Complex Optical Index Research Articles

The complex optical index of PbS nanocrystal thin films and their use for short wave infrared sensor design.

As nanocrystals (NCs) gain maturity, they become central building blocks for optoelectronics in devices such as solar cells and, more recently, infrared focal plane arrays. Now that the proof of concept of these devices has been established, their optimization requires a deeper understanding of their electronic and optical features to engineer their optoelectronic properties accurately. Though PbS NCs have been extensively investigated, the complex optical index of PbS NC thin films remains mostly unknown. Some previous works have unveiled the optical index for this type of material optimized for solar cells (excitonic peak at 940 nm), but longer wavelengths remain scarce and surface chemistry effects, which are known to be of central importance for layer doping, are simply unexplored. Here, we conduct a systematic investigation of the complex optical index of PbS NC thin films using broadband spectrally resolved ellipsometry. The obtained results are then compared with simulations combining tight-binding (TB) modeling at the NC level and the Bruggeman model to expand the results to the film scale. While TB calculation gives the NC optical indices, we extract the key NC film parameters such as the NC volume fraction and ligand indices by fitting the Bruggeman formula to ellipsometry measurements. We also bring evidence that this joint modeling method can be conducted without the need for ellipsometry data while preserving the main feature of the experimental results. Finally, the unveiled optical indices are used to model the absorption of short-wave infrared diode stacks based on PbS NCs and are relevant for state-of-the-art devices. Our electromagnetic modeling shows that the absorption within the contact is now a major limitation of the current device operated at the telecom wavelength.

CORONENE ORGANIC FILMS: OPTICAL AND SPECTRAL CHARACTERISTICS UNDER ANNEALING TEMPERATURE INFLUENCES

In this study, spin coating, which is a chemical film layer thin film deposition method, was used for coronene films that were grown on Si substrates annealed at 325, 350 and 375[Formula: see text]K to examine the impacts on the optical properties of films. This method allows for easy control of the deposition parameters such as concentration, temperature and time as well as enables the film growth at low cost. Optical (UV–Vis) spectral measurements in the wavelength range from 200[Formula: see text]nm to 800[Formula: see text]nm were used to extract the bandgap information and to calculate various optical parameters of the spin-coated coronene films. The electronic transitions on the absorption of photons of suitable energy are of indirect allowed type. The corresponding optical bandgap ([Formula: see text]) was determined. Complex dielectric constants, dissipation factor, optical and electrical conductances and refractive index of coronene films were analyzed as a function of temperature. As the film annealing temperature was increased, the dielectric constants and the refractive index values increased, whereas the optical bandgap and electrical and optical conductivity values decreased.

Complex Optical Index of HgTe Nanocrystal Infrared Thin Films and Its Use for Short Wave Infrared Photodiode Design

AbstractThe limited investigation of the optical properties of HgTe nanocrystal (NC) thin films has become a bottleneck for the electromagnetic design of devices. Using broadband ellipsometry, the refractive index (n) and the extinction coefficient (k) are determined for a series of HgTe NC films relevant to infrared sensing applications. Electromagnetic simulations reveal that the n value of HgTe NC thin films can conveniently be approximated by its mean spectral value n = 2.35 ± 0.15. This complex optical index is then used to design a diode with i) a reduced amount of Hg containing material (thin film < 150 nm) and ii) a thickness of the device better‐matched with the carrier diffusion length. It is demonstrated that introducing an aluminum grating onto the transparent conductive electrode leads to an enhanced absorption while reinforcing the work‐function difference between the two electrodes. Broadband (≈1 µm), non‐polarized, and strong absorption up to 100% is designed. This leads to a responsivity of 0.2 A W−1 and a detectivity of 2 × 1010 Jones for 2 µm cut‐off wavelength at room‐temperature, while the time response is as short as 110 ns.

