Temperature-dependent Optical Constants Research Articles

Damage mechanism and protective performance of vanadium dioxide under continuous-Wave laser irradiation

Optical limiters reduce transmittance under increasing incident optical power and protect sensitive components against high-intensity illumination. Vanadium dioxide (VO2), which is known for its thermochromic properties, is a suitable material for fabricating optical limiters. However, research on its phase transition and damage mechanisms under continuous laser irradiation remains limited. In this study, the temperature-dependent optical constants that determine the performance of VO2 were obtained using the proposed dispersion calculation model. The phase transition behavior and damage mechanism of a single-layer VO2 optical limiter exposed to 1064-nm continuous laser irradiation were investigated. The results revealed that a higher laser power on the optical limiter triggered the partial oxidation of VO2 into V2O5 within the damaged region, thus causing the transmittance before and after the phase transition in the damaged area to be higher than that in the undamaged area. Utilizing the calculation model results, a performance-enhancing VO2-based Fabry–Pérot optical limiter was developed. It exhibited a higher transmittance and a lower reflectance than single-layer VO2 in the open state and a lower transmittance in the limiting state. These findings serve as valuable references for the further optimization of VO2-based optical limiters.

Temperature-Dependent Optical Modeling of Perovskite Solar Cells

Comprehensive temperature-dependent optical modeling of perovskite solar cells (PSCs) and modules is essential to accurately predict their energy yield and quantify their energy losses under real-world operating conditions, where devices are subject to different irradiance spectra and intensities as well as operating temperatures. These models require the accurate determination of the temperature-dependent optical constants of perovskites. Here, we report on these data, empirically determined via spectroscopic ellipsometry, for triple-cation perovskites with band gaps ranging between 1.58 and 1.77 eV at temperatures between 25 and 75 °C. Using this data set, we develop a simple empirical model to obtain the temperature-dependent optical constants of perovskites of an arbitrary band gap. We validate our empirical model by comparing the measured temperature-dependent short-circuit current densities and external quantum efficiency data of single-junction PSCs with simulated results using the modeled optical constants.

Temperature-dependent optical constants of water in the thermal infrared derived from data archaeology

This paper describes a data archaeology and rescue of temperature-dependent thermal infrared (IR) optical constants for liquid water from previously published works. The data rescue is based upon digitization of a figure published by Pinkley et al. [J. Opt. Soc. Am. 67, 494 (1977)10.1364/JOSA.67.000494], which showed ratios of laboratory measured spectral reflectances taken at 4 different temperatures (1, 16, 39, and 50 °C) with those measured previously at 27°C. Using our digitization of these data, we perform Kramers-Kronig analyses to derive temperature-dependent optical constants for existing standard datasets taken at room temperatures for the entire thermal IR spectrum. These datasets are of interest for the accurate modeling of thermal IR surface emissivity and reflectance over the range of global surface temperatures.

Which factor determines the optical losses in refractory tungsten thin films at high temperatures?

Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron–phonon interaction, as well as grain-boundary and surface-scattering.Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 °C for W), the electron–phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants.

Doping and temperature-dependent UV-Vis optical constants of cubic SrTiO3: a combined spectroscopic ellipsometry and first-principles study

SrTiO3 is the model perovskite compound and has demonstrated technological relevance from being the foundation of oxide electronics to a promising candidate as a high-temperature hydrogen sensor. Here, we fully investigate the temperature-dependent UV-Vis optical constants of pristine and doped SrTiO3 in the spectral range 0.73-5.90 eV using spectroscopic ellipsometry and first-principles. The oscillator model is applied to analyze and quantify the temperature and doping effect. After heating SrTiO3 from 300 to 873 K, the amplitude of the dominant absorption peak at 4.752 eV is reduced by 11% and the shoulder of optical spectrum at 3.708 eV gradually disappears. By doping SrTiO3 with Nb element from 0.05 to 0.7% wt, extra momentum is provided to enable indirect absorption and an unconventional Urbach tail is observed due to the increased crystalline disorder. Moreover, the first-principles calculations considering the many-body excitonic effect and phonon-assisted indirect absorption are performed, which help to determine the physics origin of absorption peaks and explain the temperature influence. This work sheds light on temperature and doping dependence of UV-Vis dielectric functions of cubic SrTiO3, aiming to elucidate the novel fundamental optical phenomena of SrTiO3-based heterostructures or devices.

Temperature-dependent optical constants of monolayer {text {MoS}}_2, {text {MoSe}}_2, {text {WS}}_2, and {text {WSe}}_2: spectroscopic ellipsometry and first-principles calculations

The temperature-dependent (T = 4.5 , hbox {-} , 500 , hbox {K}) optical constants of monolayer {text {MoS}}_2, {text {MoSe}}_2, {text {WS}}_2, and {text {WSe}}_2 were investigated through spectroscopic ellipsometry over the spectral range of 0.73–6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5–4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from 6.1 times 10^{-5} to 2.6 times 10^{-4} , hbox {K}^{-1} for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron–phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.

