CoFeB thin films.

Ultrathin Ta/CoFeB/Pt trilayer structures are relevant to a wide range of spintronic applications, from magnetic tunnel junctions to skyrmionics devices. Controlling the perpendicular magnetic anisotropy, Gilbert damping and Dzyaloshinskii-Moriya interaction in the CoFeB layer is key for these applications. We examine the role of sputter gas composition during the Pt overlayer deposition of a Ta/CoFeB/Pt trilayer in Ar, Kr and Xe working gas environments during direct current magnetron sputtering. Decreasing density of the Pt layer (from 21 g/cm3 to 15 g/cm3) was apparent in specular x-ray reflectivity measurements of the trilayer films when increasing the molecular weight of the sputtering gas from Ar to Kr to Xe. Significant effects on the Gilbert damping and the interfacial Dzyaloshinskii-Moriya interaction (DMI) energy were observed, with increases in the damping from 0.037(1) to 0.042(1) to 0.048(1), and reductions in the interfacial DMI from 0.47(4) mJ/m2 to 0.45(5) mJ/m2 to 0.39(4) mJ/m2. The ability to control the perpendicular magnetization and DMI strength of these materials through judicious interfacial control is a means toward magnetic devices with better stability at smaller lateral dimension, key to device scaling for spintronic device arrays.

Calculation of Spectral Optical Constants Using Combined Ellipsometric and Reflectance Methods for Smooth and Rough Bulk Samples.

Spectra of the optical constants n and k of a substance are often deduced from spectroscopic measurements, performed on a thick and homogeneous sample, and from a model used to simulate these measurements. Spectra obtained for n and k using the ellipsometric method generally produce polarized reflectance simulations in strong agreement with the experimental measurements, but they sometimes introduce significant discrepancies over limited spectral ranges, whereas spectra of n and k obtained with the single-angle reflectance method require a perfectly smooth sample surface to be viable. This paper presents an alternative method to calculate n and k. The method exploits both ellipsometric measurements and s-polarized specular reflectance measurements, and compensates for potential surface scattering effects with the introduction of a specularity factor. It is applicable to bulk samples having either a smooth or a rough surface. It provides spectral optical constants that are consistent with s-polarized reflectance measurements. Demonstrations are performed in the infrared region using a glass slide (smooth surface) and a pellet of compressed ammonium sulfate powder (rough surface).

Cost-effective millimeter-wave measurement setup for narrowband path loss and angle-of-arrival measurements: uncertainty analysis and specular building reflection measurements

AbstractA cost-effective millimeter-wave measurement setup for narrowband path loss and angle-of-arrival measurements is presented in this paper. The setup consists of ubiquitous radio-frequency lab equipment and additional low-cost components. An algorithm is developed, which improves the measurement accuracy and reduces the required measurement time. An uncertainty analysis is performed, including a noise analysis, amplifier linearity, antenna misalignment and general system impairments. A theoretical model of the received signal plus noise is developed, which is used in Monte Carlo simulations to show the impact of snapshot averaging on the uncertainty. The estimated combined uncertainty with a 95.45% confidence level is 1.1 dB at the maximum measurable path loss and 0.3 dB in the case of low path loss, where the uncertainty due to receiver noise is negligible. The measurement setup is used in outdoor specular building reflection measurements at 24.00–24.25 GHz. The measured single-building reflections show a 1–9 dB excess loss compared to the free-space path loss. The measured excess loss is 9–20 dB for double-building reflections. These results indicate that buildings could potentially be used as effective millimeter-wave specular reflectors to extend millimeter-wave coverage.

Bidirectional reflectance capabilities of the NIST Robotic Optical Scattering Instrument.

The National Institute of Standards and Technology (NIST) Robotic Optical Scattering Instrument (ROSI) serves as the national reference instrument for specular and diffuse bidirectional reflectance measurements in the ultraviolet to short-wave infrared wavelength regions. This paper gives a comprehensive overview of the design, operation, and capabilities of ROSI. We describe measurement methods for diffuse and specular reflectance, identify and quantify the elements of the uncertainty budget, and validate the reflectance scale through comparison with NIST's previous reference instrument, the Spectral Tri-function Automated Reference Reflectometer. Examples of the range of ROSI's capabilities, including the limits for low-reflectance measurements and a research application using out-of-plane measurements of bidirectional reflectance for remote sensing reference reflectors, are also covered.

In Situ Photoacoustic Study of Optical Properties of P-Type (111) Porous Silicon Thin Films.

Porous silicon (PSi) on p-type (111) silicon substrate has been fabricated by electronically etching method in hydrofluoric acid (HF) media from 5 to 110 mA/cm of anodizing current density. The problem of determining the optical properties of (111) PSi is board through implementing a photoacoustic (PA) technique coupled to an electrochemical cell for real-time monitoring of the formation of porous silicon thin films. PA amplitude allows the calculation of the real part of the films refractive index and porosity using the reflectance self-modulation due to the interference effect between the PSi film and the substrate that produces a periodic PA amplitude. The optical properties are studied from specular reflectance measurements fitted through genetic algorithms, transfer matrix method (TMM), and the effective medium theory, where the Maxwell Garnett (MG), Bruggeman (BR), and Looyenga (LLL) models were tested to determine the most suitable for pore geometry and compared with the in situ PA method. It was found that (111) PSi exhibit a branched pore geometry producing optical anisotropy and high scattering films.

Performance and photoresponse characterizations of pyrimidine quinolone carboxylate derivatives films-based heterojunction devices

In this investigation, novel pyrimido quinolone derivative carboxylate was produced. For the first time, we attempted to obtain, a high-quality thin film of HBPQC using the thermal evaporation method and checked the structural features of the obtained films. The spectral dependence of the optical parameters was investigated depending on the measurements of both transmittance and specular reflectance. The absorption analysis was utilized to take out the energy gaps of 1.03 and 2.12 eV. The other dispersion parameters were obtained from the analysis of the spectral dependence of the refractive index based on the single oscillator model in the non-absorbing region and found to be 8.74 for the energy of oscillation, Eo and 3.17, for dispersion energy, Ed. They examined current density- voltage measurements of HBPQC – based devices confirmed that the main conduction mechanism is the space charge limited at the interface, and the improvement of the extracted parameters was obtained for the device under 100 mW/cm2. The photosensitivity of the device was prominently improved at a wavelength of 500 nm as compared with other organic-based devices. Finally, the results supported the obtainability of the HBPQC – based devices for the photodetector application.