Publications

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Optically pumped semiconductor disk lasers (SDLs) provide high beam quality with high average-power power at designer wavelengths. However, material choices are limited by the need for a distributed Bragg reflector (DBR), usually monolithically integrated with the active region. We demonstrate DBR-free SDL active regions, which have been lifted off and bonded to various transparent substrates. For an InGaAs multi-quantum well sample bonded to a diamond window heat spreader, we achieved CW lasing with an output power of 2 W at 1150 nm with good beam quality.

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Optically-pumped vertical external cavity surface emitting lasers (VECSELs) have unique characteristics that make them attractive for use in intracavity optical cooling of rare earth doped crystals. We present the development of high power VECSELs at 1020 nm for cooling ytterbium-doped yttrium lithium fluoride (Yb:YLF). The VECSEL structures use AlAs/GaAs distributed Bragg reflectors and InGaAs/GaAsP resonant periodic gain epitaxially grown by metal-organic vapor phase epitaxy. To achieve the necessary output power, we investigated thinning the substrate to improve the thermal characteristics. We demonstrated a VECSEL structure that was grown inverted, bonded to the heat sink, and the substrate removed by chemical etching. The inverted structure allows us to demonstrate 15 W output with 27% slope efficiency. Wavelength tuning of 30 nm around 1020 nm was achieved by inserting a birefringent quartz window into the cavity. The window also narrows the VECSEL emission, going from a FWHM of 5 nm to below 0.5 nm at a pump power of 40 W. © 2013 Published by Elsevier B.V.

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Topological quantum computation (TQC) has emerged as one of the most promising approaches to quantum computation. Under this approach, the topological properties of a non-Abelian quantum system, which are insensitive to local perturbations, are utilized to process and transport quantum information. The encoded information can be protected and rendered immune from nearly all environmental decoherence processes without additional error-correction. It is believed that the low energy excitations of the so-called =5/2 fractional quantum Hall (FQH) state may obey non-Abelian statistics. Our goal is to explore this novel FQH state and to understand and create a scientific foundation of this quantum matter state for the emerging TQC technology. We present in this report the results from a coherent study that focused on obtaining a knowledge base of the physics that underpins TQC. We first present the results of bulk transport properties, including the nature of disorder on the 5/2 state and spin transitions in the second Landau level. We then describe the development and application of edge tunneling techniques to quantify and understand the quasiparticle physics of the 5/2 state.

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Microsystem technologies have the potential to significantly improve the performance, reduce the cost, and extend the capabilities of solar power systems. These benefits are possible due to a number of significant beneficial scaling effects within solar cells, modules, and systems that are manifested as the size of solar cells decrease to the sub-millimeter range. To exploit these benefits, we are using advanced fabrication techniques to create solar cells from a variety of compound semiconductors and silicon that have lateral dimensions of 250 - 1000 μm and are 1 - 20 μm thick. These fabrication techniques come out of relatively mature microsystem technologies such as integrated circuits (IC) and microelectromechanical systems (MEMS) which provide added supply chain and scale-up benefits compared to even incumbent PV technologies. © The Electrochemical Society.

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We utilize quantum-confined Stark-effect in asymmetric double quantum wells (ADQW) to realize coherent detection of broadband THz pulses. For that, broadband THz transients formed by a two-color air plasma are focused onto ADQW, in turn dynamically shifting the ADQW bands, with the bandedge at ∼ 825 nm. Spectrally-resolved detection scheme analyzes absorption modulation signatures imprinted onto the transmitted NIR probe spectrum. Use of only few micron thick samples ensures large detection bandwidth, currently demonstrated up to ∼ 15 THz. Time-domain analysis of this signal shows pronounced bi-polar (coherent) as well as small unipolar components of the signal. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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InP substrates form the starting point for a wide variety of semiconductor devices. The surface morphology produced during epitaxy depends critically on the starting substrate. We evaluated (1 0 0)-oriented InP wafers from three different vendors by growing thick (5 μm) lattice-matched epilayers of InP, GaInAs, and AlInAs. We assessed the surfaces with differential interference contrast microscopy and atomic force microscopy. Wafers with near singular (1 0 0) orientations produced inferior surfaces in general. Vicinal substrates with small misorientations improved the epitaxial surface for InP dramatically, reducing the density of macroscopic defects while maintaining a low RMS roughness. GaInAs and AlInAs epitaxy step-bunched forming undulations along the miscut direction. Sulfur-doped wafers were considered for singular (1 0 0) and for 0.2° misorientation toward (1 1 0). We found that mound defects observed for InP and GaInAs layers on iron-doped singular wafers were absent for singular sulfur-doped wafers. These observations support the conclusion that dislocation termination at the surface and expansion of the step spiral lead to the macroscopic defects observed. © 2010 Elsevier B.V.

