Optical Permittivity Research Articles

Midinfrared one-dimensional photonic crystal constructed from two-dimensional electride material

Dicalcium nitride $({\mathrm{Ca}}_{2}\mathrm{N})$ is a layered material that has been recently identified as a two-dimensional electride, based on first-principles calculations, where the electronic property and the optical complex dielectric function are studied using density functional theory. We find that the optical permittivity tensor is highly anisotropic, and ${\mathrm{Ca}}_{2}\mathrm{N}$ possesses metallic characters when the in-plane dielectric function is negative in midinfrared frequency. In addition, the enhanced midinfrared transmission property and the field distribution of one-dimensional photonic crystals composed of alternating ${\mathrm{Ca}}_{2}\mathrm{N}$ layers and the dielectric material are theoretically studied using the transfer-matrix method. It is found that the ${\mathrm{Ca}}_{2}\mathrm{N}$ photonic crystals support a series of passbands and stopbands the numbers of which increase with the thickness and the permittivity of the dielectrics. The field distributions show that the transmission resonances in the passband are attributed to the coupled Fabry-P\'erot resonances of the individual reactively loaded dielectric slabs. It is also noticed that these resonances lie within certain characteristic frequency bands which are independent of the period of the photonic crystal. The low-frequency edge of the passband is highly tunable by the thickness of the electride material and the dielectric material, the permittivity of the dielectric layer, while the high-frequency edge is insensitive to the electride material thickness. Moreover, the first band gap in lower frequency is almost omnidirectional and polarization insensitive. When defect layers are introduced, the twin defect modes are found, and the frequency and the frequency interval of the two defect modes can be tuned just by changing the permittivity, the thickness and the position of the defect layer, respectively. Polarization and angular insensitive absorption bands can also be obtained for the electride-dielectric photonic crystal with a reflective substrate. These properties of electride material photonic crystals have potential applications in tunable multiband filters and absorbers in the midinfrared region.

Nanopore-Organized Nanoparticle Arrays for Tunable Optical Materials Using Nanobioplar Electrodeposition

The combination of polymers with nanopores has been of interest recently in many areas, from polymer-hybrid nanotube materials to functionalized membranes. Tightly-controlled nanopore arrays fabricated via Focused-Ion Beam (FIB) have demonstrated advantages in isolating small numbers of molecules or nanoscale objects, as with single molecule electrochemistry, and are well-suited to further engineering including multiple electrode rings and advanced surface functionalization techniques. In this work, we discuss the ability of nanopore arrays to create ordered polymer cylinder arrays for testing the formation of nanofilaments in constrained polymer conditions as well as bipolar electrodeposition using nanoparticles. Previous work examined the kinetics of nanoscale bipolar electrodeposition by creating conductive filaments through a single nanoparticle, which demonstrated unique behavior due to the inherent limitations of nanoscale mass transport. Bipolar electrochemistry has demonstrated advantages in examining electrochemical effects under high electric fields, optically-coupled experiments, and other geometries where conventionally-connected electrodes would be prohibitively complex. In macro-scale bipolar electrochemistry, the difference in electric potential between the electrode and the surrounding electrolyte provides the driving force for a chemical reaction, such as the deposition of a metal ion from solution onto a surface. If that deposition is suitably controlled, a conductive filament may be created by electrodeposition, for example, in a nanoscale gap between two electrodes to create an atomic-scale junction. This work examines the connection of multiple nanoparticles constrained in a nanopore array via the creation of conductive filaments, and the resultant change in optical behavior due to modified nanoparticle plasmonics. Preliminary theory suggests that the optical permittivity of the nanoparticle array may be tuned via formation and dissolution of the filaments. With application areas ranging from massively-parallel chemical sensing arrays to non-contact tunable metamaterials, this work uses advanced nanopore arrays to further the mechanistic understanding of nanoelectrodeposition and explore the unique challenges involved, both aspects vital to advancing the use of similar technology in real-world devices. Special thanks to the Defense Advanced Research Projects Agency (DARPA) FA8650-15-C-7546 and the NASA Space Technology Research Fellowship (NSTRF) NNX16AM45H for providing funding for this research.