A basic estimation on the light distribution and thermal behavior of the human skin through transfer matrix method coupled with Pennes’ bio-heat equation

This work is concerned with the investigation of the photo-thermal response of the human skin tissue. A four-layer, one-dimensional air–skin tissue structure has been considered. The considered structure is irradiated by the different light intensities and wavelengths. Light propagation in the skin tissue has been modeled by using a full electromagnetic method. To solve electromagnetic equations numerically, Transfer Matrix Method (TMM) has been used. Inhomogeneity of the multilayer skin tissue structure has been considered by the wavelength dependence of the Complex Optical Refractive Index (CORI). In order to determine the temporal and spatial temperature distribution inside the skin tissue, the TMM has been coupled with Pennes’ bio-heat transfer equation. The equation has been solved semi-analytically with the adiabatic condition by using the Green’s function method. Based on the preliminary findings of this study, the effect of the light intensity and wavelength on the resultant temperature distribution inside the skin tissue structure has been reported. Obtained results could be helpful for designing and improving the practical protocols through the skin photo-thermal therapy process.

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 °C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (≤150 °C) growth of ultrathin films (100-1000 Å) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films' optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 °C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (≤150 °C) growth of ultrathin films (100-1000 Å) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films' optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Spectroscopic ellipsometry study of silver nanospheres and nanocubes in thin film layers

Selective control of the optical properties of thin film layers is necessary in various domains such as photovoltaics and optoelectronics. Through localized surface plasmon resonance, silver nanoparticles exhibit selective optical properties in the visible wavelength range depending on their size and shape. The tremendous progress in nanoparticle synthesis methods, leads to easy fabrication of nanoparticles of various sizes and shapes. By embedding various nanoparticles in a thin film layer, the optical properties are engineered. Silver nanospheres and nanocubes are synthetized by a modified polyol process and randomly deposited in a non-absorbing polymer host matrix. Spectroscopic ellipsometry characterizations determine the complex optical indices of single shape nanoparticles, two shaped nanoparticles and multilayer configuration thin film layers. The spectroscopic ellipsometry data are fitted by a Cauchy law, accounting for the optical properties of the polymer host matrix, and one or several Gauss laws, accounting for the optical properties of the nanoparticles. The extinction coefficient of the blend and multilayer layers show a simple superposition of the extinction coefficients. Therefore, the nanoparticles do not interact in the thin film layers.

Degradation diagnosis of transformer insulating oils with terahertz time-domain spectroscopy

We report the frequency-dependent complex optical constants, refractive index and absorption, and complex dielectric properties over the frequency range from 0.2 to 3.0 THz for aged power transformer mineral insulating oils. These results have been obtained using terahertz time-domain spectroscopy (THz-TDS) and demonstrate the double-Debye relaxation behavior of the mineral insulating oil. The measured complex optical and dielectric characteristics can be important benchmarks for liquid molecular dynamics and theoretical studies of insulating oils. Due to clear differences in THz responses of aged mineral insulating oils, THz-TDS can be used as a novel on-site diagnostic technique to monitor the insulation condition in aged power transformers and may be valuable alternative to characterize other developing eco-friendly insulating oils and industrial liquids.

Optical Imaging and Characterization of Graphene and Other 2D Materials Using Quantitative Phase Microscopy

This article introduces an optical microscopy technique for the characterization of two-dimensional (2D) materials. The technique is based on the use of quadriwave lateral shearing interferometry (QLSI), a quantitative phase imaging technique that allows the imaging of both the intensity and the phase of an incoming light beam. The article shows how QLSI can be used to (i) image 2D materials with high contrast on transparent substrates, (ii) detect the presence of residues coming from the fabrication process, and (iii) map the 2D complex optical conductivity and complex refractive index by processing the intensity and phase images of a light beam crossing the 2D material of interest. To illustrate the versatility of this approach for 2D material imaging and characterization, measurements have been performed on graphene and MoS2.

Free-Space Characterization of Magneto-Optical Hexaferrites in the Submillimeter-Wave Range

We present the quasi-optical free-space characterization of magneto-optical hexagonal ferrites—crystalline BaFe $_{12}$ O $_{19}$ and both ceramic and crystalline SrFe $_{12}$ O $_{19}$ —in the range between 325 and 500 GHz using a vector network analyzer. This method provides us the full complex spectrum of the 2 $\times$ 2 S -matrix element of the sample. Transmission through free air and reflection of a perfect mirror have been used as references. We determined the complex optical indices, permittivities, and permeabilities of all samples by fitting a calculated model to the measured data. The perturbing effects of multiple reflections between setup components were filtered out by inverse fast Fourier transform of the measured signal and subsequent time gating in the obtained time-domain signal. The results presented in this paper show that this processing method reliably deals with all perturbing effects in free-space measurement and therefore can be applied for characterization of various types of magneto-optical samples with different sizes and significant surface roughness as well. Furthermore, it provides convincing indications regarding the usefulness of the different types of hexaferrites for nonreciprocal devices (such as isolators) for submillimeter applications.

