We are in the midst of the second quantum revolution. Research institutes and companies worldwide are working toward harnessing the power of quantum physics for technological applications. Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities, which makes them promising candidates for low-power electronic applications. Conventional III–V infrared (IR) materials have the flexibility to engineer topologically protected surface states that can be resistant to ambient environments. In particular, largely hybridized band structures provide thermodynamically stable edge currents at the higher operating temperatures, which are important for IR sensing applications. Hence, we focused on optimizing two critical components for establishing ambient topological insulator; one for enlarging the hybridization gap, Δ, and the other for reducing bulk conduction in InAsSb/InGaSb structures. We performed a modelling study, and achieved an approximately 79 meV from InAs/InGaSb superlattices (SLs) lattice matched to AlSb, which is one of the largest reported value by far. Based on this modeling study, we selected a baseline SL design of InAsSb/GaSb on GaSb with Δ of ~62 meV to address key material issues such as finite bulk carrier conduction in undoped region of SLs. Systematic growth/processing optimization was performed in order to reduce the bulk charge carriers. The origin of constrained carrier dynamics in largely hybridized SL system and their effects on the designed topological structure were discussed.
To test whether conventional infrared materials can be used to control the electronic wavefunction to form a topological state, a 6.2 Å metamorphic (InAs/InGaSb/InAs) quantum well (QW) absorber with ~60 meV of hybridization gap (Δ) was investigated. We developed a thick metamorphic InGaSb buffer layer on GaAs wafer to create a 6.2 Å lattice constant for the QW growths. The lattice constant of virtual substrates (VSs) was very close to the target value of 6.2 Å, however the resulting crystalline quality of the VSs was inefficient for topological insulator. The cross-sectional transmission electron microscopy image revealed that the dislocation density in the InGaSb buffer layer was high closer to the GaAs substrate and gradually reduced upon continued growth. However some mismatch-related defects were propagated into the absorber region, consequently degraded the transport quality of absorber. The QW absorber grown on VS had a low mobility. The mobility was dramatically improved by selecting pseudomorphic QW or superlattice absorber with a small Δ that was grown on a lattice-matched GaSb substrate. Hence, in order for the proposed 6.2 Å materials to be viable for sensing applications, a critical effort will be the development of better optimized metamorphic buffers for the design or of highlyhybridized psedomorphic designs that can be grown on lattice-matched substrates.
Quantum photonics opens doors for applications in sensing, data transfer, and quantum computing. Application areas in many of these technologies require in some manner tunable single photon sources. Hyperbolic metamaterials, composed of metallic building blocks embedded in dielectric media control emission lifetime by modifying the photon density of states. However, no previous efforts have explored the transient modification of metamaterials to modulate emission. Antimony-based semiconductor hyperbolic metamaterials (SHMMs) offer a route to modulation of these resonances at the mid-infrared (IR) wavelength range, which would modulate emission. In this work, we demonstrate the ability to create an ultrafast hyperbolic momentum state in metallic InAsSb/dielectric GaSb stacks and explore the possibility of transient modification of metamaterials by controlling the optical properties of photon emission. If successful, this study will establish a new platform for deterministic single photon emission that can be integrable into opto-electronic platforms and dramatically advance optical quantum technologies. Properly engineered quantum well structures are grown by molecular beam epitaxy with Si-doping in order to convert the InAsSb layers from dielectric to metallic at IR frequencies. The carrier excitation scheme of the engineered hyperbolic stacks is investigated in a variety of excitation levels using pump–probe measurements. The photo-excited carriers in the structure with a metal fraction of ∼0.5 show a polarization dependent reflectivity change, which indicates a transient hyperbolic metamaterial state in the heterostructure induced by the pump laser.
