Measurement Of Optical Constants Research Articles

Electron backscattering coefficients for Cr, Co, and Pd solids: A Monte Carlo simulation study

We have calculated electron backscattering coefficients, η(Ep), at primary electron energies Ep of 0.1–100 keV for three elemental and intermediate atomic number solids, Cr, Co and Pd, with an up-to-date Monte Carlo simulation model. A relativistic dielectric functional approach is adopted for the calculation of the electron inelastic cross section, where several different datasets of optical energy loss function (ELF) are adopted. The calculated backscattering coefficient is found to be substantially affected by the ELF, where the influence can be seen to follow the f- and ps-sum rules and the resultant energy dependence of electron inelastic mean free path. To understand the uncertainties involved in a comparison with experimental data both the theoretical uncertainty due to the elastic cross-section model and the experimental systematic error for the contaminated surfaces are investigated. A total of 192 different scattering potentials are employed for the calculation of Mott's electron elastic cross section and this theoretical uncertainty is confirmed to be small. On the other hand, the simulation of contaminated Co and Pd surfaces with several carbonaceous atomic layers can well explain the experimental data. The present results indicate that accurate backscattering coefficient data should be either measured from fully cleaned surfaces or obtained from modern Monte Carlo theoretical calculations involving reliable optical constants data. With the recent progress in the accurate measurement of optical constants by reflection electron energy loss spectroscopy technique, constructing a reliable theoretical database of electron backscattering coefficients for clean surfaces of elemental solids is highly hopeful.

Generic characterization method for nano-gratings using deep-neural-network-assisted ellipsometry.

As a non-destructive and rapid technique, optical scatterometry has gained widespread use in the measurement of film thickness and optical constants. The recent advances in deep learning have presented new and powerful approaches to the resolution of inverse scattering problems. However, the application of deep-neural-network-assisted optical scatterometry for nanostructures still faces significant challenges, including poor stability, limited functionalities, and high equipment requirements. In this paper, a novel characterization method is proposed, which employs deep-neural-network-assisted ellipsometry to address these challenges. The method processes ellipsometric angles, which are measured by basic ellipsometers, as functional signals. A comprehensive model is developed to profile nano-gratings fabricated by diverse techniques, by incorporating rounded corners, residual layers, and optical constants into an existing model. The stability of the model is enhanced by implementing several measures, including multiple sets of initial values and azimuth-resolved measurements. A simple compensation algorithm is also introduced to improve accuracy without compromising efficiency. Experimental results demonstrate that the proposed method can rapidly and accurately characterize nano-gratings fabricated by various methods, with relative errors of both geometric and optical parameters well controlled under 5 %. Thus, the method holds great promise to serve as an alternative to conventional characterization techniques for in-situ measurement.

Ellipsometric studies of CH3NH3PbI3 perovskite thin film on different underlayers and its effect on photovoltaic device performance

Measurement of optical constants of semiconducting thin films is an ongoing strive for optoelectronics and related applications. The optical constants (refractive index (n) and extinction coefficient (k)) plays a vital role in optimizing the thicknesses of different layers for improved performance in optoelectronic devices such as solar cells, photodetectors, light-emitting diodes, etc. In the present work, we measured the optical constants of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) deposited on five different underlying layers/substrates and estimated the error in simulated current density estimation for a device with the architecture of Glass/ITO/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au. The optical modeling and performance analysis of this device has been done using transfer matrix method while spectroscopic ellipsometry has been used to measure the optical constants of MAPbI3 film on five different types of underlayer stacks. A maximum error of around 10.6% has been observed when the optical constants are taken from MAPbI3 films deposited on the different underlying stacks. Additionally, experimental aspects of these films such as surface morphology, structural, and optical properties have also been studied to better understand and correlate the observed differences.

