Lyuba Kuznetsova

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.

Optical mode confinement in the Al/SiO 2 disk nanocavities with hyperbolic dispersion in the infrared spectral region

Abstract. This paper presents the results of a numerical study of the optical mode confinement in whispering gallery mode disk nanocavities with hyperbolic dispersion using nanolayered Al/SiO2 hyperbolic metamaterial with different Al fill fractions. The fundamental properties of the optical modes and resonance frequencies for the disk nanocavities are studied using the numerical finite-element method. Numerical simulations show that light can be well confined in a disk nanocavity with a radius of up to an order of magnitude smaller than free-space resonant wavelength. This paper will also focus on how Purcell factor and quality factor of the disk nanocavities are affected by the fill fraction of the aluminum in the nanolayered metamaterial. Potential future applications for disk nanocavities with hyperbolic dispersion include silicon photonics optical communications networks, ultrafast LEDs, and biological nanoparticles sensing.

Theoretical design of nano-layered Al/SiO2 metamaterial with hyperbolic dispersion with minimum losses

Motivated by a greater need for increased performance in modern-day technology, this paper shows the results of theoretical calculations for the optical properties of Al/SiO2 nano-layered metamaterial with hyperbolic dispersion. Our main focus is on designing a metamaterial with low losses, since losses might outweigh any increase in speed of photonic devices. We have investigated the effect of three major variables (number/thickness of the Al layers and Al fill fraction) on inherent losses and hyperbolic dispersion using the effective medium approximation with non-local corrections. Our model predicts a variation of the dielectric permittivity only in the perpendicular direction as the number of Al layers changes. First, we present the results of the detailed study of varying the number of Al layers, N, in attempt to find the “saturation limit” of non-local corrections in Al/SiO2 layers. Next, we changed Al fill fraction in a sample of N= 20 layers to find parameters for the material with minimized losses. We found that both of these effects determine the transition wavelength to hyperbolic dispersion, which allows for fine-tuning of the optical properties for future applications.

Numerical modeling of photoluminescence in anisotropic nano-layered aluminum-doped zinc-oxide metamaterial with hyperbolic dispersion

Aluminum-doped ZnO (AZO), a wide direct bandgap semiconductor which emits laser light in the ultraviolet range at room temperature, presents a promising optical gain material for creating lasers for applications in photonics, information storage, biology and medical therapeutics. AZO exhibits an excitonic photoluminescence peak in the ultraviolet region and a defect related photoluminescence peak in the visible region. In addition, a recently developed aluminum-doped ZnO nano-layered structure has a unique optical property namely that the dispersion of the dielectric constant exhibits an optical topological transition in the isofrequency surface from an ellipsoid to a hyperboloid. This unusual optical property provides a unique opportunity for creating nanoscale cavities with dimensions significantly smaller than the wavelength of light which could lead to potential applications such as efficient and compact ultraviolet lasers and LEDs. In this work, we investigate the photoluminescence properties of the anisotropic nano-layered aluminum-doped zinc oxide. In order to describe the influence of the aluminum dopants, a complete model for photoluminescence based on the set of rate equations for electron-hole recombination is developed. The set of coupled rate equations is solved numerically using the fourth order Runge Kutta technique for various optical pump intensities. Our calculations predict that the near-band-edge intensity increases with the addition of aluminum (aluminum filling factor up to ~3%) which indicates that the band gap energy increases as the aluminum content is increased.

Spectroscopic ellipsometry for anisotropic nano-layered Al/SiO2 metamaterial with hyperbolic dispersion

A special class of nano-layered hyperbolic metamaterials (HMMs) has received special attention recently due to their unique optical property, namely that the dispersion of the dielectric constant for HMMs exhibits a topological transition in the iso-frequency surface from an ellipsoid to a hyperboloid. Using aluminum in metal-dielectric nano-layered structures offers several advantages over currently used noble metals. The plasma frequency of the aluminum is higher than that of gold or silver. As a result, aluminum exhibits metallic characteristics over a broader spectral range than gold and silver. In addition, SiO2 is used as the dielectric for this hyperbolic metamaterial because it could be easily integrated into current CMOS technology and has near-zero losses in the UV region. In this investigation, we use generalized spectroscopic ellipsometry to study the distribution of Al within nano-layered samples fabricated using the RF sputtering technique under varying fabrication parameters with a goal of achieving hyperbolic dispersion. In our work, we developed an approach to analyzing generalized spectroscopic ellipsometry data for anisotropic Al/SiO2 structures with strong absorption, which uses the 4x4 transfer matrix approach, also known as the Berreman-formalism. This developed approach allows obtaining permittivity in all three dimensions and importing theoretical permittivity models which are tailored to the Al/SiO2 material’s optical and electrical properties. In this work, we investigate the methods of reducing Al oxidation during fabrication by means of varying the fabrication temperatures and pressure by fitting data from RC2 Ellipsometer (A.C. Woollam Co.), which has dual rotating compensators. Applications for this Al/SiO2 hyperbolic metamaterial will also be discussed.

