Anisotropic Optical Constants Research Articles

Anisotropic Optical Properties of Monolayer Aligned Single‐Walled Carbon Nanotubes

The 2D materials are the fundamental building blocks for modern optoelectronics and photonics. Optically anisotropic monolayers give even more flexibility in device design and performance. However, the random orientation of optical axes in the large‐scale samples prevents anisotropic monolayers from widespread use. The alternative structure is a monolayer of aligned single‐walled carbon nanotubes (SWCNTs) with an anisotropic dielectric tensor. Herein, aligned SWCNTs monolayer anisotropic optical constants in a broad spectral range (250–1700 nm) are measured for the first time. It is discovered that it has a large birefringence of Δn ≈ 0.2 and a high dichroism of Δk ≈ 0.4. Moreover, it is demonstrated that aligned SWCNTs monolayer optical response can be described by an effective medium approximation using the graphene dielectric function. In addition, it gives a universal approach for a determination of carbon concentration in nanotubes structures. It also applies for other types of carbon nanotubes, such as multi‐walled and randomly oriented carbon nanotubes arrays. Hence, in the results, aligned SWCNTs monolayer optical constants are added to the optical anisotropy database, which facilitates the longstanding challenge of using 1D structures in two dimensions, and a rapid characterization method for carbon nanotubes is provided.

Highly anisotropic layered crystal AgBiP2Se6: Growth, electronic band-structure and optical properties

A centimeter size crystal of silver- and bismuth-bearing selenophosphate, AgBiP2Se6, was effectively grown for the first time by Bridgman technique and characterized via different experimental and theoretical techniques to explore its quality and some physicochemical properties. The present experimental and theoretical data reveal a well-defined layered structure of the AgBiP2Se6 compound and its highly anisotropic optical constants. In particular, the AgBiP2Se6 crystal was studied to estimate binding energies of core-level electrons of the atoms constituting the crystal and to explore the energy distribution of the valence electronic states for both as-grown and treated with middle-energy Ar+ ions surfaces. The XPS data reveal high hygroscopicity of the AgBiP2Se6 crystal. The SEM/EDX results feature a composition of the crystal to be very close to a nominal stoichiometric content. In the present work we probe various models to gain the best correspondence of theoretical total density of states curve and the experimental XPS spectrum. We have established theoretically that contributions of Se p states prevail in the upper section of the valence band, its central section is prevailed by Ag d states, while the bottom is formed from contributions of P p states. Following the band-structure findings, we examine in detail the optical constants of the compound under study. In particular, the present calculations yield that AgBiP2Se6 is a promising optoelectronic semiconductor with essential anisotropic behavior for two main non-zero diagonal constituents of the second order dielectric tensor in special energy regions for all the optical constants, whereas the energy-loss spectrum is isotropic.

Giant terahertz polarization rotation in ultrathin films of aligned carbon nanotubes

For easy manipulation of polarization states of light for applications in communications, imaging, and information processing, an efficient mechanism is desired for rotating light polarization with a minimum interaction length. Here, we report giant polarization rotations for terahertz (THz) electromagnetic waves in ultrathin ( ∼ 45 n m ), high-density films of aligned carbon nanotubes. We observed polarization rotations of up to ∼ 20 ∘ and ∼ 110 ∘ for transmitted and reflected THz pulses, respectively. The amount of polarization rotation was a sensitive function of the angle between the incident THz polarization and the nanotube alignment direction, exhibiting a “magic” angle at which the total rotation through transmission and reflection becomes exactly 90°. Our model quantitatively explains these giant rotations as a result of extremely anisotropic optical constants, demonstrating that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz polarization devices.

Optical Theory-Based Simulation of Attenuated Total Reflection Infrared Spectra of Montmorillonite Films

AbstractInfrared analyses of clay mineral samples are usually performed by transmission techniques. While transmission measurements are easy and inexpensive, the sample preparation plays a critical role in the quality of the data. Alternatively, attenuated total reflection (ATR) provides a powerful and often simpler analysis method. However, the ATR spectra reveal significant differences when compared to transmission spectra sometimes leading to confusion in the interpretations. Indeed, optical effects play a prominent role in the ATR spectral profile and their identification is mandatory for obtaining quantitative information regarding molecular/particle orientation or film thickness. The objective of the present study was to perform exact spectral simulations of montmorillonite films by making use of optical theory, including the determination of the anisotropic optical constants from the experimental reflectance spectra by Kramers-Kronig (KK) transformation. This methodology was used: (1) to choose the appropriate optical conditions for advanced and reliable characterization of clay minerals; (2) to extract quantitative information such as the estimation of the film thickness; and (3) to discriminate optical phenomena (optical interferences) from chemical/structural features of the sample.

