Accurate Optical Constants Research Articles

Infrared Spectroscopic and Physical Properties of Methanol Ices—Reconciling the Conflicting Published Band Strengths of an Important Interstellar Solid

Infrared spectroscopic observations have established the presence of solid methanol (CH3OH) in the interstellar medium and in solar system ices, but the abundance of frozen CH3OH cannot be deduced without accurate band strengths, optical constants, and reference spectra. In this paper we identify disagreements, omissions, and gaps in the literature on infrared (IR) intensities of methanol ices, including unaddressed concerns that reach back several decades. New spectra are presented with intensity measurements aided by new data on the index of refraction and density of solid CH3OH. The result is that the large discordant results from different laboratory groups can now be reconciled. Multiple ices have been used to determine, apparently for the first time, IR intensities of H2O + CH3OH mixtures of accurately known composition for use with observations of interstellar ices. Also for the first time, measurements on CH3OH ices with different thicknesses have allowed us to report both near-IR band strengths and optical constants for two near-IR features used by planetary scientists. We have used our new IR results to determine vapor pressures of solid CH3OH and have compared them to measurements made with a quartz-crystal microbalance. Thermal annealings of methanol ices have been carried out and phase changes in the solid state examined. Comparisons of our results to earlier work are presented where possible, and electronic versions of our new results are made available.

Optimizing the Design of the Vapor‐Deposited CsPbCl3‐Based Optoelectronic Devices via Simulations and Experiments

AbstractWide‐bandgap CsPbCl3 perovskite has fueled studies on applications in a variety of newly developing optoelectronic devices, including solar cells, lasers, light‐emitting diodes, and photodetectors. Most of them are now focused on film fabrication and device performance optimization, technology transfer of these proof‐of‐concept devices to the market will necessitate optimization of the device structure to minimize the optical loss and fabrication cost. While device model‐based electro‐optics simulations are considered a powerful method to attain the purpose, during which accurate optical constants are preconditions. Herein, for the first time, the optical constants of CsPbCl3 film through Mueller matrix spectroscopic ellipsometry (SE) are reported. Taking optoelectronic devices in vertical structure as a demonstration, simulations on the distributions of 2D electromagnetic field and photo‐generated electron field are arranged, successfully demonstrating the profile of the photons and electrons inside the devices. The evolution trend of device performance in the experiments matched with the simulations, for device performance is closely related to the photons and photo‐generated electrons. Benefitting from the simulations and experiments, general design criteria of the CsPbCl3‐based optoelectronic and photonic devices are posed, inevitably accelerating their development and commercialization.

Effect of rate of deposition on opto-electrical characteristics of vapor deposited MoO3/Ag/MoO3 transparent electrode for photovoltaic applications

Following an approach combining optical modeling and experimental study in this proposed work, we report designing and fabrication of MoO3/Ag/MoO3 transparent electrode (MAM-TE) for perovskite solar cell (PvSC) applications. We started with the optical design of the TE to determine the optimized thickness of the alternating layers. Variable angle spectroscopic ellipsometry was employed to find the accurate optical constants of each layer which works as the input parameter for the Transfer Matrix Method (TMM) based Matlab code. With the help of optical modeling, the optimum thickness of the undercoat and overcoat oxide layer is calculated. The thickness of the metal layer (Ag) is kept fixed at 9 nm as it is found to be thick enough to provide good electrical conductivity. In this work, we investigated the effect of the rate of deposition (RoD) of sandwiched Ag layer on the optical and electrical behavior of MAM-TE. For a better understanding of the effect of RoD on Ag, characteristics of Ag films are performed via scanning electron microscopy (SEM) and optical absorption spectroscopy (UV–Visible). Transmittance spectra revealed that as the RoD of the Ag layer is increased from 0.5 Å/s to 5 Å/s, the films become more transparent, which is in good agreement with SEM images of the deposited Ag films. In this way, keeping the RoD of the Ag layer fixed at 5 Å/s, we successfully fabricate a highly transparent MAM-TE with an average visible transmittance > 77 %, along with a sheet resistance < 5 Ω/□. After the complete optimization of fabricated TE, it is finally incorporated into a perovskite solar cell device architecture consisting of FTO/TiO2/MAPI/Spiro/MAM. The preliminary results of the fabricated PvSC give a power conversion efficiency (PCE) of 4.79 %. Hence, the fabricated TE can integrate routes for its revolutionizing applications in diverse fields, including optoelectronic industries and bifacial photovoltaics applications.