Hyperbolic metamaterials are valuable potential single-photon emitters because of their large density of states at phase boundaries. We grow metamaterial stacks using alternating layers of undoped GaSb (dielectric) and Si-doped InAsSb (metallic). This combination can act as a dielectric in the sample plane, but a metal perpendicular to the plane, forming a hyperbolic metamaterial (HMM) state depending on the density of free electrons. We demonstrate this behavior by injecting free electrons using an ultrafast 1300-nm pump laser, while probing the differential reflectivity and transmittivity with a linearly polarized probe in the range of 4-5 µm. The difference in results for s- and p-polarized probes demonstrates the anisotropic nature of the hyperbolic state, suggesting that single photons at mid infrared (MIR) frequencies may be efficiently emitted in a highly directional manner. The HMM state is also dependent on the metal fraction, which we control via the relative thicknesses of the layers. Additionally, spectroscopic ellipsometry reveals that the metal fraction is consistently lower than the nominal value, a phenomenon we attribute to doped carriers being squeezed to the center of the InAsSb layers. Our analysis of the pump-related shift of the metal/dielectric/HMM phase diagram shows that our sample structure is a highly tailorable avenue to MIR spontaneous photon emission.
We engineer the nonlinear response of multilayered hyperbolic metamaterials systems by employing the optical response of Arsenide semiconductor materials. The photoexcited carriers lead to spectral, temporal, and polarization dependent optical response that is described in this talk. The response is suggestive of generating a transient hyperbolic materials response in this system. Exploration of this response and the use of these materials as a basis for nonlinear metasurfaces will be discussed.
We study theoretically and numerically high density of states for hyperbolic bilayered metamaterials (HMM). It reveals that density response of HMMis reminiscent of Fermi electronic band structure of metal or semiconductors. By the method of Green function a van Hove type singularity is found in photonic density spectra of HMM with saddle
point localization on photonic Fermi surface (FS) of metamaterial Similar to the electronic systems, the photonic FS
close to Van Hove singularity experiences instabilities induced by the changes in volume fractions of its constituents
that leads to the Lifshitz type zero-temperature phase transition between FS of types I and II hyperbolic states at the
protected by topology critical point.
The epitaxially grown system comprising semimetallic ErAs (erbium arsenide) nanoparticles (MNP) and InAs (indium arsenide) single quantum dots (QDs) hold great promise for plasmonic applications. The distinguishing feature of the ErAs-InAs hybrid is that the hybrid separation can be varied and accurately controlled in the molecular-beam epitaxial growth. In order to assess the potential of this system for plasmonic applications, this paper aims to estimate and optimize the expected magnitude of the plasmonic enhancement. We use the Sun–Khurghin theory to estimate the expected absorption and photoluminescence (PL) enhancement. Using a carefully selected set of materials’ parameters as input, we predict about 500-fold plasmonic PL enhancement for this system under resonant conditions.
We experimentally demonstrate a structured thin film that selectively absorbs incident electromagnetic waves in discrete bands, which by design occur in any chosen range from near UV to far infrared. The structure consists of conducting islands separated from a conducting plane by a dielectric layer. By changing dimensions and materials, we have achieved broad absorption resonances centered at 0.36, 1.1, 14, and 53 microns wavelength. Angle-dependent specular reflectivity spectra are measured using UV-visible or Fourier spectrometers. The peak absorption ranges from 85 to 98%. The absorption resonances are explained using the model of an LCR resonant circuit created by coupling between dipolar plasma resonance in the surface structures and their image dipoles in the ground plane. The resonance wavelength is proportional to the dielectric permittivity and to the linear dimension of the surface structures. These absorbers have application to thermal detectors of electromagnetic radiation.
This work investigates properties of surface plasmons on doped metal oxides in the 2-20 μm wavelength regime. By
varying the stoichiometry in pulse laser deposited Ga and Al doped ZnO, the plasmonic properties can be controlled via a fluctuating free carrier concentration. This deterministic approach may enable one to develop the most appropriate stoichometry of ZnAlO and ZnGaO in regards to specific plasmonic applications for particular IR wavelengths. Presented are theoretical and experimental investigations pertaining to ZnAlO and ZnGaO as surface plasmon host materials. Samples are fabricated via pulsed laser deposition and characterized by infrared ellipsometry and Hall-effect measurements. Complex permittivity spectra are presented, as well as plasmon properties such as the field propagation lengths and penetration depths, in the infrared range of interest. Drude considerations are utilized to determine how the optical properties may change with doping. Finite element simulations verify these plasmonic properties. These materials not only offer potential use as IR plasmon hosts for sensor applications, but also offer new integrated device possibilities due to stoichiometric control of electrical and optical properties.