Mueller matrix ellipsometric study of multilayer spin-VCSEL structures with local optical anisotropy

Spin-laser structures such as spin-polarized vertical-cavity surface-emitting lasers are semiconductor devices in which the radiative recombination processes involving spin-polarized carriers result in an emission of circularly polarized photons. Nevertheless, additional linear in-plane anisotropies in the cavity, e.g., interfacial and surface anisotropies, generally lead to preferential linearly polarized laser emission and to possible coupling between modes. We present Mueller matrix ellipsometric study of non-intentionally doped InGaAs/GaAsP laser structures devoted for optical pumping operations in the spectral range from 0.73 to 6.4 eV in order to disentangle surface and quantum wells contributions to the linear optical birefringence of the structures. The measurement of full 4 × 4 Mueller matrix for multiple angles of incidence and in-plane azimuthal angles in combination with proper parametrization of optical functions has been used for extraction of optical permittivity tensor components along [110] and [110] crystal axis of surface strained layers and quantum wells grown on [001]-substrate. Such spectral dependence of optical tensor elements is crucial for modeling of spin-laser eigenmodes, resonance conditions, and also for understanding of sources of structure anisotropies.

First-principles momentum distributions and vibrationally corrected permittivities of hexagonal and cubic ice

Three-dimensionally resolved proton momentum distributions and end-to-end distributions have been calculated for hexagonal and cubic water ice. First-principles quantum nuclear wave functions have been used to investigate the impact of vibrational anisotropy, anharmonicity, proton and stacking disorder, temperature, and pressure on these distributions. Moreover, the effects of vibrations on the electronic density in hexagonal ice are shown to lead to a 5% vibrational correction with respect to the static-lattice optical permittivity, and proton disorder is found to be crucial in explaining its experimentally observed temperature dependence.

Properties-Adjustable Alumina-Zirconia Nanolaminate Dielectric Fabricated by Spin-Coating.

In this paper, an alumina-zirconia (Al2O3-ZrO2) nanolaminate dielectric was fabricated by spin-coating and the performance was investigated. It was found that the properties of the dielectric can be adjusted by changing the content of Al2O3/ZrO2 in nanolaminates: when the content of Al2O3 was higher than 50%, the properties of nanolaminates, such as the optical energy gap, dielectric strength (Vds), capacitance density, and relative permittivity were relatively stable, while the change of these properties became larger when the content of Al2O3 was less than 50%. With the content of ZrO2 varying from 50% to 100%, the variation of these properties was up to 0.482 eV, 2.12 MV/cm, 135.35 nF/cm2, and 11.64, respectively. Furthermore, it was demonstrated that the dielectric strength of nanolaminates were influenced significantly by the number (n) of bilayers. Every increment of one Al2O3-ZrO2 bilayer will enhance the dielectric strength by around 0.39 MV/cm (Vds ≈ 0.86 + 0.39n). This could be contributed to the amorphous alumina which interrupted the grain boundaries of zirconia.

Dynamically controlled Purcell enhancement of visible spontaneous emission in a gated plasmonic heterostructure

Emission control of colloidal quantum dots (QDs) is a cornerstone of modern high-quality lighting and display technologies. Dynamic emission control of colloidal QDs in an optoelectronic device is usually achieved by changing the optical pump intensity or injection current density. Here we propose and demonstrate a distinctly different mechanism for the temporal modulation of QD emission intensity at constant optical pumping rate. Our mechanism is based on the electrically controlled modulation of the local density of optical states (LDOS) at the position of the QDs, resulting in the modulation of the QD spontaneous emission rate, far-field emission intensity, and quantum yield. We manipulate the LDOS via field effect-induced optical permittivity modulation of an ultrathin titanium nitride (TiN) film, which is incorporated in a gated TiN/SiO2/Ag plasmonic heterostructure. The demonstrated electrical control of the colloidal QD emission provides a new approach for modulating intensity of light in displays and other optoelectronics.

Study of granular flow in silo based on electrical capacitance tomography and optical imaging

This paper presents a comparison between optical images and two different modalities of electrical capacitance tomography (ECT), namely an off-the-shelf ECT system and a model one combining an inductance (L) - capacitance (C) - resistance (R) meter with a multiplexer. This comparison was performed to analyse the degree of measurement accuracy and to estimate the spatial resolution of the ECT systems. In this case, the detection of the position of flow boundaries during silo discharging and its translation when discharging changes from concentric to eccentric were investigated. A versatile rectangular silo model was used to generate different types of funnel flow of rice material, which was designated as the most adapted granular material to create both optical and electrical permittivity contrasts between stagnant and flowing zones. When a high temporal resolution is not requested, the model ECT system reduces the measurement error of flow boundary detection between 3% and 9% when compared with optical measurements. Moreover, the system is able to capture boundary translation of about 5% of the silo bin width, which is below the assumed spatial resolution of standard commercial ECT systems.