Ellipsometric determination of anisotropic optical constants of single phase Ga2O3 thin films in its orthorhombic and monoclinic phases

The anisotropic optical constants of high-quality, single crystalline Ga2O3 films in both orthorhombic (ε) and monoclinic (β) phases were obtained from ellipsometry study. The (0001)-oriented ε-Ga2O3 and a (−201) β-Ga2O3 epilayers were grown on (0001) α-Al2O3 substrates by mist-chemical vapor deposition and pulsed laser deposition, respectively. The experimental bandgaps derived from optical constants were found to be related with the allowed optical transitions from valence sub-bands to conduction band minimum in terms of the polarization-selection rules in such asymmetric structures. The pseudo-ordinary (a-b plane) and extraordinary (c axis) optical bandgaps of the ε-Ga2O3 film were estimated to be 4.85 ± 0.02 and 4.76 ± 0.02 eV, respectively. The pseudo ordinary (b axis) and extraordinary (a-c plane) optical bandgaps of the β-Ga2O3 film were estimated to be 4.92 ± 0.02 and 4.57 ± 0.02 eV, respectively. The energy difference between the two bandgaps of the ε-Ga2O3 is about 0.09 eV, which matches the theoretically calculated energy difference (about 0.06 eV) of the highest two valence bands.

Non-standard inversion method of ellipsometric equations for uniaxially anisotropic 2D materials on semiconductor or metallic substrates

A new analytic approach for solving the ellipsometric inversion problem for uniaxially anisotropic 2D materials has been developed. It is based on the classic point-by-point calculation method which treats the points of the spectrum independently, without correlation to other points. The method can be an efficient alternative to the common regression analysis in cases where the dispersion law of 2D materials has complex features. The important advantage of the method is that it does not require the initial guesses for anisotropic optical constants and the time taken for data processing is negligible. It is shown that the new technique is sensitive enough to determine the extraordinary dielectric constant of the monographene. The method offers significant practical interest since the anisotropic properties of 2D materials have not yet been explored.

Quantitative resonant soft x-ray reflectivity from an organic semiconductor single crystal.

Resonant soft X-ray reflectivity at the carbon K-edge was applied to a trigonal tetracene single crystal. The angular resolved reflectivity was quantitatively simulated describing the tetracene crystal in terms of its dielectric tensor, which was derived from the anisotropic absorption cross section of the single molecule, as calculated by density functional theory. A good agreement was found between the experimental and theoretically predicted reflectivity. This allows us to assess the anisotropic optical constants of the organic material, probed at the carbon K-edge, in relation to the bulk/surface structural and electronic properties of the crystal, through empty energy levels.

Anisotropic optical properties of a-axis AlN films: a spectroscopic ellipsometry study

We report the uniaxial anisotropic optical constants of wurtzite-type a-axis oriented AlN films deposited on Si (100) using DC reactive magnetron sputtering as a function of growth temperature (Ts, 35 to 600 °C) using the spectroscopic ellipsometry (SE) technique. The thickness and roughness of these films are determined from the regression analysis of SE data, which are corroborated using TEM and AFM techniques. Highly oriented a-axis AlN film grown at 400 °C, exhibits high n and low k at 210 nm (deep-UV region) with small birefringence (−0.01) and dichroism (0.03) near the band edge. All these AlN films exhibit transparent nature from near-infrared (NIR) to 354 nm, where optical band gap energies vary between 5.7 to 6.1 eV.