The strain distribution across interfaces in InAs/GaSb superlattices is investigated by scanning transmission electron
microscopy (STEM), using an aberration corrected probe. Atomic resolution images of the superlattices (grown on
(100)-GaSb substrates) were acquired using the high-angle annular dark field (HAADF) imaging technique. For
quantitative strain analysis, the peak-pair algorithm was used to determine the local atomic displacements across
interfaces and within individual layers in the structure. The measured displacements were then used to calculate the
strain map with respect to a reference lattice in the GaSb-substrate region. To precisely identify the local regions in the
strain map Fourier transformation of the HAADF-STEM image was performed to obtain the chemically-sensitive (200)-
Fourier component of the image. A comparison of these images with strain profiles determined from the strain maps
revealed that the GaSb-on-InAs interface is GaAs-like, with a tensile strain of - 0.018 ± 0.003, whereas the overall strain
at the InAs-on-GaSb interface was negligible. In addition, the strain within the GaSb layers was found to be
compressive, with a magnitude of 0.008 ± 0.003, indicating In incorporation in these layers.
Quantum dots have the potential to produce devices with enhanced properties. However, many
quantum dot devices require the quantum dots to have a precise size and a precise location for
optimum operation. So far approaches such as directed assembly and self assembly have failed due
to the random effects resulting during nucleation of the quantum dots. InAs grown under metal
rich conditions can remain planar as opposed to forming the self assembled quantum dot
morphology. Recently we have demonstrated that planar InAs when patterned via tip-based
scribing and then annealed under an As pressure typical for self-assembled quantum dot growth
reorganizes and assumes a 3D morphology. We have been studying this process as a potential
method to precisely locate quantum dots with definable sizes. In this work we report change in the
morphology for different thickness of planar InAs for various pattern dimensions and annealing
temperatures. We have analyzed the composition of the films after annealing to determine the
effect induced in the films from patterning resulting from scribing. Using this approach, arrays of
3D InAs mounds have been formed with mounds having base dimensions of 800, 500, and 350Å.
These results demonstrate that the smaller patterns are less stable and coarsening becomes more
dominant.
The past decade has witnessed rapid progress in the development of techniques for correcting lens aberrations in high-resolution
transmission electron microscopy (HRTEM), resulting in significant enhancement in the directly interpretable
spatial resolution in HRTEM images. Furthermore, in combination with advanced image processing and analysis, it is
now possible to employ HRTEM as a quantitative technique for structural and chemical analysis at the atomic scale. In
this paper we have applied these developments to investigate interfaces in InAs/GaSb superlattices, the main objectives
being the mapping of changes in chemical composition and strain at each interface. For examining changes in
composition we use the focal series reconstruction technique, which retrieves the quantum-mechanical electron wave
function at the exit surface of the sample. The phase images of the electron wave function are then analyzed by linear
multivariate statistical analysis to independently quantify change in the In/Ga and As/Sb contents across each interface.
The strain profiles across interfaces are determined from HRTEM images, obtained from a TEM equipped with a
spherical aberration corrector, employing the "peak-pair analysis" (PPA) algorithm. Finally, the high-angle annular
dark-field imaging technique (HAADF), using a monochromated and probe corrected TEM, is also employed to examine
interfaces.
The unique properties of quantum dots should allow enhanced or
novel devices to be fabricated. However, the typical method of
formation is to self-assemble quantum dots. This process causes
quantum dots to have a distribution in properties such as size and to
form at near random location. Since many of these possible devices
require near exact positioning of the quantum dots with given sizes,
most of these potential devices have been unrealized or exist in far
from optimum conditions. In this work, we present a new method
which is being examined for its potential to form uniform quantum dot
structures. This technique is surface tension driven restructuring of a
nano-patterned surface. In particular, we have formed a planar 5nm
thick InAs film under metal rich conditions. The sample pattern was
formed using a 3mg load measured with a Hysitron nano-indentor and
maintained using STM scan electronics. The pattern consisted of a
grid of 150 lines in x and y directions in nominal 9μm x 9μm square
area. AFM analysis showed a series of lines which are spaced ~180
nm lines apart in the y direction and lines spaced ~60 nm and 120 nm
in the x-direction. The patterned sample was annealed under a high As
flux, near 5 x 10-6 torr, after removal of the surface oxides. The
resulting structure clearly shows the reorganization of the InAs in
regions defined by the original patterning in AFM images. AFM
analysis indicates large features with 80nm base width were formed.