Terahertz spectroscopy of charge transport in films of pristine and doped single-wall carbon nanotubes

Electrical transport mechanisms of 2D carbon nanotube networks are presently under intensive studies. The related experimental data are ambiguous and controversial. We report on terahertz-infrared spectra of optical conductivity and dielectric permittivity of thin transparent films composed of pristine and CuCl- or iodine-doped single-walled carbon nanotubes (SWCNTs) measured in the frequency range from 7 to 25 000 cm−1 and at temperatures from 5 to 300 K. Controversially to the existing results, we have not observed a clear signature of the so-called terahertz conductivity peak. Instead, a typical metallic-like frequency- and temperature-dependent behavior of the conductivity and permittivity has been discovered. It was attributed to the high quality interconnected SWCNT network providing the almost free pathways for charge carriers. Applying Drude conductivity model, we have determined the temperature and doping dependences of effective parameters of the carriers in the films: plasma frequency, scattering rate, mobility, mean-free path. The obtained results demonstrate a great potential of the material in the field of electromagnetic applications at frequencies up to few terahertz.

Eigenmodes of spin vertical-cavity surface-emitting lasers with local linear birefringence and gain dichroism

We present a general method for the modeling of semiconductor lasers such as a vertical-cavity surface-emitting laser and a vertical-external-cavity surface-emitting laser containing multiple quantum wells and involving anisotropies that may reveal (i) a local linear birefringence due to the strain field at the surface or (ii) a birefringence in quantum wells due to phase amplitude coupling originating from the reduction of the biaxial ${D}_{2d}$ symmetry group to the ${C}_{2v}$ symmetry group at the III-V ternary semiconductor interfaces. From a numerical point of view, a scattering S-matrix recursive method is implemented using a gain or amplification tensor derived analytically from the Maxwell-Bloch equations. It enables one to model the properties of the emission (threshold, polarization, and mode splitting) from the laser with multiple quantum well active zones by searching for the resonant eigenmodes of the cavity. The method is demonstrated on real laser structures and is presently used for the extraction of optical permittivity tensors of surface strain and quantum wells in agreement with experiments. The method can be generalized to find the laser eigenmodes in the most general case of circular polarized pumps (unbalance between the spin-up and spin-down channels) and/or dichroism allowing an elliptically polarized light emission as recently demonstrated experimentally when the linear birefringence is almost compensated [Joly et al., Opt. Lett. 42, 651 (2017)].

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 Dispersion, Permittivity Spectrum and Thermal-Lensing Effect in Nickel-Doped Zinc Sulfide Nanoparticles

In this paper, Ni-doped ZnS (ZnS:Ni2+) nanoparticles (NPs) have been prepared through a chemical method. The average size of the particle is 45 nm. Thin films of the particles have been prepared by using the spin-coating method. The linear and nonlinear optical properties of Ni-doped ZnS thin films and the colloidal solution of them have been studied widely. Using a precise numerical method, the refractive index curve (dispersion curve), absorption coefficient and optical permittivity of Ni-doped ZnS film have been obtained. Using these values, the absorption coefficient of the colloidal solution of Ni-doped ZnS particles has been simulated and compared with experimental results. Finally, using the z-scan method at low laser irradiation, the thermo-optical effect has been studied and the nonlinear refractive index due to this effect has been reported.

Computational and experimental studies on dielectric relaxation and dipole moment of some anilines and phenol

In the present study, the dielectric relaxation behaviour of four polar molecules namely 2-Nitroaniline, 4-Bromoaniline, 4-Chloroaniline and 4-Chlorophenol were studied in dilute solutions of benzene using microwave bench at 9.59GHz frequency. The different parameters like, dielectric constant (ε′), dielectric loss (ε″), static dielectric constant (ε0) and optical permittivity (ε∞) have been determined. Further from these values, dipole moment (μ) and relaxation time (τ) were calculated for all the molecules following Gopalkrishna, microwave conductivity and the Higasi method. The dipole moment for all the molecules were also estimated theoretically from ab initio computations by using Density Functional Theory (DFT) calculations with the help of Gaussian 09W software. Dipole moments determined experimentally and through DFT computations are in good agreement with the dipole moments estimated by taking vector sum of group moments of Aniline, Nitrobenzene, Bromobenzene, Chlorobenzene and Phenol by neglecting interaction between pendant groups. The experimentally determined relaxation times were analyzed in terms of Stokes-Einstein-Debye (SED) theory and the results are compared with molecular radii estimated from DFT and Edward's atomic increment methods. Further, in order to understand the chemical stability and reactivity of all these molecules, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and HOMO-LUMO energy gap were also estimated from DFT computations. Our experimental findings suggest that, 2-Nitroaniline exhibits simple Debye type relaxation behaviour while the other three samples exhibit Higasi's distribution of relaxation times which may be due to both overall and intra-molecular rotation of the molecules. Further, the HOMO-LUMO studies revealed that, 2-Nitroaniline is more chemically reactive having least energy gap and 4-Chlorophenol is least chemically reactive having highest energy gap among the four samples.