Semiconductors for high selectivity thermal emitters

Selective emitters limit thermal radiation to a narrow spectral band enabling applications including energy conversion and sensing. While applications demand high spectral selectivity, the materials used for selective emitters limit the ultimate selectivity. At high temperatures (>600 K), the optical properties of materials degrade, resulting in poor spectral selectivity, and thus leaving an open question—what material platform makes a good selective thermal emitter? Here, we show that doped semiconductors make a good choice by predicting their temperature dependent optical constants in a wide doping range. We build semi-empirical models to predict the temperature dependences of microscopic processes in GaAs and Si, a polar and a non-polar semiconductor respectively, and demonstrate that semiconductors with small and tunable optical losses enable very high spectral selectivity in thermal emitters. Our predictions agree reasonably well with more complex and accurate methods such as variational and second order perturbation theories, and serve as a guide in the discovery and development of materials for high temperature nanophotonics.

Temperature-dependent optical constants of liquid isopropanol, n-butanol, and n-decane.

Liquid isopropanol, n-butanol, and n-decane are combustible organic compounds that are frequently used in theoretical and experimental researches on fuel combustion. In this work, the temperature-dependent optical constants of liquid isopropanol, n-butanol, and n-decane in the region 500-5500 cm-1 at ambient pressure are measured using the combined ellipsometry-transmission method. In the combined method, the optical constants are first measured by a modified ellipsometry method, and then the absorption indices for weak absorption regions are obtained by the transmission method using the refractive indices measured by the modified ellipsometry method. The refractive indices of liquid isopropanol, n-butanol, and n-decane are within the range from 1.3 to 1.45 in the studied wavelength and temperature region. The absorption indices of these liquids range from 10-5 to 10-1. In the temperature range studied, the refractive indices decrease with increasing temperature in an approximately linear manner, but the effects of the temperature on the absorption indices are much smaller. The characteristic wavenumbers of the main absorption peaks are consistent with the vibrational frequencies of major functional groups.

Temperature-dependent optical constants of highly transparent solids determined by the combined double optical pathlength transmission-ellipsometry method.

The optical constants of five highly transparent substrates (polycrystalline BaF2, CaF2, MgF2, ZnSe, and ZnS) were experimentally determined based on a combined technique using both the double optical pathlength transmission method and the ellipsometry method within temperature range 20°C-350°C in the ultraviolet-infrared region (0.2-20μm). The results show that the refractive index spectra of polycrystalline BaF2, CaF2, and MgF2 are similar, but differ from that of polycrystalline ZnSe and ZnS. The thermo-optic coefficient of these highly transparent substrates increases with increasing temperature. The absorption indices show a significant temperature-dependent behavior, which increases with increasing temperature from 20°C to 350°C over the transparent region. For the sake of application, the fitted formulas of the refractive index of the five highly transparent substrates as a function of wavelength and temperature are presented.

Measurement of Wavelength and Temperature-Dependent Optical Properties of Thermochromic Pigments.

Thermochromic material is a substance that is capable of changing reversibly the color as the temperature rises. Therefore, the optical spectrum of thermochromic material is responsive to the environmental temperature. In this study, the temperature-dependent optical constants of thermochromic pigments over the wavelength of 350-1800 nm were investigated. Three kinds of thermochromic pigments featured with black, blue, and red colors at room temperature were suspended in water and the light reflection and transmission of the suspensions at different temperatures were measured by a multifunctional spectrophotometer. It was found that below the transition temperature of thermochromic material, the refractive index was 2.1-2.5, 2.2-2.6, and 2.0-2.4 over the wavelength range of 350-1800 nm for black, blue, and red thermochromic pigment, respectively, while above the transition temperature it reached 2.3-2.7, 2.4-2.9, and 2.4-2.7, respectively. It was also observed that the relationship between refractive index of thermochromic pigment and wavelength follows the cubic polynomial function. Furthermore, the extinction coefficient is in the range of 1 × 10-5-1.2 × 10-4 for all thermochromic pigments and remains approximately stable at different temperatures. The determination of optical constants of thermochromic pigments provides essential parameters in the modeling of light scattering and absorption by pigment particles to further fine-tune the optical properties of thermochromic coating.

