A fully automated multi-target reactive magnetron sputtering (MS) process is presented in which real-time modeling and in situ standard or Mueller matrix ellipsometry is combined demonstrating growth of nanoscale multi-layer optical thin films having desired properties such as thickness, while observing properties such as index of refraction (n), extinction coefficient (k), and complex permittivity throughout growth. For each material layer isotropic or anisotropic properties as required can be modeled automatically in real-time, allowing for the development of hyperbolic metamaterials. In situ use of an RC2 ellipsometer from JA Woollam is presented, having a spectral range of 210nm - 2500nm. TEM measurements of the thin films are presented.
In this work, we used effective medium approximation and the Drude-Lorentz model with one oscillator for transparent conducting oxide optical permittivity to design multilayered stacks of transparent conducting oxides and dielectrics. The influence of the material parameters for the multilayered metamaterial on nonlinearity and higher-order dispersion are presented. The finite-difference time domain (FDTD) numerical simulations are used to investigate the pulse propagation at the ENZ spectral point. The results based on a full wave analysis study performed using FDTD numerical algorithm show a dramatic pulse shaping for femtosecond pulses at the epsilon-near-zero spectral point.
The FDTD numerical simulations are used to investigate the time evolution of the two-level system of conduction and valence bands. Free carriers’ population dynamics in the conduction band for large intensities of the ultrashort pulse (100 fs) is calculated. The results show that time-dependent real part of the optical permittivity at ENZ spectral point is saturated and resembles a step function as the ultrashort pulse amplitude reaches ~1010 V/m. The calculated value of the intensity-dependent refractive index is n2~-2×10-11 cm2/W. The results demonstrate that increasing initial carrier concentration from ~2×1020cm-3 to ~4×1020cm-3 leads to a significant n2 magnitude change.
In this work, we will present the results of the reflectance calculations at various polarizations and angle of incidences for multilayered aluminum-doped zinc oxide metamaterials. The optical permittivity data was deduced from ellipsometry measurements of the multilayered AZO/ZnO metamaterials. We found that reflectance for TM polarized light exhibits smooth minimum near ENZ spectral point and sharp increase in magnitude beyond ENZ wavelength. The various multilayered samples analysis shows that reflectance for TM-polarized incident light for samples with lower optical losses lead to a faster transition from zero to high reflectance.
Recent advances in understanding of ultrashort pulse propagation in multilayered AZO/ZnO ENZ metamaterial are presented. The influence of the material parameters for AZO/ZnO metamaterial on chromatic dispersion are discussed. Numerical approach based on a full wave analysis of the ultrashort pulse propagation in the presence of enormous second-order dispersion was used to investigate ultrashort pulse propagation through ENZ AZO/ZnO metamaterial. An approach using an adaptive pre-shaping algorithm for ultrashort pulse distortion compensation during the propagation at the ENZ spectral point is introduced. The results based on auxiliary differential equation finite-difference time domain method that show a dramatic change in shape for the probe pulse modulated using pump pulses of various duration (100-500 fs) and amplitude (106−1010 V/m) are presented.
Aluminum-doped zinc oxide metamaterial emerged as a promising plasmonic material due to its low optical loss and high conductivity. The Hong-Ou-Mandel effect is a result of two photons interfering on a beam-splitter. The coincidence rate of the detectors will drop to zero when the identical input photons overlap perfectly in time which results in the Hong-Ou-Mandel dip. If the time delay is scanned, the position of the HOM dip can be measured with femtosecond precision. Therefore, this two-photon interference effect has the potential for applications in precision measurement of time delays. Here, we experimentally observed Hong-Ou-Mandel interference for multilayered AZO/ZnO metamaterial. The Hong-Ou-Mandel effect was observed using a biphoton source with a periodically-poled Potassium Titanyl Phosphate crystal and two single photon counting modules monitoring the output from a beam splitter at 810 nm wavelength. The coincidence probability for separable photons, as a function of time delay τ, was fitted using least square method. The multilayered AZO/ZnO sample (carrier concentration 1020 -1021cm-3) on quartz substrate was used for delay. Our measurements show that the extracted time delay τ=25 µm for multilayered AZO/ZnO was about two orders of magnitude larger than expected from the thickness of the sample.