Competition between subwavelength and deep-subwavelength structures ablated by ultrashort laser pulses

Femtosecond laser-induced periodic subwavelength and deep-subwavelength structures (SWS; DSWS) have attracted attention due to their subdiffraction resolution of surface and inner volume patterning. Understanding of the richness of laser–matter interaction during formation of SWS and DSWS is another quest which can help to find control for nanoscale fabrication. Lack of control over SWS and DSWS formation has impacted their wider use and calls for a deeper insight into the relationship between them. Herein we present a systematic study defining a criterion for imprinting either SWS or DSWS, which is based on a competition and their mutual incompatibility discriminated by the laser fluence and pulse accumulation. Structure evolution of SWS and DSWS is highly dependent on the localized effective laser fluence, which determines the instantaneous optical permittivity by the laser-excited electrons creating an active plasma layer. The proposed universal SWS and DSWS competition mechanism involving the laser-induced plasma wave at the plasma–substrate interface ties together many previous observations and unifies the discussed mechanisms of surface nanoripple formation.

Longitudinal magnetoconductivity and magnetodielectric effect in bilayer graphene

It was recently shown that a finite imbalance between electron densities in the K and K′ valleys of bilayer graphene induces a magnetoelectric coupling. Here we explore ramifications of this electronically tunable magnetoelectric effect for the optical conductivity and dielectric permittivity of this material. Our results augment current understanding of longitudinal magnetoresistance and magnetocapacitance in unconventional materials.

Optical permittivity, thermo-optical and electrical properties of nickel oxide nanostructures with heavy cupper doping

In this research, physical properties of nickel oxide nano-structured layers doped with various amount of Cu atoms (20–60 at.%) is studied using spray pyrolysis method on the glass substrate. The FESEM images show formations of nanostructures of about 20–60 nm and the XRD patterns show layers have a polycrystalline cubic structure nature with (111) as the preferred direction that its intensity reduces as the doping density increases. Analyzing transmittance UV. Vis spectra shows the variations of optical band gap of the samples are due to occurrence of doping atoms and quantum confined effects. Optical permittivity of different doped films have been compared using new numerical method and show the prominent effect of doping percent to real and imaginary parts of electrical permittivity. Also Hall effect results shows that Cu atoms substituted by Ni atoms sites play as acceptor atoms in the crystalline lattice. Finally, thermo-optical properties of the films have been studied using Nd–YAG laser illumination.

The effect of chemical variations on the structural polarity of relaxor ferroelectrics studied by resonance Raman spectroscopy

Resonance Raman spectroscopy was applied to doped PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 relaxor ferroelectrics, to better understand the effect of composition disorder on the mesoscopic-scale polar order in complex perovskite-type (ABO3) ferroelectrics. The excitation photon energy used was 3.8 eV, which is slightly above the energy gap and corresponds to the maximum of the optical dielectric permittivity. Group-theory analysis reveals that the resonance Raman scattering (RRS) observed under these conditions is allowed only in polar crystal classes. Therefore, RRS is dominated by the atomic dynamics of nanoregions with coherent polar distortions, which considerably facilitates the comparison of polar order in various compounds. The results show that A-site doping (Ba2+, Sr2+, La3+, Bi3+) has significantly stronger effect on the structural polarity than the introduction of a third element at the B site (Nb5+ or Sn4+ doped in PbSc0.5Ta0.5O3). The A-site substitution by cations that in contrast to Pb2+ have isotropic outermost electron shells disturbs the system of lone-pair electrons, thus reducing the correlation length of coupled polar distortions and the strength of the electric field associated with the mean polarization of polar nanoregions. A-site doping with larger cations (Ba2+) augments the polar deformation of the individual BO6 octahedra due to local elastic fields. As a result, such A-site doping intensifies the initial structural polarity at high temperatures and prevails the enlargement of the polar fraction at low temperatures. A-site doping with smaller cations (Sr2+, La3+), regardless if they are isovalent or aliovalent to Pb2+, increases the correlation length of antiferrodistortive order (BO6 tilts), which in turn assists the development of double-perovskite structure with coherent local polar distortions. A-site doping with aliovalent cations (Bi3+) having the same outermost electron shell and ionic radius as the host A-site Pb2+ cations leads to stronger coupling along the –B–O–B– bond linkages due to enhanced random local electric fields.