Designing optimal nanofocusing with a gradient hyperlens

AbstractWe report the design of a high-throughput gradient hyperbolic lenslet built with real-life materials and capable of focusing a beam into a deep sub-wavelength spot of λ/23. This efficient design is achieved through high-order transformation optics and circular effective-medium theory (CEMT), which are used to engineer the radially varying anisotropic artificial material based on the thin alternating cylindrical metal and dielectric layers. The radial gradient of the effective anisotropic optical constants allows for matching the impedances at the input and output interfaces, drastically improving the throughput of the lenslet. However, it is the use of the zeroth-order CEMT that enables the practical realization of a gradient hyperlens with realistic materials. To illustrate the importance of using the CEMT versus the conventional planar effective-medium theory (PEMT) for cylindrical anisotropic systems, such as our hyperlens, both the CEMT and PEMT are adopted to design gradient hyperlenses with the same materials and order of elemental layers. The CEMT- and PEMT-based designs show similar performance if the number of metal-dielectric binary layers is sufficiently large (9+ pairs) and if the layers are sufficiently thin. However, for the manufacturable lenses with realistic numbers of layers (e.g. five pairs) and thicknesses, the performance of the CEMT design continues to be practical, whereas the PEMT-based design stops working altogether. The accurate design of transformation optics-based layered cylindrical devices enabled by CEMT allow for a new class of robustly manufacturable nanophotonic systems, even with relatively thick layers of real-life materials.

Anisotropic optical constants, birefringence, and dichroism of wurtzite GaN between 0.6 eV and 6 eV

We report the room-temperature anisotropic dielectric functions (DFs), refractive indices, and absorption coefficients as well as birefringence and dichroism of wurtzite GaN in the spectral range between 0.6 eV and 6 eV. They have been determined by combined spectroscopic ellipsometry, optical retardation, and transmission measurements on a series of m- and c-plane bulk substrates prepared from crystals grown by hydride vapor phase epitaxy. The accuracy of the derived DFs is estimated by investigation of the role of mosaicity-related crystal imperfections, self-consistency test based on a Kramers-Kronig analysis, and examination of the influence of kind of overlayer. We also briefly discuss optical properties of a highly defective near-surface layer of GaN crystals introduced by their mechanical polishing.

Structural and optical properties of alumina passivated amorphous Si slanted columnar thin films during electrochemical Li-ion intercalation and deintercalation observed by in situ generalized spectroscopic ellipsometry

The authors report on the structural and optical property changes of alumina passivated amorphous Si slanted columnar thin films during electrochemical Li-ion intercalation and deintercalation determined by in situ generalized spectroscopic ellipsometry. The cyclic voltammetry investigations versus Li/Li+ are performed at a rate of 1 mV/s, while Mueller matrix generalized spectroscopic ellipsometry data are collected. Through a best-match model analysis utilizing the homogeneous biaxial layer approach, temporal anisotropic optical constants are obtained. The authors observe a strong anisotropic electrochromic response with maximum changes of ∼18% in the anisotropic refractive indices and ∼750% in the anisotropic extinction coefficients. Furthermore, the thin films reversibly expand and contract by ∼35%. A comparative analysis of the temporal optical constant response to the changes in overall optical anisotropy of the electrode reveals six transient regions throughout the Li-ion intercalation and deintercalation cycle of the highly ordered three-dimensional nanostructures. The transients correspond to electrochemical potential regions which show limited charge transfer, metalization or demetalization, and swelling or deswelling of the nanostructures. Furthermore, the electrochemical potential regions in which the transients are observed here are very similar to those previously reported for Li-ion intercalation and deintercalation of silicon nanowires using structural analysis techniques, where four distinct phases of Li-Si alloy formation were revealed. The authors find that at low Li contributions, swelling and deswelling occur preferentially along the slanted columns, while at high Li contributions, swelling and deswelling occur preferentially within the intercolumnar space.

Control of slanting angle, porosity, and anisotropic optical constants of slanted columnar thin films via in situ nucleation layer tailoring