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We have successfully designed, built and operated a microlaser based on a AlGaInAs multiple quantum well (MQW) semiconductor saturable absorber (SESA). Optical characterization of the semiconductor absorber, as well as, the microlaser output is presented. © 2010 Ontical Society of America.

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We report the first observation of coherent plasmon emission of THz radiation from arrays of semiconductor nanowires. The THz signal strength from InAs nanowires is comparable to a planar substrate, indicating the nanowires are highly efficient emitters. This is explained by the preferential orientation of plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. Using a bulk Drude model, we identify the average donor density and mobility in the nanowires in a non-contact manner. Contact IV transconductance measurements provide order of magnitude agreement with values obtained from the THz spectra. © 2009 SPIE.

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The three-dimensional confinement inherent in InAs self-assembled quantum dots (SAQDs) yields vastly different optical properties compared to one-dimensionally confined quantum well systems. Intersubband transitions in quantum dots can emit light normal to the growth surface, whereas transitions in quantum wells emit only parallel to the surface. This is a key difference that can be exploited to create a variety of quantum dot devices that have no quantum well analog. Two significant problems limit the utilization of the beneficial features of SAQDs as mid-infrared emitters. One is the lack of understanding concerning how to electrically inject carriers into electronic states that allow optical transitions to occur efficiently. Engineering of an injector stage leading into the dot can provide current injection into an upper dot state; however, to increase the likelihood of an optical transition, the lower dot states must be emptied faster than upper states are occupied. The second issue is that SAQDs have significant inhomogeneous broadening due to the random size distribution. While this may not be a problem in the long term, this issue can be circumvented by using planar photonic crystal or plasmonic approaches to provide wavelength selectivity or other useful functionality.

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Alternative solutions are desired for mid-wavelength and long-wavelength infrared radiation detection and imaging arrays. We have investigated quantum dot infrared photodetectors (QDIPs) as a possible solution for long-wavelength infrared (8 to 12 {mu}m) radiation sensing. This document provides a summary for work done under the LDRD 'Infrared Detection and Power Generation Using Self-Assembled Quantum Dots'. Under this LDRD, we have developed QDIP sensors and made efforts to improve these devices. While the sensors fabricated show good responsivity at 80 K, their detectivity is limited by high noise current. Following efforts concentrated on how to reduce or eliminate this problem, but with no clear path was identified to the desired performance improvements.

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We report a semiconductor based mechanism for electrically controlling the frequency of light transmitted through extraordinary optical transmission gratings. In doing so, we demonstrate active control over the surface plasmon (SP) resonance at the metal/dielectric interface. The gratings, designed to operate in the midinfrared spectral range, are fabricated upon a doped GaAs epilayer. Tuning of over 25 cm{sup -1} is achieved, and the devices are modeled to investigate the physical origin of the tuning mechanism. Though our structures are designed for the midinfrared, the tuning mechanism demonstrated could be applied to other wavelength ranges, especially the visible and near infrared.

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Electroluminescence from self-assembled InAs quantum dots in cascade-like unipolar heterostructures is demonstrated. Initial results show weak luminescence signals in the mid-infrared from such structures, though more recent designs exhibit significantly stronger luminescence with improved designs of the active region of these devices. Further studies of mid-infrared emitting quantum dot structures have shown anisotropically polarized emission at multiple wavelengths. A qualitative explanation of such luminescence is developed and used to understand the growth morphology of buried quantum dots grown on AlAs layers. Finally, a novel design for future mid-infrared quantum dot emitters, intended to increase excited state scattering times and, at the same time, more efficiently extract carriers from the lowest states of our quantum dots, is presented,.