The refractive index is a key characteristic of polymer materials in optical applications. For organic
polymers, typical refractive indices are in the range of 1.35 to 1.65. Extending the refractive index beyond
the limits is of fundamental scientific interest and would enhance the utility of polymers in many
applications. Polymeric thin films fabricated by plasma enhanced chemical vapor deposition (PECVD)
have been investigated in the fields of electronics and optics and their utility is becoming more widespread
in a variety of applications. Outstanding attributes of the PECVD photonic films include a smooth surface,
dense crosslinking structure, robustness, environmental resistance, optical transparency in either visible or
IR regions, and good adhesion to many optical window and substrate materials. In recent years, our
laboratory has fabricated novel polymer optical coatings and films by PECVD. One focus of this research
has been to expand the achievable maximum refractive index. This goal has been sought using two
approaches including increasing the conjugation and crosslinking of chemical moieties of the bulk film and
incorporation of metal ions into the structure. The techniques of XPS, FTIR, HRSEM, and ellipsometry
were used to characterize both the optical properties and the chemical structure of plasma polymerized
benzene, ferrocene, and metal-phthalocyanine thin films. The structure-property relationship and the effect
of PECVD processing conditions are also discussed in this presentation.
Recently quantum dots (QDs) have been the topic of extensive research. Unique properties arise in QDs due to a combination of the localized nature of their wavefunctions and a singularity in the associated density of states. Many strained III-V semiconductor film-substrate systems form QDs via a self-assembly process by means of a Stranski-Krastanov process. The strain relief responsible for the 3D nucleation causes a variation in the in-plane lattice constant which allows subsequent QD layers separated by thin spacer layers to be vertically stacked. Recently this concept has been extended to allow the formation of a heterojunction quantum dot (HeQuaD). In this structure an initial self-assembled QD (SAQD) is formed and then a different similarly strained material is nucleated on the initial SAQDs forming a crown on the underlying QD. This crown is also of a size appropriate to cause quantum confinement. In particular a stack of 4 layers of a HeQuaD structure of a GaSb crowned InAs SAQD on GaAs with GaAs spacer layers has been formed. The top HeQuaDs have been left uncapped to allow AFM analysis of the morphology. Photoluminescence of the HeQuaD has 3 peaks at ~0.95eV, 1.15eV, and 1.35eV. We have measured the intensity and temperature dependence of these PL peaks.
Quantum dots (QDs) have been receiving considerable attention due to the unique properties, which arise due to the confinement of the electron and holes in a lower band gap material. The InAs on GaAs material system is one of the most studied combinations in which quantum dots form during epitaxy. These QDs form in a Stranski Krastanov manner via a self-assembly process in which the dots nucleate at a critical adatom coverage on a wetting layer of InAs. QDs may be vertically aligned by using the residual strain above a buried dot layer to enhance the nucleation of the second layer of dots. In this work, we show the formation of QDs, which are composed of multiple materials, can be formed through a marriage of these two concepts. In this particular demonstration, we formed InAs dots on GaAs andcrowned the QDs with GaSb and encapsulated the entire structure with GaAs. Atomic Force Microscopy shows additional nucleation between the InAs layers has been minimized and cross-sectional transmission electron microscopy shows the formation the composite structure. Transmission electron microscopy indicated a clear boundary between the GaSb and InAs regions. AFM analysis of the HeQuaD structure shows that GaSb material grows mainly on the two (1 1 0) inclined facets. Thus, the HeQuaD is elongated along the (1 1 0) direction. We have also obtained preliminary photoluminescence (PL) from a 3 layer GaS/InAs HeQuaD structure with a peak around 1.3 microns.
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