Reference-free self-calibrating tip-based scattering-type THz near-field microscopy

In this work, we present a quantitative analysis model based on reference-free self-calibration to analyze scattered fields and approach curves on a dielectric substrate for terahertz scattering-type scanning near-field optical microscopy. The results of our model are compared with experimentally measured data and a fully numerical analysis based on a line dipole image method and a quasi-electrostatic approximation. The model is used to extract the effective radius of the tip and the relative permittivity of the silicon substrate to the near-field scattering signal. The measured approach curves on Au and silicon substrates show good agreement with the calculated approach curves, and the refractive index for silicon is precisely determined to be 3.42. For a tip-based THz scattering-type scanning near-field optical microscope, the proposed analysis model allows for the extraction of the effective probe radius and dielectric functions, thereby enabling conclusive measurements of geometric parameters and optical constants.

Infrared spectra and optical constants of astronomical ices: V. cyclopropane and ethylene oxide

Infrared (IR) spectra of solid cyclopropane (c-C3H6) and ethylene oxide (c-C2H4O), two cyclic aliphatic compounds, have been recorded in their amorphous and crystalline states. The spectra have been combined with measurements of density and refractive index (λ = 670 nm) for each form of each solid to compute IR intensities (i.e., absorption coefficients and band strengths) and optical constants for use in laboratory experiments and astronomical observations. These are the first such measurements of absolute IR intensities and optical constants for each solid at temperatures relevant to the outer Solar System and the interstellar medium, with all results coming from a single laboratory. Two literature band strengths for ethylene oxide's strongest IR feature, one value a calculation and the other a gas-phase measurement, need to be increased by ∼25% to match the solid-phase value reported here. Suggestions are made for applications and future work related to the chemistry of icy bodies in the Solar System and beyond, including Pluto's surface, Titan's atmosphere, and the formation of cometary C3.

Optical measurements on a budget: A 3D-printed ellipsometer

Ellipsometry is an optical analysis technique that is useful for characterizing the physical properties of a thin-film system. Light reflected from a sample surface undergoes a change in polarization due to phase delay and anisotropic reflection. This enables one to perform non-destructive measurements of film thickness, surface roughness, refractive index, and other optical constants. Ellipsometric techniques are particularly convenient for characterizing coatings or films in the semiconductor and optics industries. However, these techniques may be inaccessible to undergraduate students and educators due to the prohibitive cost of ellipsometers and similar instrumentation. In response to this roadblock, we describe the construction of a simple, inexpensive, manually operated, rotating analyzer ellipsometer (RAE). Required materials include a laser pointer, polarizing film, photometric detector, and a 3D-printed opto-mechanical framework, which are all readily accessible at most institutions. The instrument's performance was evaluated by comparing thickness measurements of tetraethyl orthosilicate films to those determined by a commercially available reflectometer. An average film thickness difference of 0.77% was measured using the two instruments.

Pseudoheterodyne near-field imaging at kHz repetition rates via quadrature-assisted discrete demodulation

Scattering-type scanning near-field optical microscopy enables the measurement of optical constants of a surface beyond the diffraction limit. Its compatibility with pulsed sources is hampered by the requirement of a high-repetition rate imposed by lock-in detection. We describe a sampling method, called quadrature-assisted discrete (quad) demodulation, which circumvents this constraint. Quad demodulation operates by measuring the optical signal and the modulation phases for each individual light pulse. This method retrieves the near-field signal in the pseudoheterodyne mode, as proven by retraction curves and near-field images. Measurement of the near-field using a pulsed femtosecond amplifier and quad demodulation is in agreement with results obtained using a CW laser and the standard lock-in detection method.

Study of structural, optical and electrical properties of SnO2 doped TiO2 thin films prepared by a facile Sol-Gel route

This manuscript deals with sol-gel derived SnO2-TiO2 thin films deposited on silicon wafer type Si (100) substrate, which are processed by spin coating technique. Heat treatments of the as-deposited films are then performed at four different temperatures (200, 400, 600, and 800 °C). The first characterization via XRD identifies the nano-crystalline phase begins to form at 400 °C from the long range order and becomes well crystallized into larger size with the increase of annealing temperature. Morphological and compositional studies of these samples are carried out, followed, for the first time, by spectrophotometric measurement of optical constants (extinction coefficient (k) and refractive index (n)). Then the high-temperature measurement of dielectric properties for which the data interpretation in the form of log-log plots of real and imaginary components of the complex permittivity are found to exhibit universal many body response over a broad frequency range 1 Hz–1 MHz.Different presentations of the universal dielectric behaviour by means of impedance, modulus, and conductivity analyses are given as well.