Multilayered Al:ZnO/ZnO metamaterial, a material that exhibits unique optical properties such as hyperbolic dispersion, attracted a high research interest due to its low optical loss and high conductivity. Combination of the optical gain and strong anisotropy for the Al:ZnO/ZnO metamaterial provide novel opportunities to control spontaneous emission. High doping concentrations (1020- 1021 cm-3 ) for Al:ZnO/ZnO require the inclusion the effect of the band filling. While ZnO has a large bandgap of ~3.3 eV, it has been suggested that in Al:ZnO the Burstein-Moss effect results in an increase in bandgap and thus a decrease in emitted wavelength, which may partially explain the suppression of visible photoluminescence and increase in ultraviolet photoluminescence observed in highly doped Al:ZnO. Here, we investigated the interplay between bandgap renormalization and band filling (Burstein-Moss effect). The results of our calculations show that the energy shift due to the Burstein-Moss effect (blue-shift) and bandgap renormalization (redshift) strongly depends on carrier concentration in multilayered Al:ZnO/ZnO. We found that the energy blue-shift due to BursteinMoss compensates the red-shift from the bandgap renormalization when a carrier concentration reaches 1020 cm-3 .
In this work, we investigate the guided mode properties of planar anisotropic aluminum-doped zinc oxide waveguides (air/metamaterial/silicon oxide) at the epsilon-near-zero spectral point. Our calculations predict two fundamentally different propagation regimes for the lowest order guided TM and TE modes over a broad spectral range (400-2000 nm). Our study shows that excitation of the TM guided mode is possible for wavelengths higher than epsilon-near-zero spectral point. However the propagation distance for such a TM mode will strongly depend on the spectral position of the mode’s eigenfrequency. E.g., propagation distance is maximized for eigenfrequencies near epsilon-near-zero point and reduced by an order of magnitude as the eigenfrequency is tuned by ~50 nm.
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 .
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.
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.
Transparent conductive oxide materials have shown unique optical properties, such as negative refraction, hyperbolic dispersion, and epsilon-near-zero dispersion. In particular, aluminum-doped zinc oxide (Al:ZnO) has shown the most promising results over traditionally used noble metals. Pulsed layer deposition is a popular technique due to its fast and controlled growth rate, as well as the stoichiometric target-to-substrate material transfer. But, since it uses large and inhomogeneous kinetic energy, samples could be prone to macro- and microscopic defects. In this work, we investigate multilayered samples of Al:ZnO/ZnO grown by pulsed laser deposition with the goal of developing a low-loss metamaterial with hyperbolic dispersion. Different fabrication conditions, such as Al:ZnO/ZnO ratio, the thickness of an individual layer, different substrates, and deposition temperatures, were investigated. Results of the ellipsometry analysis, based on fitting spectroscopy data using the Berreman formalism, show that the hyperbolic dispersion transition (Re ε∥>0, Re ε⊥< 0) is achieved at λc=1868 nm wavelength (Im (ε⊥)~0.03) for samples with 1:4 Al:ZnO/ZnO deposition ratio. The fitted dielectric functions for samples with various parameters show that a lower deposition temperature leads to a shorter transition wavelength.
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.
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.
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.
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.
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.
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.
Three-dimensional finite-element-method simulations are used to investigate a system consisting of a subwavelength
silicon microdisk cavity for the detection of different viruses of the same type. This is done by observing the effects that
a spherical nanoparticle had on the frequency resonances of WGMs of the silicon microdisk. Results show that the
observed spectral shift vary for the TM15 mode with an attached nanoparticle of radiuses between 100-300 nm. This
frequency shift size-dependence makes it possible the for mature and immature HIV-1 viruses to be identified by the
resonant frequency change in the transmission spectrum.
Silicon emission in and out resonant coupling with a high Q optical mode is studied in a microdisk cavity with
emissive W-centers. Results show that the W-centers photoluminescence intensity in a silicon microdisk is one order of
magnitude higher than that in the substrate. It exhibits a maximum when emission line and cavity mode frequencies are
matched.
The compensation of nonlinear phase shifts by dispersion in femtosecond fiber amplifiers is explained. Contrary to previous understanding, a chirped-pulse fiber amplifier with mismatched stretcher and compressor can out-perform a matched system when the pulse acquires a significant nonlinear phase shift.
Recent advances in femtosecond fiber lasers are described. Self-similar evolution of parabolic pulses in a modelocked laser can be exploited to substantially increase the pulse energy and peak power that can be achieved without wave-breaking. Experimentally, pulse energies as high as 10 nJ and peak powers as high as 80 kW are obtained from Yb fiber lasers operating in the wave-breaking-free regime.
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