Molecular-Based Theory for Electron-Transfer Reorganization Energy in Solvent Mixtures.

Using statistical-field techniques, we develop a molecular-based dipolar self-consistent-field theory (DSCFT) for charge solvation in liquid mixtures under equilibrium and nonequilibrium conditions, and apply it to compute the solvent reorganization energy of electron-transfer reactions. In addition to the nonequilibrium orientational polarization, the reorganization energy in liquid mixtures is also determined by the out-of-equilibrium solvent composition around the reacting species due to preferential solvation. Using molecular parameters that are readily available, the DSCFT naturally accounts for the dielectric saturation effect and the spatially varying solvent composition in the vicinity of the reacting species. We identify three general categories of binary solvent mixtures, classified by the relative optical and static dielectric permittivities of the solvent components. Each category of mixture is shown to produce a characteristic local solvent composition profile in the vicinity of the reacting species, which gives rise to the distinctive composition dependence of the reorganization energy that cannot be predicted using the dielectric permittivities of the homogeneous solvent mixtures.

Feasibility demonstration of a massively parallelizable optical near-field sensor for sub-wavelength defect detection and imaging.

To detect and resolve sub-wavelength features at optical frequencies, beyond the diffraction limit, requires sensors that interact with the electromagnetic near-field of those features. Most instruments operating in this modality scan a single detector element across the surface under inspection because the scattered signals from a multiplicity of such elements would end up interfering with each other. However, an alternative massively parallelized configuration, capable of interrogating multiple adjacent areas of the surface at the same time, was proposed in 2002. Full physics simulations of the photonic antenna detector element that enables this instrument, show that using conventional red laser light (in the 600 nm range) the detector magnifies the signal from an 8 nm particle by up to 1.5 orders of magnitude. The antenna is a shaped slot element in a 60 nm silver film. The ability of this detector element to resolve λ/78 objects is confirmed experimentally at radio frequencies by fabricating an artificial material structure that mimics the optical permittivity of silver scaled to 2 GHz, and “cutting” into it the slot antenna. The experimental set-up is also used to demonstrate the imaging of a patterned surface in which the critical dimensions of the pattern are λ/22 in size.

Combined optical–electrical finite-element simulations of thin-film solar cells with homogeneous and nonhomogeneous intrinsic layers

A two-dimensional finite-element model was developed to simulate the optoelec- tronic performance of thin-film, p-i-n junction solar cells. One or three p-i-n junctions filled the region between the front window and back reflector; semiconductor layers were made from mixtures of two different alloys of hydrogenated amorphous silicon; empirical relation- ships between the complex-valued relative optical permittivity and the bandgap were used; a transparent-conducting-oxide layer was attached to the front surface of the solar cell; and a metallic reflector, either flat or periodically corrugated, was attached to the back surface. First, frequency-domain Maxwell postulates were solved to determine the spatial absorption of photons and thus the generation of electron-hole pairs. The AM1.5G solar spec- trum was taken to represent the incident solar flux. Second, drift-diffusion equations were solved for the steady-state electron and hole densities. Numerical results indicate that increasing the number of p-i-n junctions from one to three may increase the solar-cell efficiency by up to 14%. In the case of single p-i-n junction solar cells, our simulations indicate that efficiency may be increased by up to 17% by incorporating a periodically corrugated back reflector (as opposed to a flat back reflector) and by tailoring the bandgap profile in the i layer. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original pub- lication, including its DOI. (DOI: 10.1117/1.JPE.6.025502)

Designing optical metamaterial with hyperbolic dispersion based on an Al:ZnO/ZnO nano-layered structure using the atomic layer deposition technique

Nano-layered Al:ZnO/ZnO hyperbolic dispersion metamaterial with a large number of layers was fabricated using the atomic layer deposition (ALD) technique. Experimental dielectric functions for Al:ZnO/ZnO structures are obtained by an ellipsometry technique in the visible and near-infrared spectral ranges. The theoretical modeling of the Al:ZnO/ZnO dielectric permittivity is done using effective medium approximation. A method for analysis of spectroscopic ellipsometry data is demonstrated to extract the optical permittivity for this highly anisotropic nano-layered metamaterial. The results of the ellipsometry analysis show that Al:ZnO/ZnO structures with a 1:9 ALD cycle ratio exhibit hyperbolic dispersion transition change near 1.8 μm wavelength.