Investigation of Optical and Electrical Properties of Different Compositions of As-S-Se Thin Films at Thickness 725 nm With High Precision Using a Wedge-Shaped Optical Model

Different compositions of as-obtained As40S60-xSex thin films (x = 0 at.%, 20 at.%, 40 at.%, and 60 at.%) with fixed thicknesses were deposited by a thermal evaporation technique. Inheterogeneities of thin-film thickness is a problem that includes significant errors of optical calculations unless there is an optical model that prevents these errors, and the consequent gross errors, in the measurement of optical constants. If not taken into account, this may lead to rather large calculated values for the absorption coefficient or the incorrect presence of the absorption-band tail, as well as to significant errors in the calculated values of the refractive index and film thickness. The optical properties of As40S60-xSex thin films have been determined utilizing measurements of the optical transmission spectra. Owing to the shrinking of the transmission spectra in both the medium and strong absorption regions, we have resorted to applying the optical wedge model for the determination of the film thickness with high precision that equals approximately 725 nm. This paper therefore presents formulae for the transmittance spectrum of a thin dielectric film of selected thickness covering a thick, non-absorbing substrate as well as its upper and lower envelopes. The effect of the content variation on the interference fringes of the transmittance spectrum is analyzed in detail. The electrical properties of the As40S60-xSex thin films have been studied in terms of measuring the temperature-dependent AC conductivity. Both the dielectric constants and dielectric modulus were investigated and are discussed for applications in optoelectronic devices. The change in electrical properties of As40S60-xSex thin films has been interpreted in terms of changed morphological and structural properties. The ratios of the elements were analyzed by comparing them with the actual weight ratios of the bulk material using EDX technology, in addition to the assessment of the Amorphic structure and composition characteristics of the films examined by the x-ray and scanning electron microscopy.

Near-infrared optical properties and proposed phase-change usefulness of transition metal disulfides

The development of photonic integrated circuits would benefit from a wider selection of materials that can strongly control near-infrared (NIR) light. Transition metal dichalcogenides (TMDs) have been explored extensively for visible spectrum optoelectronics; the NIR properties of these layered materials have been less-studied. The measurement of optical constants is the foremost step to qualify TMDs for use in NIR photonics. Here, we measure the complex optical constants for select sulfide TMDs (bulk crystals of MoS2, TiS2, and ZrS2) via spectroscopic ellipsometry in the visible-to-NIR range. We find that the presence of native oxide layers (measured by transmission electron microscopy) significantly modifies the observed optical constants and need to be modeled to extract actual optical constants. We support our measurements with density functional theory calculations and further predict large refractive index contrast between different phases. We further propose that TMDs could find use as photonic phase-change materials, by designing alloys that are thermodynamically adjacent to phase boundaries between competing crystal structures, to realize martensitic (i.e., displacive, order–order) switching.

Free-space transfer of comb-rooted optical frequencies over an 18\u2009km open-air link

Phase-coherent transfer of optical frequencies over a long distance is required for diverse photonic applications, including optical clock dissemination and physical constants measurement. Several demonstrations were made successfully over fiber networks, but not much work has been done yet through the open air where atmospheric turbulence prevails. Here, we use an 18 km outdoor link to transmit multiple optical carriers extracted directly from a frequency comb of a 4.2 THz spectral width. In stabilization to a high-finesse cavity with a 1.5 Hz linewidth, the comb-rooted optical carriers are simultaneously transferred with collective suppression of atmospheric phase noise to −80 dBc Hz−1. Microwaves are also delivered by pairing two separate optical carriers bound with inter-comb-mode coherence, for example a 10 GHz signal with phase noise of −105 dBc Hz−1 at 1 Hz offset. Lastly, an add-on demonstration is given for multi-channel coherent optical communications with the potential of multi-Tbps data transmission in free space.