Optical response of heterogeneous polymer layers containing silver nanostructures.

This work is focused on the study of the optical properties of silver nanostructures embedded in a polymer host matrix. The introduction of silver nanostructures in polymer thin films is assumed to result in layers having adaptable optical properties. Thin film layers with inclusions of differently shaped nanoparticles, such as nanospheres and nanoprisms, and of different sizes, are optically characterized. The nanoparticles are produced by a simple chemical synthesis at room temperature in water. The plasmonic resonance peaks of the different colloidal solutions range from 390 to 1300 nm. The non-absorbing, transparent polymer matrix poly(vinylpyrrolidone) (PVP) was chosen because of its suitable optical and chemical properties. The optical studies of the layers include spectrophotometry and spectroscopic ellipsometry measurements, which provide information about the reflection, transmission, absorption of the material as well as the complex optical indices, n and k. Finite difference time domain simulations of nanoparticles in thin film layers allow the visualization of the nanoparticle interactions or the electric field enhancement on and around the nanoparticles to complete the optical characterization. A simple analysis method is proposed to obtain the complex refractive index of nanospheres and nanoprisms in a polymer matrix.

Characterizing the tunable refractive index of vanadium dioxide

Vanadium dioxide undergoes a reversible metal-insulator phase change near 68°C. This transition is often recognized for its electronic properties; however, the concomitant change in optical properties is also attractive. One application is to exploit the optical properties within this transition region to control and tune the refractive index between its insulating and metallic states. To achieve this, we used atomic layer deposition to grow high-quality, low-temperature (≤150°C), ultrathin films of vanadium dioxide on sapphire substrates. Measurements of optical transmittance and reflectance as a function of temperature are then used to create wavelength and temperature dependent models of the complex optical refractive index. Our models create a foundation for developing vanadium dioxide as a tunable refractive index material.

New approach for the determination of aerosol refractive indices – Part I: Theoretical bases and numerical methodology

The knowledge of aerosol complex refractive indices on wide spectral range with high spectral resolution is important for many research fields and applications. Various combinations of experimental/theoretical/numerical approaches have been employed to determine the optical indices of aerosol particles. However, each approach has its own advantages and limitations that restrict its generalization. This article is first part of a work aimed at proposing a new technique for determining the optical constants of aerosols. Experimentally, the method is based on recording transmittance spectra of an aerosol flow from thermal infrared to UV–visible combined with the size distribution measurements. Herein, we present the theoretical and numerical bases of the algorithm developed to retrieve the imaginary and real parts of refractive indices. This model associates the Mie theory, the single subtractive Kramers-Kronig relations, and the optimal estimation method with an iterative process. In order to quantify the capabilities of the algorithm to retrieve complex refractive indices, inverse calculations are performed from simulated extinction spectra of Quartz particles whose some of optical properties are available in the literature. We have detailed each step of the procedure and performed some comparisons with the most currently employed methods. The impact of experimental accuracy and numerical simulation are investigated in terms of errors, and uncertainties on the retrieved real and imaginary parts of the complex optical index.

Optical index measurement of InAs/GaSb type-II superlattice for mid-infrared photodetection at cryogenic temperatures

The complex optical index ñ (=n + i κ) of InAs/GaSb type-II superlattice (T2SL) was determined in the 1–6 μm range from reflectivity measurements of resonant structures at 80 K. Several air/superlattice/gold cavities of various thicknesses were fabricated and their reflectivity was measured. As the large variations induced by the cavity resonances in the studied range can be accurately modeled, we were able to determine the complex optical index ñ of InAs/GaSb T2SL with an error of only a few percent. We found that the in-plane refractive index n of the T2SL lies between 3.65 and 3.77 in the mid-infrared wavelength range, and the absorption coefficient κ decreases from 0.23 to 0.001 as the wavelength increases from 1.25 to 5.65 μm. The subtractive Kramers-Kronig relation allowed us to verify our determination of ñ.