Hexagonal boron nitride nanophotonics: a record-breaking material for the ultraviolet and visible spectral ranges.

A global trend towards miniaturization and multiwavelength performance of nanophotonic devices drives research on novel phenomena, such as bound states in the continuum and Mietronics, as well as surveys for high-refractive index and strongly anisotropic materials and metasurfaces. Hexagonal boron nitride (hBN) is one of the promising materials for future nanophotonics owing to its inherent anisotropy and prospects of high-quality monocrystal growth with an atomically flat surface. Here, we present highly accurate optical constants of hBN in the broad wavelength range of 250-1700 nm combining imaging ellipsometry measurements, scanning near-field optical microscopy and first-principles quantum mechanical computations. hBN's high refractive index, up to 2.75 in the ultraviolet (UV) and visible range, broadband birefringence of ∼0.7, and negligible optical losses make it an outstanding material for UV and visible range photonics. Based on our measurement results, we propose and design novel optical elements: handedness-preserving mirrors and subwavelength waveguides with dimensions of 40 nm operating in the visible and UV ranges, respectively. Remarkably, our results offer a unique opportunity to bridge the size gap between photonics and electronics.

Label-free brain tissue imaging using large-area terahertz metamaterials

Terahertz (THz) imaging technology has shown significant potential for use in biomedical imaging owing to its non-ionizing characteristics by its low photon energy and its ultrabroadband spectral comparability with many molecular vibrational resonances. However, despite the significant advantage of being able to identify bio-materials in label-free configurations, most meaningful signals are buried by huge water absorption, thus it is very difficult to distinguish them using the small differences in optical constants at THz regime, limiting the practical application of this technology. Here, we demonstrate advanced THz imaging with enhanced color contrast by the use of THz field that is localized and enhanced by a nanometer-scale slot array. THz images of a biological specimen, such as mouse brain tissue and fingerprint, on a nano-slot array-based metamaterial sensing chip, which is elaborately fabricated in large-area, show a higher contrast and clearer boundary information in reflectance without any labeling. A reliable numerical solution to find accurate optical constants using THz nano-slot resonance for the quantitative analysis of target bio-specimens is also introduced. Finally, the precise optical properties of real bio-samples and atlas information are provided for specific areas where amyloid beta proteins, known to cause dementia, have accumulated in a mouse brain.

Fast Determination of Optical Constants and Sample Thickness of Thin Liquid Samples in Terahertz Time-Domain Spectroscopy

The accuracy of the extracted optical constants by terahertz time-domain spectroscopy critically depends on the accuracy of the sample thickness. However, due to the large number of optimization steps, it is time consuming to extract the optical constants over a broad frequency range and determine the sample thickness simultaneously using conventional algorithms. With particular emphasis on thin liquid samples, a three-dimensional optimization algorithm was used to obtain the optical constants and sample thickness simultaneously, significantly reducing the calculation time. In the experiments, the THz signals of 200-μm-thick water in a known cuvette were measured, and its accurate thickness and optical constants were determined by the algorithm, validating the effectiveness of the approach.

Optical modeling of wide-bandgap perovskite and perovskite/silicon tandem solar cells using complex refractive indices for arbitrary-bandgap perovskite absorbers

Wide-bandgap perovskites are attractive top-cell materials for tandem photovoltaic applications. Comprehensive optical modeling is essential to minimize the optical losses of state-of-the-art perovskite/perovskite, perovskite/CIGS, and perovskite/silicon tandems. Such models require accurate optical constants of wide-bandgap perovskites. Here, we report optical constants determined with ellipsometry and spectrophotometry for two new wide-bandgap, cesium-formamidinium-based perovskites. We validate the optical constants by comparing simulated quantum efficiency and reflectance spectra with measured cell results for semi-transparent single-junction perovskite cells and find less than 0.3 mA/cm2 error in the short-circuit current densities. Such simulations further reveal that reflection and parasitic absorption in the front ITO layer and electron contact are responsible for the biggest optical losses. We also show that the complex refractive index of methylammonium lead triiodide, the most common perovskite absorber for solar cells, can be used to generate approximate optical constants for an arbitrary wide-bandgap perovskite by translating the data along the wavelength axis. Finally, these optical constants are used to map the short-circuit current density of a textured two-terminal perovskite/silicon tandem solar cell as a function of the perovskite thickness and bandgap, providing a guide to nearly 20 mA/cm2 matched current density with any perovskite bandgap between 1.56 and 1.68 eV.

