A. R. Albrecht

Optically Pumped Frequency Reconfigurable Antenna Design

This letter presents a novel frequency reconfigurable antenna design using photoconductive silicon elements as optical switches. By illuminating these silicon elements with light of suitable wavelength, their physical properties can be altered from that of a semiconductor to almost metal-like, which in turn alters the radiation properties of the antenna structure. Our work builds on similar work conducted in the past, but goes further by demonstrating a new geometry for coupling the light energy onto the silicon switches, thereby facilitating conformal integration of such reconfigurable antennas into next-generation wireless devices. In this letter, we first present a theoretical model characterizing the behavior of silicon substrate under light illumination. We then present experimental results on a stripline circuit employing a single silicon switch under light illumination and compare the theoretical model to experimental measurements. Finally, a novel frequency reconfigurable antenna design utilizing our new coupling geometry is designed, and its experimentally measured RF performance is compared to numerical simulations.

Room-Temperature Operation of Buffer-Free GaSb–AlGaSb Quantum-Well Diode Lasers Grown on a GaAs Platform Emitting at 1.65

Buffer-free growth of GaSb on GaAs using interfacial misfit (IMF) layers may significantly improve the performance of antimonide-based emitters operating between 1.6 and 3 mum by integrating III-As and III-Sb materials. Using the IMF, we are able to demonstrate a GaSb-AlGaSb quantum-well laser grown on a GaAs substrate and emitting at 1.65 mum, the longest known operating wavelength for this type of device. The device operates in the pulsed mode at room temperature and shows 15-mW peak power at -10degC and shows high characteristic temperature (To) for an Sb-based active region. Further improvements to IMF formation can lead to high-performance lasers operating up to 3 mum.

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This paper presents a reconfigurable radio front-end antenna scheme suitable for cognitive radio communications. Our scheme comprises of an UWB antenna structure (antenna-1) and a reconfigurable antenna structure (antenna-2) incorporated together on the same substrate. The UWB antenna-1 is used for channel sensing while the reconfigurable antenna-2 is designed to frequency-hop between pre-determined communication channels. The reconfiguration of antenna-2 is achieved by using photoconductive switches which are selectively illuminated by laser light from a series of integrated laser diodes. A prototype was fabricated and tested to prove the applicability of our proposed radio front-end scheme.

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This paper presents a new reconfigurable antenna design using optical switching which overcomes any biasing associated with standard MEMs and PIN switches. In our approach, the switching elements comprises of doped silicon, and the change in the elements RF conductivity from that of a semiconductor material to that of a metal-like material is achieved upon exposure to a laser light coupled through an optical fiber cable. Two prototype antennas are fabricated and tested to demonstrate the proposed approach. Good qualitative agreement is observed between the simulated and measured data.

Implementation of a cognitive radio front-end using optically reconfigurable antennas

We demonstrate the ability to form either coupled or isolated patterned quantum dot (PQD) ensembles on nanopatterned GaAs pyramidal buffers. The coupled PQD “clusters” consist of close-spaced PQDs with inter-QD spacing of 5nm. The isolated PQD “pairs” are comprised of two PQDs well separated by 110nm. The photoluminescence behavior, measured in integrated intensity, linewidth, and emission peak as a function of excitation intensity and temperature, indicates lateral coupling within the QD clusters and an isolated nature for QD pairs. The ability to tailor PQD formation and subsequent carrier recombination characteristic may prove useful in developing PQD-based devices for optical computing applications.

Optically pumped reconfigurable antenna systems (OPRAS)

Optical refrigeration (solid-state laser cooling) is based on the principle of anti-Stokes fluorescence where incident light from a coherent (low entropy) source such as laser is upconverted into high entropy fluorescence via absorption or removal of vibrational energy (phonons) [1]. This phenomenon has been termed “a laser running in reverse” as it mimics the reverse operation of a solid-state laser. Since its first experimental observation in 1995 [2], optical refrigeration in a variety of rare-earth doped glasses and crystals has been demonstrated [3]. A major milestone was achieved in 2010 by cooling a 5% Yb:YLF crystal to an absolute temperature of 155K from room temperature [4]. This represented the first demonstration of an all-solid-state cryocooler, whereby achieving lower temperatures than that of standard multi-stage Peltier coolers. Exploiting the E4-E5 electronic resonance in the Yb<;sup>3+<;/sup> Stark manifold was essential in achieving these results [4]. Subsequently, the Yb:YLF cooler was employed to cool a semiconductor load (GaAs) to 165K [5]. Most recently, we have cooled a 10% doped Yb:YLF to115K from room temperature. Figure 1(a) reflects this progress history since 1995 by depicting the lowest temperature achieved in Yb doped glasses and crystals versus year. Fig. 1(b) shows the expected cooling efficiency in the record 10% doped Yb:YLF sample as a function of excitation wavelength and temperature. The minimum achievable temperature in this sample is ~<;90K and is expected to be further lowered towards 70K by improving the purity during crystal growth [6].In this talk, we present an overview of the recent advances in optical refrigeration and outline a prospective study for practical application of this technology. Various pumping schemes (intracavity [7] and external cavity [8] enhancements), implementation of a practical thermal link, and various techniques for improving the overall efficiency will be discussed. As the only all-solid-state cryocooling technology with no moving parts (vibration free), and immune from electromagnetic interference, it can boost numerous applications ranging from spaceborne sensors to high-Tc superconducting electronics prevalent, for example, in the medical devices.

Optical properties of patterned InAs quantum dot ensembles grown on GaAs nanopyramids

InAs quantum dots embedded in InGaAs quantum well (DWELL) structures grown by metal-organic chemical-vapor deposition on nano-patterned GaAs pyramids and planar GaAs (001) substrate are comparatively investigated. Photoluminescence (PL), PL excitation, and time-resolved PL measurements demonstrate that the DWELL grown on the GaAs pyramids has a broad QW PL band (FWHM ~ 90 meV) and a better QD emission efficiency than the DWELL structure grown on the planar GaAs (001) substrate. These properties are attributed to the InGaAs QW with distributed thickness profile on the faceted GaAs pyramid, which introduces tapered energy band structure and assists the carrier capture into the QDs. This research provides useful data for further improving the performance of DWELL structures for device applications.

Solid state optical crycoolers: Developments and prospective

We demonstrate the capability to control patterned quantum dot formation through adjusting the growth parameters inside the MOCVD reactor. Effects of altered crystallographic structure are measured using photoluminescence and SEM images