Optical Constants and Effective-Medium Origins of Large Optical Anisotropies in Layered Hybrid Organic/Inorganic Perovskites.

Hybrid organic/inorganic perovskites (HOIPs) are of great interest for optoelectronic applications due to their quality electronic and optical properties and the exceptional ease of room-temperature synthesis. Layered HOIP structures, e.g., Ruddlesden-Popper phases, offer additional synthetic means to define self-assembling multiple quantum well structures. Measurements of Ruddlesden-Popper HOIP optical constants are currently lacking, but are critical for both a fundamental understanding as well as optoelectronic device design. Here, we use momentum-resolved optical techniques to measure error-constrained complex uniaxial optical constants of layered lead-iodide perovskites incorporating a variety of organic spacer molecules. We demonstrate how large optical anisotropies measured in these materials arise primarily from classical dielectric inhomogeneities rather than the two-dimensional nature of the electronic states. We subsequently show how variations among these materials can be understood within a classical effective-medium model that accounts for dielectric inhomogeneity. We find agreement between experimentally inferred dielectric properties and quantum-mechanical calculations only after accounting for these purely classical effects. This work provides a library of optical constants for this class of materials and clarifies the origins of large absorption and photoluminescence anisotropies witnessed in these and other layered nanomaterials.

A Simplified Approach for Obtaining Optical Properties of VO2 Thin Films, and Demonstration of Infrared Shape‐Shifting Devices

AbstractMicrometer‐sized VO2‐based devices with integrated resistive heaters of different configurations are fabricated. Quality of the VO2 films is confirmed by measuring the characteristic drop in transmittance and negative differential emissivity for these films. A two‐interface model for optical transmittance, reflectance, and absorbance is presented. This method and analytic model presents an advantage over most typically used approaches in that it does not require direct measurements of the material's optical constants to estimate transmittance. By combining the substrate and the VO2 film into one layer with a reduced optical admittance, the two‐interface model is reduced to a single‐layer model. Moreover, the present work demonstrates the implementation of the developed VO2‐based devices in adaptive camouflage and shape‐converting applications. Electrical pulses are used to program different emissivity states to convert geometric shapes inside a fully integrated VO2‐based electro‐optical window. This results in the reconfiguring of thermal images to either create new shapes, or shift from one to another.

Measurement of Optical Constants of TiN and TiN/Ti/TiN Multilayer Films for Microwave Kinetic Inductance Photon-Number-Resolving Detectors

We deposit thin titanium-nitride (TiN) and TiN/Ti/TiN multilayer films on sapphire substrates and measure the reflectance and transmittance in the wavelength range from 400 nm to 2000 nm using a spectrophotometer. The optical constants (complex refractive indices), including the refractive index n and the extinction coefficient k, have been derived. With the extracted refractive indices, we propose an optical stack structure using low-loss amorphous Si (a-Si) anti-reflective coating and a backside aluminum (Al) reflecting mirror, which can in theory achieve 100% photon absorption at 1550 nm. The proposed optical design shows great promise in enhancing the optical efficiency of TiN-based microwave kinetic inductance photon-number-resolving detectors.

An Ellipsometric Technique with an ATR Module and a Monochromatic Source of Radiation for Measurement of Optical Constants of Liquids in the Terahertz Range

Measuring the optical constants of liquids, especially water-containing solutions, is very difficult in the terahertz range. The well-known methods of measurement of the refractive index and absorption coefficient in this range include time-domain spectroscopy and Fourier-transform spectrometry. We have developed a highly sensitive ellipsometry method for measuring the optical constants of liquids using the tunable monochromatic radiation from the Novosibirsk free-electron laser. The ellipsometer is supplemented with an internal reflection module for the measurement of highly absorbing samples. The angle of incidence on the sample in the silicon prism of the module has been optimized for maximum sensitivity to parameters to measure. Measurements of the optical constants of various liquids have been performed, and a sensitivity of 0.01 has been demonstrated.