Grain boundary effects on the optical constants and Drude relaxation times of silver films

Silver demonstrates the unique optical properties that make it suitable for various plasmonic applications. Determining the accurate optical constants for silver and investigating its dependence on grain structures are necessary for reliable theoretical predictions. This work investigates the effects of different deposition conditions and the resultant variable grain sizes, upon the optical properties of the silver films in conjunction with ellipsometry and optical characterization. Annealing conditions of substrate SiNx films were also studied to ensure good quality of deposited polycrystalline silver films. The effects of grain sizes of silver on its electronic relaxation times were studied and shown to be consistent with the theoretical electrical resistivity model. This study shows that voids and surface layers can affect the optical constant values and that reproducible results can be obtained by avoiding such artefacts. The results will have implications on the analysis of different silver based nanostructures like nanowires and nanorods where grain boundary scattering can affect the optical and electrical properties like reflection and resistivity.

Model-blind characterization of thin-film optical constants with momentum-resolved reflectometry.

Determining optical constants of thin material films is important for characterizing their electronic excitations and for the design of optoelectronic devices. Spectroscopic ellipsometry techniques have emerged as the predominant approach for measuring thin-film optical constants. However, ellipsometry methods suffer from complications associated with highly model-dependent, multi-parameter spectral fitting procedures. Here, we present a model-blind, momentum-resolved reflectometry technique that yields accurate and precise optical constants, with quantifiable error estimates, even for film thicknesses less than 50 nm. These capabilities are demonstrated by interrogating an optical absorption resonance in films of the polymer P(NDI2OD-T2). We show that this approach produces exceptional agreement with UV-Vis-NIR absorption measurements, while simultaneously avoiding the need to construct complicated multi-oscillator spectral models. Finally, we use this procedure to resolve subtle differences in the out-of-plane optical properties of different film morphologies that were previously obscured in ellipsometry measurements.

Infrared spectra and optical constants of astronomical ices: II. Ethane and ethylene

Infrared spectroscopic observations have established the presence of hydrocarbon ices on Pluto and other TNOs, but the abundances of such molecules cannot be deduced without accurate optical constants (n, k) and reference spectra. In this paper we present our recent measurements of near- and mid-infrared optical constants for ethane (C2H6) and ethylene (C2H4) in multiple ice phases and at multiple temperatures. As in our recent work on acetylene (C2H2), we also report new measurements of the index of refraction of each ice at 670nm. Comparisons are made to earlier work where possible, and electronic versions of our new results are made available.

Accurate and Facile Determination of the Index of Refraction of Organic Thin Films Near the Carbon1sAbsorption Edge

A practical and accurate method to obtain the index of refraction, especially the decrement δ, across the carbon 1s absorption edge is demonstrated. The combination of absorption spectra scaled to the Henke atomic scattering factor database, the use of the doubly subtractive Kramers-Kronig relations, and high precision specular reflectivity measurements from thin films allow the notoriously difficult-to-measure δ to be determined with high accuracy. No independent knowledge of the film thickness or density is required. High confidence interpolation between relatively sparse measurements of δ across an absorption edge is achieved. Accurate optical constants determined by this method are expected to greatly improve the simulation and interpretation of resonant soft x-ray scattering and reflectivity data. The method is demonstrated using poly(methyl methacrylate) and should be extendable to all organic materials.