Effect of substrate orientation on terahertz optical transmission through VO_2 thin films and application to functional antireflection coatings

We report studies of the terahertz (THz) transmission through vanadium dioxide (VO2) thin films grown on c-, m-, and r-plane sapphire substrates. Our results revealed THz amplitude modulation as large as 84% for VO2 films grown on r-plane sapphire substrates upon crossing the metal–insulator phase transition temperature. Complex optical conductivity and refractive indices were determined for all investigated samples in the metallic state. Results are consistent with electrical resistivity measurements and described based on the Drude model. The real and imaginary parts of the optical conductivity and refractive index are obtained, and associations with variations in the grain morphology and crystal quality are described. We show that VO2 thin films can be used as tunable broadband THz frequency antireflecting coatings.

Complex optical index of single wall carbon nanotube films from the near-infrared to the terahertz spectral range

We retrieve the complex optical index of single-walled carbon nanotube (CNT) films in the 0.6-800 μm spectral range. Results are obtained from a complete set of optical measurements, reflection and transmission, of free-standing CNT films using time domain spectroscopy in the terahertz (THz) and Fourier transform infrared (IR) spectroscopy in the visible-IR. Based on a Drude-Lorentz model, our results reveal a global metallic behavior of the films in the IR, and confirm their high optical index in the THz range.

Picosecond ultrasonics on a single micron carbon fiber

A pump-probe picosecond technique is used to reveal the opto-acoustic response of micrometric carbon fibers. Two carbon fibers having the hexagonal symmetry are experimentally studied. A dedicated experimental setup allows to generate acoustic waves that propagate in a cross section of the fiber. The elastic properties in the transverse direction of a single carbon fiber are thus evaluated. Complex optical refractive index is also measured. The results provide interesting perpectives for the non-destructive evaluation of elastic and optical properties at a micron scale.

Diffraction microtomography with sample rotation: influence of a missing apple core in the recorded frequency space

Abstract Diffraction microtomography in coherent light is foreseen as a promising technique to image transparent living samples in three dimensions without staining. Contrary to conventional microscopy with incoherent light, which gives morphological information only, diffraction microtomography makes it possible to obtain the complex optical refractive index of the observed sample by mapping a three-dimensional support in the spatial frequency domain. The technique can be implemented in two configurations, namely, by varying the sample illumination with a fixed sample or by rotating the sample using a fixed illumination. In the literature, only the former method was described in detail. In this report, we precisely derive the three-dimensional frequency support that can be mapped by the sample rotation configuration. We found that, within the first-order Born approximation, the volume of the frequency domain that can be mapped exhibits a missing part, the shape of which resembles that of an apple core. The projection of the diffracted waves in the frequency space onto the set of sphere caps covered by the sample rotation does not allow for a complete mapping of the frequency along the axis of rotation due to the finite radius of the sphere caps. We present simulations of the effects of this missing information on the reconstruction of ideal objects.

Multi-scale Modeling of Radiation Heat Transfer through Nanoporous Superinsulating Materials

In this contribution, the extraordinarily high level of thermal insulation produced by nanoporous materials, which can achieve thermal conductivities down to a few mW·m−1·K−1 when they are evacuated to a primary vacuum, is highlighted. The objective here is to quantify the level of radiation heat transfer traveling through a nanoporous material in relation with its composition. The model used here is based on the “non-gray anisotropically scattering Rosseland approximation,” which allows the definition of a “radiation thermal conductivity” expressed as a function of the optical properties (complex optical index spectra), mean sizes and volume fractions of the different populations of particles constituting the material. With the help of this simple model, one can draw interesting conclusions concerning the impacts of different parameters related to the microstructure of the nanoporous material on the amplitude of the radiation heat transfer. In the future, this model should help to orient the formulation of new nanoporous materials with optimized radiative properties.

Light scattering by coated spheres: Experimental results and numerical simulations

This work presents and compares experimental and numerical results obtained for the light scattered by micron-sized spherical particles (glass, silica) coated with non-absorbing and absorbing mantles (graphite, organic black paint). The comparison validates the use of light scattering studies for the determination of the physical parameters of dust particles, especially the determination of complex optical indices and thickness of absorbing mantles. This approach is of interest to validate experimental or numerical results, and to interpret remote light scattering observations, particularly those related to compact particles possibly covered with mantles.