Electron-beam evaporation at a glancing angle of 85° is utilized to fabricate highly ordered, spatially coherent titanium slanted columnar thin films. Prior to deposition of the slanted columnar thin films, a titanium nucleation layer is deposited using electron-beam deposition at normal incidence with various intended nucleation layer thicknesses of 0nm, 5nm, 7.5nm, 10nm, 20nm, and 50nm. Structural and optical properties of the anisotropic porous thin films are studied by scanning electron microscopy, atomic force microscopy, and Mueller matrix generalized spectroscopic ellipsometry in the near infrared to ultra-violet spectral regions. An anisotropic effective medium approximation is employed for analysis of the ellipsometry data in order to determine slanting angle and porosity model parameters. We find that the slanting angle and titanium volume fraction are strongly influenced by the nucleation layer thickness. Structural parameters of SCTFs deposited on 50nm nucleation layers are similar to those from SCTFs with no nucleation layer. For small nucleation layer thicknesses, the corresponding SCTF slanting angle and titanium volume fraction decrease reaching a minimum of ≈33° and ≈12%, respectively, at 10nm nucleation layer thickness. In accordance with the strong decrease in volume fraction we observe substantial reduction of the effective anisotropic thin film optical constants. We find the slanting angle and porosity variation reproducible and suggest use of a nucleation layer for control of slanting angle and porosity of slanted columnar thin films.

Optical properties of graphene oxide and reduced graphene oxide determined by spectroscopic ellipsometry

We report the optical constants of graphene oxide and reduced graphene oxide determined by spectroscopic ellipsometry. The dynamic changes in optical properties and thickness of a drop-cast graphene oxide layer during reduction by long-term exposure to focused broad-band white light are monitored in situ. The anisotropic optical constants of the graphene oxide layer and the isotropically averaged optical constants of the reduced layer are precisely determined from a multiple-location analysis of spatially resolved data across the exposed location and a multiple-time-step analysis of the dynamic data, respectively. Observed inter-band transitions in the graphene oxide layer are discussed in relation to theoretical predictions for different coverage levels of the graphene oxide sheets with oxygen containing functional groups. The derived optical constants of the reduced graphene oxide layer are compared to reported values of graphene and thermally reduced graphene oxide.

Optical and magneto-optical characterization of thin films of functionalized tetraphenylporphyrins

The anisotropic optical constants of vacuum deposited films of H2TMPP, CuTMPP, and NiTMPP (TMPP=5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin) in the thickness range of 50 to 150nm were obtained by spectroscopic ellipsometry measurements and subsequent modelling. The angle of the planar molecular backbone with respect to the substrate plane was determined from the degree of optical anisotropy of the optical constants. The resulting angles were compared to those of near edge X-ray absorption fine structure (NEXAFS) spectroscopy on 2 to 5nm thick films. Gold and natively oxidised silicon were used as substrates. The molecular tilt angle on Si (~38°) was found to be slightly smaller than on gold (~41°) for the thick films. This trend was confirmed by NEXAFS. Magneto-optical Kerr effect spectroscopy was applied in order to obtain the off-diagonal elements of the dielectric tensor under a magnetic field. These results were then used to calculate magnetic circular dichroism spectra for comparison with literature results and the assignment of Faraday terms to the particular absorption features.

Morphology and molecular orientation of ethyl-substituted dicyanovinyl-sexithiophene films for photovoltaic applications

Enhancement of the efficiency of organic solar cell devices requires knowledge about the structure of the organic layers involved. Films of the donor material dicyanovinyl-sexithiophene bearing four ethyl side-chains at thiophenes two and five DCV6T-Et(2,2,5,5) (DCV6T-Et) are prepared by thermal evaporation in high vacuum at various thicknesses and substrate temperatures. Infrared spectroscopic ellipsometry is used for determination of the molecular orientation in the thin films grown on room temperature (RT) substrate. From simulation of the IR ellipsometric data, the film thickness and the anisotropic optical constants of the DCV6T-Et films are determined. It is found that the optical constants strongly depend on the film thickness. Different average molecular orientations are determined for a few molecules thin (4nm) and somewhat thicker (20nm) films. Furthermore, the evolution of the surface morphology of films deposited at elevated substrate temperatures (80°C, 100°C) is studied in comparison to the thick RT-film. Atomic force microscopy images indicate that the growth on heated substrate is accompanied by an increase in grain size and surface roughness of the films. Simultaneously, the measured optical absorption spectra display structured and increased absorption in the red spectral region for the DCV6T-Et films deposited at higher substrate temperatures. The changes in surface topography and optical response relate to improved molecular arrangement induced by the substrate heating. To demonstrate the morphological influence on solar cell performance, we finally discuss DCV6T-Et/C60 planar heterojunction solar cells composed of DCV6T-Et films deposited at different substrate temperatures.