From Plasmon Spectra of Metallic to Vibron Spectra of Dielectric Nanoparticles

Light interacts surprisingly differently with small particles than with bulk or gas phase materials. This can cause rare phenomena such as the occurence of a "blue moon". Spectroscopic particle phenomena of similar physical origin have also spawned countless applications ranging from remote sensing to medicine. Despite the broad interest in particle spectra, their interpretation still poses many challenges. In this Account, we discuss the challenges associated with the analysis of infrared, or vibron, extinction spectra of small dielectric particles. The comparison with the more widely studied plasmon spectra of metallic nano-particles reveals many common features. The shape, size, and architecture of particles influence the band profiles in vibron and plasmon spectra in similar ways. However, the molecular structure of dielectric particles produces infrared spectral features that are more diverse and detailed or even unique to vibron spectra. More complexity means higher information content, but that also makes the spectra more difficult to interpret. Conventional models such as classical electromagnetic theory with a continuum description of the wavelength-dependent optical constants are often no longer applicable to these spectra. In cases where accurate optical constants are not available and for ultrafine particles, where the molecular structure and quantum effects become essential, researchers must resort to molecular models for light-particle interaction that do not require the prior knowledge of optical constants. In this Account, we illustrate how vibrational exciton approaches combined with molecular dynamics simulations and solid-state density functional calculations provide a viable solution to these challenges. Molecular models reveal two important characteristics of vibron spectra of small molecularly structured particles. The band profiles in vibron spectra are largely determined by transition dipole coupling between the molecules in a particle. Below a specific particle size limit, conventional models fail. Molecular models explain many other phenomena in particle spectra, such as size, shape, and mixing effects, providing the foundation for a better understanding of the interaction of solar radiation with aerosols and clouds and for the design of dielectric nanomaterials.

Accurate determination of optical coefficients from chemical samples using terahertz time-domain spectroscopy and effective medium theory

A technique for calculating the optical constants of powdered materials that have been pressed into pellets along with a diluent such as polyethylene, as is commonly used in the spectroscopy community, is introduced. The simple Beer-Lambert law typically used to calculate the optical constants has the inherent weakness that it treats the absorbing medium as a single nonporous solid, as opposed to a dielectric material embedded within a medium. This leads to a systematic underestimation of both the absorption coefficient and the refractive index, especially at low filling factors. Effective medium theory provides a way to calculate more accurate optical constants and produces similar optical constants across a wide range of filling factors, including crucially at the low filling factors commonly used experimentally. The technique can also be extended to estimate the porosity of the samples and provide a true, preparation-independent value for the optical properties of the sample material.

Terahertz time-domain attenuated total reflection spectroscopy in water and biological solution

We demonstrate time-domain attenuated total reflection spectroscopy in terahertz frequency region. Geometry of the reflection measurement is well optimized to obtain accurate optical constants of water or aqueous biomolecular system. We determine the dielectric constants of distilled water and sucrose solutions with this technique. This technique will open new aspects in the research field of biological systems in water.

Accurate determination of optical constants of textured SnO2 using low incidence angle spectroscopic ellipsometry

Accurate optical constants (n and k) of textured SnO2 films were obtained in the range of 0.85–4.6eV using spectroscopic ellipsometry. The angular dependence of the spectroscopic ellipsometer measurement was determined at incident angles 30°, 50°, and 70° from normal to the sample surface. Depolarization components due to texturing and substrate backside reflection were separately analyzed using the measurement on polished and unpolished samples. Depolarization due to substrate backside reflection was modeled accurately. By performing the measurements at a low angle of incidence (30°), where the depolarization due to texturing is at a minimum and therefore can be neglected in the optical model, one can obtain the accurate values of n and k.

Characterizing Optical Constants of Thin Films for Vacuum Ultraviolet Lithography Applications

The determination of optical constants of thin films is an important requirement for the study of masks, antireflective coatings, and resist materials used in the vacuum ultraviolet (VUV) lithography process. We demonstrate a simple method for determining optical constants by measuring the reflectance of thin films with different incidence angles. In order to obtain accurate optical constants, the measurement of reflectance, incidence angles and thickness should be confirmed carefully. In addition, thickness measurements are not important for thick high-absorption films. The measured reflectance should be modified by the surface roughness of thin films particularly in vacuum ultraviolet regimes. The optical constants of various materials were obtained experimentally by this method without using complicated mathematics models.