Exact interband transition energies of VO2 films

The temperature dependent optical constants of VO2 thin films with different thicknesses on variable substrates were measured by spectroscopic ellipsometry. The exact interband transition energies crossover metal-to-insulator transition of VO2 were obtained using a standard critical point model fitting based on the measured optical constants. The dynamic image of all critical points was obtained during the heating and cooling processes within the measured energy region. By introducing the fine orbital structure into the band model, every critical point was assigned to the corresponding band transition. The critical point of 4.7eV predicted theoretically, which is response to the transition from O2P to upper a1g, was obtained experimentally for the first time. Our work provides a new view for the complex band scheme of the monoclinic and rutile phase VO2. Moreover, the insulator-to-metal transition temperature is modified by regulating both film thickness and substrate.

Temperature dependent optical constants for SiO2 film on Si substrate by ellipsometry

In situ ellipsometry measurements in the spectra range from 246 to 1000 nm were performed on a SiO2/Si system at temperatures varying from 25 to 600 °C. By using Cauchy dispersion model to describe SiO2 layer and B-spline to parameterize Si substrate, the temperature dependent optical constants of SiO2 film and Si substrate were precisely determined and analyzed which is helpful to understand the temperature effect of substrate for ellipsometry measurement. The temperature coefficient of refractive index of SiO2 at elevated temperature was obtained by ellipsometry. The results indicate that the temperature coefficient increases with increasing wavelength and decreasing temperature.

Application of emitter-sample hybrid terahertz time-domain spectroscopy to investigate temperature-dependent optical constants of doped InAs.

We investigate temperature-dependent carrier dynamics of InAs crystal by using reflection-type terahertz time-domain spectroscopy, particularly with a recently developed emitter-sample hybrid structure. We successfully obtain the optical conductivity in a terahertz frequency of bulk InAs whose dc conductivity is in the range of 100-150 Ω-1 cm-1. We find that both real and imaginary parts of the optical conductivity can be fit well with the simple Drude model, and the free-carrier density and the scattering rate obtained from the fit are in good agreement with corresponding values obtained by using other techniques, such as the Hall measurement and the dc-resistivity measurement. These results clearly demonstrate that the proposed technique of adopting the emitter-sample hybrid structure can be exploited to determine temperature-dependent optical constants in a reflection geometry and hence to investigate electrodynamics of bulk metallic systems.

Temperature dependent optical properties of silver from spectroscopic ellipsometry and density functional theory calculations

A detailed analysis of temperature dependence of optical constants of silver thin film measured by spectroscopic ellipsometry in the energy range 1.4 to 5 eV for temperatures ranging from 300 K to 650 K in steps of 50 K is presented in this paper. A microstructure modeling based on Bruggeman effective medium approximation is carried out on the temperature dependent optical constants. A red shift of ∼300 meV accompanied by a broadening of the interband transition is observed and an additional weak absorption edge emerges from the lower energy side of main interband absorption transition at 550 K. Drude model is applied to perform the analysis of dielectric function in the low energy region. The energy loss function −Im(1/ε) shows large damping with increasing temperature and this is a consequence of increasing magnitude of ε2 resulting from the composite transition in the region where ε1 is very small. These experimental results are corroborated with first principles electronic structure calculations using the full potential linearized augmented plane wave method within the frame-work of density functional theory.

Optical constants of refractory oxides at high temperatures

Many cosmic dust species, among them refractory oxides, form at temperatures higher than 300 K. Nevertheless, most astrophysical studies are based on the room-temperature optical constants of solids, such as corundum and spinel. A more realistic approach is needed for these materials, especially in the context of modeling late-type stars. We aimed at deriving sets of optical constants of selected, astrophysically relevant oxide dust species with high melting points. A high-temperature-high-pressure-cell and a Fourier-transform spectrometer were used to measure reflectance spectra of polished samples. For corundum (alpha-Al$_2$O$_3$), spinel (MgAl$_2$O$_4$), and alpha-quartz (SiO$_2$), temperature-dependent optical constants were measured from 300 K up to more than 900 K. Small particle spectra were also calculated from these data. All three examined oxides show a significant temperature dependence of their mid-IR bands. For the case of corundum, we find that the 13$\mu$m emission feature - seen in the IR spectra of many AGB stars - can very well be assigned to this mineral species. The best fit of the feature is achieved with oblate corundum grains at mean temperatures around 550 K. Spinel remains a viable carrier of the 13$\mu$m feature as well, but only for T < 300 K and nearly spherical grain shapes. Under such circumstances, spinel grains may also account for the 31.8$\mu$m band that is frequently seen in sources of the 13$\mu$m feature and which has not yet been identified with certainty.