Morphology and molecular orientation in sexithiophene and N,N′-bis (n-octyl)-dicyanoperylenediimide heterostructures

We report on the growth mode of N,N′-bis (n-octyl)-dicyanoperylenediimide (PDI-8CN2) on sexithiophene (T6) thin films, studied with different structural, morphological and optical techniques. We aim to individuate the most favorable conditions for the realization of heterostructure devices. The crystalline quality was established by X-ray patterns and Atomic Force Microscopy (AFM) images, and found to be generally high. The anisotropic optical constants extracted from ellipsometry measurements shed light on the mean molecular orientation in the PDI-8CN2 film. AFM images evidence two different growth modes: at T6 thickness less than 2 monolayers (ML), the growth of PDI-8CN2 on T6 is favored with respect to SiO2, while, at higher thickness (2–6 ML), the situation is reversed. An optimum T6 underlayer thickness of approximately 1 ML provides the best quality of PDI-8CN2 layer corresponding to the highest island dimension, the highest molecular order parameter, and the lowest roughness. Spectrum broadening was observed for extinction coefficient of PDI-8CN2 in the heterostructures, as compared with a sole material film, and explained by two effects: increase in molecular disorder and formation of charge transfer complexes.

Optical properties of hybrid titanium chevron sculptured thin films coated with a semiconducting polymer

Optical and structural properties of a hybrid metallic chevron sculptured thin film from titanium coated with the semiconducting polymer poly(3-dodecylthiophene) (P3DDT) are reported. The nanostructured thin film with two subsequent layers of oppositely slanted nanocolumns was fabricated by glancing angle deposition and coated with P3DDT by a spin-cast process. Spectroscopic generalized ellipsometry is employed to determine geometrical structure properties and the anisotropic optical constants of the coated and uncoated film in the spectral range from 400 to 1700nm. The nanostructured thin films before and after hybridization show highly anisotropic optical properties. The complex refractive indices along major polarizability directions of the hybridized chevrons are increased in the entire investigated spectral range with respect to the as-deposited chevrons. Changes in birefringence and dichroism upon polymer infiltration are observed.

Anisotropic optical constants of in-plane oriented polyfluorene thin films on rubbed substrate

We present a photometric method to determine the anisotropic optical constants of several aligned polyfluorene films. These polymers exhibit liquid crystal characteristic under heat treatment and polymer chains are preferentially in-plane oriented on a rubbed alignment layer. A self-consistent dispersion formula of Forouhi–Bloomer model is introduced to fit the measured polarized reflectance and transmittance curves by a global optimization algorithm. The very good agreements between the experimental and theoretical spectra allow us to shed light on the parallel and perpendicular components of optical constant. On this basis, light-emitting devices are fabricated using the anisotropic active films. The measured polarized electroluminescence spectra confirm the optical birefringence.

Optical, structural, and magnetic properties of cobalt nanostructure thin films

We report on optical, structural, and magnetic properties of two substantially different cobalt nanostructure thin films deposited at an oblique angle of incidence of 85° away from the substrate normal. Comparison is made between an achiral columnar thin film grown without substrate rotation and a chiral nanocoil sculptured thin film by glancing angle deposition with substrate rotation. Generalized spectroscopic ellipsometry is employed to determine geometrical structure properties and the anisotropic optical constants of the films in the spectral range from 400 to 1000 nm. The magnetic properties are analyzed with a superconducting quantum interference device magnetometer. Both nanostructure thin films show highly anisotropic optical properties such as strong form birefringence and large dichroism. In particular, Co slanted columnar thin films are found to possess monoclinic optical properties. Magnetic measurements at room temperature show hysteresis anisotropy with respect to a magnetic field either parallel or perpendicular to the nanostructures’ long axis. We find extremely large coercive fields of approximately 3 kOe for our achiral columnar nanostructures.

Uniaxial anisotropy of organic thin films determined by ellipsometry

AbstractOrganic thin films frequently exhibit strong anisotropic optical constants, which in many cases are uniaxial with the optic axis oriented along the surface normal. The data analysis presented here to obtain the anisotropic optical constants of thin films on silicon substrates is based on the model system diindenoperylene (DIP), but nevertheless applies to all uniaxial films on SiO2. In addition to variable angle spectroscopic ellipsometry, different substrates are used to perform a multiple sample analysis. This way of analysing data increases the sensitivity to uniaxial anisotropy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)