Optical mode confinement in three-dimensional Al/SiO2 nano-cavities with hyperbolic dispersion

Today’s technological needs are demanding for faster and smaller optical components. Optical microcavities offer a high confinement of electromagnetic field in a small volume, with dimensions comparable to the wavelength of light, which provides a unique system for the enhancement of light-matter interactions on the nanoscale. However, further reducing the size of the optical cavity (from microcavity to nanocavity) is limited to the fundamental diffraction limit. In hyperbolic metamaterials, large wave vectors can be achieved. Therefore, optical cavities, created from hyperbolic metamaterials, allow the confinement of the electromagnetic field to an extremely small volume with dimensions significantly smaller than the wavelength of light. This paper presents the results of numerical study of the optical mode confinement in nanocavities with hyperbolic dispersion using nanolayered Al/SiO2 hyperbolic metamaterial with different Al fill fractions. The fundamental properties of the optical modes and resonance frequencies for the nanocavities are studied using the finite-elementmethod numerical technique. Numerical simulations show that the light can be well confined in a disk with radius up to λ/65. This paper will also focus on other variables such as Q-factor and Al fill fraction. Potential future applications for three-dimensional nanocavities with hyperbolic dispersion include: silicon photonics optical communications networks, ultrafast LEDs and biological nanoparticles sensing.

Optical mode properties for nano-layered aluminum-doped zinc oxide rectangular waveguides at the epsilon-near-zero spectral point

The optical mode properties of an anisotropic nano-layered aluminum-doped zinc oxide rectangular waveguides at the epsilon-near-zero spectral point are numerically investigated. The finite element method is used for a numerical study of the optical resonance frequencies for a square Al:ZnO/ZnO waveguide (1 μm width/height). Optical permittivity for multilayered Al:ZnO/ZnO is described using an effective medium approximation. Our numerical finite element method calculations predict a significant spectral shift, a modified free spectral range, and an asymmetric electric field distribution for lower order optical modes. Those modes have resonance wavelengths at the epsilon-near-zero point (~ 1800 nm). We show that the resonant frequency for the lower order TE11 mode increases dramatically compared to the non-doped zinc-oxide waveguides, while the higher order modes (e.g, TE21) remain almost at the same frequency. This results in less than a 5% difference in resonance frequencies for these two modes for Al:ZnO/ZnO square waveguide.

Ultrafast dynamics of the ultraviolet and visible photoluminescence in the aluminum-doped zinc oxide metamaterial

The emission properties of aluminum-doped zinc oxide are numerically investigated. A complete model for photoluminescence, based on the set of rate equations for electron-hole recombination, is used to study the influence of carrier concentration (1017-1020 cm-3 ) on the visible and ultraviolet (UV) emission. The set of coupled rate equations is solved numerically using the fourth order Runge-Kutta technique for various optical pump intensities and pulse durations. The results for low carrier concentration (~1017 cm-3 ) show that at low pump intensity (0.01 mJ/cm2 ) visible emission is dominant in the emission spectrum and, as the pump intensity increases (~1 mJ/cm2 ), the UV emission becomes dominant. The study of ultrafast dynamics shows that for pump pulse durations of less than ~ 1 ns the intensity of the UV emission is an order of magnitude larger compared to the visible intensity for aluminum-doped ZnO samples with carrier concentration ~1018 cm-3 .

Finite-difference time-domain numerical study of ultrashort pulse propagation across sub-micron scale distances in Al:ZnO/ZnO at the epsilon near-zero spectral point

The epsilon-near-zero (ENZ) spectral region in metamaterials has shown unique opportunities for enhancing light-matter interactions, particularly due to the large variation of dielectric permittivity over a small frequency range. In this work, ultrashort pulse propagation at the ENZ point is investigated using both the split-step method approach to solving Nonlinear Schrödinger’s equation (NLSE) and the one-dimensional finite-difference time-domain (FDTD) method. We use an estimation for chromatic dispersion at the ENZ for the NLSE, and low input powers for the initial pulse to minimize nonlinearities for both methods. The permittivity for the AZO/ZnO structure was varied only in the AZO layer, which we estimated using Drude model. We found that the damping frequency, γ, in the Drude model has the most influence on pulse shaping during propagation as it relates to losses within the material. Results from our 1D FDTD simulations have shown soliton-like behavior for incoming ultrashort pulses with duration 100 fs in the ENZ region up to 300 nm lengths for γ = 1x1011 and 1x1012 Hz.

Subwavelength silicon disk whispering-gallery-mode microcavities for size-dependent nanoparticles detection in the mid-infrared

Abstract. Three-dimensional finite-element-method numerical simulations are used to investigate a size-dependent sensing technique by observing the effects that a spherical nanoparticle had on the frequency resonances of whispering-gallery modes of a subwavelength silicon microdisk. Results show that the observed spectral shift varies significantly (∼2 to 8 nm) for the TM1,2 optical mode with an attached nanoparticle with radii between 150 and 400 nm. This frequency shift size-dependence makes it possible to identify viruses of different sizes by the resonant frequency change in the transmission spectrum in the mid-infrared.