Variation of the infrared spectra of nitric acid hydrates with formation conditions: Impact on PSC identification

Despite years of effort, the composition of polar stratospheric clouds (PSCs) is still uncertain. Future studies may be able to use FTIR extinction measurements to determine PSC composition. These studies require accurate optical constants for PSC materials. It has been suggested that accurate infrared (IR) optical constants for aerosols can only be derived from aerosol spectra. We have examined the IR spectra of nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) films and aerosols formed using a variety of experimental conditions. We conclude that several spectroscopically different types of NAD and NAT may exist. Alternatively, NAT could be birefringent and its spectra depend strongly on crystallographic orientation. The important causes for the different types or orientations seem to be the formation conditions and temperature, rather than whether the material is a film or aerosol.

Optical properties of cobalt oxide films deposited by spray pyrolysis

The optical constants of Co3O4 films were determined by analyzing variable angle of incidence spectroscopic ellipsometry data and normal incidence transmittance data, between 3500 and 17 000 Å. Absorption was taken to be nonzero for the soda–lime–silica float glass substrate. The absorption of the soda–lime–silica float glass was fitted by an ensemble of four Gaussian oscillators over the entire measured spectral range. Surface roughness of the film was measured with atomic force microscopy and included in the optical model. The films were deposited by spray pyrolysis of cobalt acetylacetonate onto heated soda–lime–silica float glass and fused silica substrates. Fused silica was used to observe film absorption without the effect of substrate absorption. The oxide film phase was identified with thin-film x-ray diffraction analysis. Simultaneous fits of ellipsometry and transmittance data from coated samples were computed to obtain the film thickness, optical constants, and the amplitudes of the four Gaussian oscillators representing the glass substrate absorption. This analysis was done by treating the optical constants at each wavelength as simple fitting parameters or as variables with a Kramers–Kronig consistent parametric model. Spectra for the most accurate optical constants of Co3O4 films deposited by spray pyrolysis onto both substrates are presented.

Study of the optical constants determination of thin films: Dependence on theoretical assumptions

We present a critical study of the known reflectance (R) and transmittance (T) method to extract the optical constants of thin film–substrate systems. In order to study the effect of the usual assumptions (homogeneous layers, semi-infinite substrate, normal incidence angle for reflectance measurement, etc.) in the loss of solution during the numerical inversion process, we have developed a theoretical model that overcomes these approximations. Simulated film–substrate systems as well as RT measurements from a ZnSe film onto a CaF2 substrate have been used to show how accurate film thickness and optical constants can be obtained using a more complete optical model.

Infrared Intensities of Liquids XVIII: Accurate Optical Constants and Molar Absorption Coefficients between 6500 and 800 cm−1 of Dichloromethane at 25°C, from Spectra Recorded in Several Laboratories

This is the last of four papers that present the detailed measnrements and results that led to the acceptance by the International Union of Pure and Applied Chemistry of Secondary Infrared Absorption Intensity Standards for liquids. In this paper accurate infrared absorption intensities of liquid dichloromethane at 25°C are presented. The accuracy was estimated from the ±1.5% average agreement of integrated intensities over specific wavenumber ranges between spectra measured by five spectroscopists in four laboratories. The use of data from different instruments in different laboratories has significantly included the effect of systematic instrumental errors. The spectra from the different spectroscopists have been averaged, unweighted, to give intensity spectra of dichloromethane that are presented as the best available. The results obtained agree with the only measurements that have been made against a primary standard, the estimated accuracy of which is about 5.5%. The spectra of the molar absorption coefficient and or the real and imaginary refractive indices are reported as tables and graphs between 6500 and 800 cm−1. Also reported are the peak heights and the areas under band groups in the molar absorption coefficient and imaginary refractive index spectra. The imaginary refractive index, k(ν˜), and molar absorption coefficient, Em(ν˜), values are believed to be accurate to an average ±2.3% over the 36 measured bands. The baseline k(ν˜) values are believed to be accurate to ∼8% below 6000 cm−1, ∼1% below 4500 cm−1, and ∼25% above 6000 cm−1, where the absorption is extremely weak. The areas under band groups in the k(ν˜) and Em(ν˜) spectra are believed to be accurate to 1.5% averaged over the 15 measured band groups and to 1.0% over the 10 band groups below 3600 cm−1. The real refractive index, n(ν˜), values are believed to be accurate to 0.2%.