Temperature of mesospheric ice particles simultaneously retrieved from 850 cm−1libration and 3200 cm−1vibration band spectra measured by ACE-FTS

[1] We report a new approach to the retrieval of ice particle temperature in the atmosphere from midinfrared remote sensing observations. Two different in physical nature spectral bands of water ice, the libration band near 850 cm−1 and O-H stretch band at 3200 cm−1, are used simultaneously. The use of both bands in the unified least squares fitting procedure enables one to enhance the stability of fitting, as well as to improve the reliability and accuracy of retrieved ice temperature values. A set of reference spectra for the least squares minimization is compiled from experimental temperature-dependent optical constants for water ice that are available in the literature. The method is applied to the observations made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on board the SCISAT-1 satellite. According to our results, a combination of the vibration (3200 cm−1) and rotation (850 cm−1) bands within a unified minimization procedure can significantly, by about 4K, reduce the upper estimate of the total uncertainty in the retrieved temperatures from 12 K down to 8 K. The average temperature of bright mesospheric ice clouds observed by ACE-FTS during July 2005 is found to be about 131K, in contrast to the value of 135K retrieved from the vibration band alone.

Influence of emissivity on behavior of metallic dust particles in plasmas

Influence of thermal radiation emissivity on the lifetime of a dust particle in plasmas is investigated for different fusion relevant metals (Li, Be, Mo, and W). The thermal radiation is one of main cooling mechanisms of the dust in plasmas especially for dust with evaporation temperature higher than 2500K. In this paper, the temperature- and radius-dependent emissivity of dust particles is calculated using Mie theory and temperature-dependent optical constants for the above metallic materials. The lifetime of a dust particle in uniform plasmas is estimated with the calculated emissivity using the dust transport code DUSTT [A. Pigarov et al., Physics of Plasmas 12, 122508 (2005)], considering other dust cooling and destruction processes such as physical and chemical sputtering, melting and evaporation, electron emission etc. The use of temperature-dependent emissivity calculated with Mie theory provides a longer lifetime of the refractory metal dust particle compared with that obtained using conventional emissivity constants in the literature. The dynamics of heavy metal dust particles are also presented using the calculated emissivity in a tokamak plasma.

Temperature-dependent optical constants and birefringence of nematic liquid crystal 5CB in the terahertz frequency range

We have measured the frequency dependence and temperature dependence of the optical constants of a liquid crystal 4′-n-pentyl-4-cyanobiphenyl (5CB) in both nematic and isotropic phases by using terahertz time-domain spectroscopy. The extinction coefficient of 5CB is less than 0.02 and without sharp absorption features in the frequency range of 0.2–1.0 THz. The extraordinary and ordinary indices of refraction at 25 °C are around 1.77 and 1.58, respectively, giving rise to a birefringence of 0.20±0.02 in this frequency range. The temperature-dependent order parameter extracted from birefringence agrees with the results from the visible region quite well.

Effects of temperature on the surface plasmon resonance at a metal–semiconductor interface

The effects due to elevated temperatures on the surface plasmon (SP) at a metal–semiconductor interface are studied both experimentally and theoretically. In particular, a junction made of silver and amorphous silicon is fabricated and the interfacial plasmon is excited optically via the Kretschmann geometry. Both the reflectance and phase monitoring of the response of the junction have been studied as a function of temperature from 300 K to 380 K. Theoretical simulations have been carried out to understand the observed data, using a previously established model for the temperature-dependent optical constants of the metal, together with empirically fitted data for those of the semiconductor. Reasonable qualitative comparison between experimental data and simulation is obtained. It is found that the strength of the SP at the junction will decrease as temperature increases, and the methodology of the present experiment may provide a way to quantify such a decrease in the operation efficiency of the junction. In addition, it is shown that, by monitoring the resonance angle, such a junction may act as a temperature sensor with sensitivity possibly higher than the previous ones which employed a bare metal film in the Kretschmann geometry.

Measurement of the temperature-dependent optical constants of water ice in the 15–200 µm range

The real and imaginary refractive indices of water ice in the far infrared (IR) are used in the satellite interpretation of cloud properties as well as to obtain information on ice throughout the solar system. However, few measurements of these values exist. We have measured the real and imaginary refractive indices of water ice in the far IR every 10 deg over the temperature range of 106-176 K. Ice films ranging from 0 to 140 microm thick were grown by the condensation of water vapor onto a cold silicon substrate, and the film transmission was measured from 650 to 50 cm(-1). The thickness of the ice films was determined using optical interference from a reflected He-Ne laser (lambda = 623.8 nm). The optical constants were then determined by simultaneously fitting the calculated spectra of films of varying thickness to their respective measured transmission spectra with an iterative Kramers-Kronig technique. The results are compared with previously measured data and show large discrepancies at some wavelengths while good agreement exists at others. Possible reasons for the differences are discussed. Our data clearly distinguish crystalline and amorphous ice. In addition, we note a slight shoulder in our spectra, which can be used to distinguish between cubic and hexagonal ice, although this distinction is difficult.