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The potential for implementing quantum coherence in semiconductor self-assembled quantum dots has been investigated theoretically and experimentally. Theoretical modeling suggests that coherent dynamics should be possible in self-assembled quantum dots. Our experimental efforts have optimized InGaAs and InAs self-assembled quantum dots on GaAs for demonstrating coherent phenomena. Optical investigations have indicated the appropriate geometries for observing quantum coherence and the type of experiments for observing quantum coherence have been outlined. The optical investigation targeted electromagnetically induced transparency (EIT) in order to demonstrate an all optical delay line.

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The thermal interdiffusion of AlSb/GaSb multiquantum wells was measured and the intrinsic diffusivities of Al and Ga determined over a temperature range of 823-948 K for 30-9000 s. The 77-K photoluminescence (PL) was used to monitor the extent of interdiffusion through the shifts in the superlattice luminescence peaks. The chemical diffusion coefficient was quantitatively determined by fitting the observed PL peak shifts to the solution of the Schroedinger equation, using a potential derived from the solution of the diffusion equation. The value of the interdiffusion coefficient ranged from 5.2 x 10{sup -4} to 0.06 nm{sup 2}/s over the conditions studied and was characterized by an activation energy of 3.0 {+-} 0.1 eV. The intrinsic diffusion coefficients for Al and Ga were also determined with higher values for Al than for Ga, described by activation energies of 2.8 {+-} 0.4 and 1.1 {+-} 0.1 eV, respectively.

We have investigated InAs quantum dots (QD) formed on GaAs(1 0 0) using metal-organic chemical vapor deposition. Through a combination of room temperature photoluminescence and atomic force microscopy we have characterized the quantum dots. We have determined the effect of growth rate, deposited thickness, hydride partial pressure, and temperature on QD energy levels. The window of thickness for QD formation is very small, about 3Å of InAs. By decreasing the growth rate used to deposit InAs, the ground state transition of the QD is shifted to lower energies. The formation of optically active InAs QD is very sensitive to temperature. Temperatures above 500°C do not form optically active QDs. The thickness window for QD formation increases slightly at 480°C. This is attributed to the thermal dependence of diffusion length. The AsH3 partial pressure has a non-linear effect on the QD ground state energy. © 2003 Elsevier B.V. All rights reserved.

n-type GaSb has been prepared by metal-organic chemical vapour deposition with tellurium donors using diethyltelluride as the dopant precursor. The maximum carrier concentration achieved was 1.7 x 10{sup 18} cm{sup -3}, as measured by van der Pauw-Hall effect measurements, for an atomic tellurium concentration of 1.8 x 10{sup 19} cm{sup -3}. The apparent low activation of tellurium donors is explained by a model that considers the effect of electrons occupying both the {Lambda} and L bands in GaSb due to the small energy difference between the {Lambda} and L conduction band minima. The model also accounts for the apparent increase in the carrier concentration determined by van der Pauw-Hall effect measurements at cryogenic temperatures.

Quantum dot nanostructures were investigated experimentally and theoretically for potential applications for optoelectronic devices. We have developed the foundation to produce state-of-the-art compound semiconductor nanostructures in a variety of materials: In(AsSb) on GaAs, GaSb on GaAs, and In(AsSb) on GaSb. These materials cover a range of energies from 1.2 to 0.7 eV. We have observed a surfactant effect in InAsSb nanostructure growth. Our theoretical efforts have developed techniques to look at the optical effects induced by many-body Coulombic interactions of carriers in active regions composed of quantum dot nanostructures. Significant deviations of the optical properties from those predicted by the ''atom-like'' quantum dot picture were discovered. Some of these deviations, in particular, those relating to the real part of the optical susceptibility, have since been observed in experiments.

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