Optical constants of iron and nickel metal and an assessment of their relative influences on silicate mixture spectra from the FUV to the NIR

We report new measurements of the optical constants of iron and nickel metal in the ultraviolet, visible, and near-infrared portions of the electromagnetic spectrum (∼0.16–3.59 µm), building upon the measurements of Cahill et al. (2012). These values were determined from metal films vapor-deposited onto fused-silica prisms. Our measurement of optical constants employed ellipsometry performed within the prism, sensing the side of the metal film unexposed to the ambient atmosphere. The data we report have important implications for modeling planetary reflectance and emittance spectra, especially in relation to space-weathering effects observed in remotely sensed data for the surfaces of the Moon, Mercury, and asteroids.

Optical Characterization of the SPT-3G Camera

The third-generation South Pole Telescope camera is designed to measure the cosmic microwave background across three frequency bands (95, 150 and 220 GHz) with ~16,000 transition-edge sensor (TES) bolometers. Each multichroic pixel on a detector wafer has a broadband sinuous antenna that couples power to six TESs, one for each of the three observing bands and both polarization directions, via lumped element filters. Ten detector wafers populate the focal plane, which is coupled to the sky via a large-aperture optical system. Here we present the frequency band characterization with Fourier transform spectroscopy, measurements of optical time constants, beam properties, and optical and polarization efficiencies of the focal plane. The detectors have frequency bands consistent with our simulations, and have high average optical efficiency which is 86%, 77% and 66% for the 95, 150 and 220 GHz detectors. The time constants of the detectors are mostly between 0.5 ms and 5 ms. The beam is round with the correct size, and the polarization efficiency is more than 90% for most of the bolometers

Revealing the role of oxygen vacancies on the phase transition of VO2 film from the optical-constant measurements.

Vanadium dioxide (VO2) material shows a distinct metal–insulator transition (MIT) at the critical temperature of ∼340 K. Similar to other correlated oxides, the MIT properties of VO2 is always sensitive to those crystal defects such as oxygen vacancies. In this study, we investigated the oxygen vacancies related phase transition behavior of VO2 crystal film and systematically examined the effect of oxygen vacancies from the optical constant measurements. The results indicated that the oxygen vacancies changed not only the electron occupancy on V 3d–O 2p hybrid-orbitals, but also the electron–electron correlation energy and the related band gap, which modulated the MIT behavior and decreased the critical temperature resultantly. Our work not only provided a facile way to modulate the MIT behavior of VO2 crystal film, but also revealed the effects of the oxygen vacancies on the electronic inter-band transitions as well as the electronic correlations in driving this MIT process.

Focus on materials, semiconductors, vacuum, and cryogenics

Focus on materials, semiconductors, vacuum, and cryogenics

High-temperature solar-thermal conversion by semiconducting β-FeSi2 absorbers with thermally stabilized silver layers

In-situ high-temperature measurements of optical constants (refractive indices n and extinction coefficients k) for each layer of solar selective absorbing multilayers were carried out to simulate solar-thermal conversion efficiency at high temperatures. Conventional Mo layers showed a significant rise in infrared emissivity and a degradation of selective absorbing behavior with the increasing temperature. In contrast, thermally stabilized Ag layers maintained low emissivity even at temperatures below 700°C. In these layers, agglomeration and vaporization were suppressed by SiNx nano-particles dispersed in the Ag matrix and interfacial W adhesive layers, respectively. The interband absorption of β-FeSi2 layers at higher temperatures revealed that larger n and k in the infrared region due to thermally excited electrons induced no rise in emissivity of the multilayers, and the narrower band gap based on Einstein's model shifted the absorptance cut-off wavelength closer to the ideal value and hence resulted in a higher solar absorptance. The solar-thermal conversion efficiencies of the multilayers consisting of low emissivity Ag layers with β-FeSi2 absorbers were found to be greater than 75.9%, as estimated from the temperature dependence of n and k spectra.