C. A. Wang

Room-temperature diode-pumped Yb:YAG laser

We have developed an efficient room-temperature ytterbium-doped YAG laser operating at 1.03 microm pumped by an InGaAs strained-layer diode laser operating at 968 nm. The threshold was 234 mW and 23 mW of output power was obtained for an absorbed pump power of 345 mW. This laser offers a number of advantages over AlGaAs pumped Nd:YAG lasers, such as broader absorption features, longer fluorescent lifetime, and lower thermal loading of the gain medium.

Recombination processes in doubly capped antimonide-based quaternary thin films

Recombination processes in antimonide-based materials for thermophotovoltaic (TPV) devices have been investigated using a radio-frequency (rf) photoreflectance technique, in which a Nd–YAG pulsed laser is used to excite excess carriers, and the short-pulse response and photoconductivity decay are monitored with an inductively coupled noncontacting rf probe. Both lattice-matched AlGaAsSb and GaSb have been used to double cap InGaAsSb active layers to evaluate bulk lifetime and surface recombination velocity with different active layer thicknesses. With an active layer doping of 2×1017 cm−3, effective bulk lifetimes of 95 ns and surface recombination velocities of 1900 cm/s have been obtained. As the laser intensity is increased the lifetime decreases, which is attributed to radiative recombination under these high-level injection conditions. Similar measurements have been taken on both TPV device structures and starting substrate materials for comparison purposes.

InGaAs/AlGaAs strained single quantum-well diode lasers with extremely low-threshold current density and high efficiency

Graded‐index separate‐confinement heterostructure InGaAs/AlGaAs single quantum well diode lasers emitting at 1.02 μm have been fabricated from structures grown by organometallic vapor phase epitaxy. Under pulsed operation, threshold current densities as low as 65 A/cm2, the lowest reported for InGaAs/AsGaAs lasers, have been obtained for a cavity length L of 1500 μm. Differential quantum efficiencies as high as 90% have been obtained for L=300 μm. Output powers as high as 1.6 W per facet and power conversion efficiencies as high as 47% have been obtained for continuous operation of uncoated lasers with L=1000 μm.

High‐power strained‐layer InGaAs/AlGaAs tapered traveling wave amplifier

High power, nearly diffraction‐limited cw performance has been obtained from a traveling wave amplifier, fabricated in a strained‐layer InGaAs/AlGaAs laser structure, with a laterally tapered gain region and with a cavity‐spoiling feature to prevent laser oscillation. The input beam diffracts as it propagates, efficiently filling the tapered active region. For input optical power of 85 mW from a Ti:sapphire laser, total cw output of 1.44 W has been achieved with 1.28 W in a central lobe with width less than 1.2 times the diffraction limit at 977 nm wavelength. Only 15 mW of power incident on the amplifier was sufficient to provide 1 W output into the central lobe.

High-power, strained-layer amplifiers and lasers with tapered gain regions

Laterally tapered gain regions designed to accommodate the diffraction of narrow single-lobe beams that have been used in both optical amplifiers and lasers are described. Amplifier output power of 3.5 W with 3.1 W in a 1.05 times diffraction-limited lobe and laser output power of over 4 W with approximately half the power in a 1.7 times diffraction-limited lobe have been achieved.<<ETX>>

High-quantum-efficiency 0.5 eV GaInAsSb/GaSb thermophotovoltaic devices

We report high-performance lattice-matched GaInAsSb/GaSb thermophotovoltaic (TPV) devices with a 0.5 eV band gap. The TPV structures were grown on GaSb substrates by organometallic vapor phase epitaxy at a lower temperature (525 °C compared to 550 °C) to improve the quality of the metastable GaInAsSb alloy. The 0.5 eV TPV devices exhibit external quantum efficiency as high as 60%, which corresponds to an internal quantum efficiency of 90%, assuming 35% reflection losses. This efficiency is comparable to the value measured for 0.53 eV devices. The ratio of the open circuit voltage to band-gap energy ratio decreases from 0.57 for 0.53 eV devices to 0.48 for 0.5 eV devices.

Flow visualization studies for optimization of OMVPE reactor design

Abstract Gas flow visualization studies have been performed in a vertical reactor tube incorporating a rotating disk susceptor. Smoke particles of TiO 2 transported in a stream of He gas are illuminated by a sheet of laser light. This technique permits detailed observation of gas flow patterns. To study the influence of the method of gas injection, the gas was injected through (1) a vertical pipe inlet coaxial with the tube, (2) four inlets tangential to the tube and (3) a radial inlet above a porous plug. The method of injection was found to have a critical effect on vortex formation, which is minimized by using the porous plug. Disk rotation can be effective in creating a uniform boundary layer at the disk surface. When the susceptor is heated, the flow is strongly perturbed by thermally induced convection, which can be reduced by lowering the operating pressure.

Femtosecond measurements of the nonresonant nonlinear index in AlGaAs

Time‐division interferometry with 430 fs tunable laser pulses is used for direct femtosecond measurements of the wavelength dependence of the nonresonant nonlinear index of refraction, n2, in AlGaAs waveguides at room temperature. Below band‐gap n2 values of ∼10−12 cm2/W are observed with resonant enhancement as the laser wavelength is tuned toward the band edge.

Quaternary InGaAsSb Thermophotovoltaic Diodes

InxGa1-xAsySb1-y thermophotovoltaic (TPV) diodes were grown lattice matched to GaSb substrates by metal-organic vapor phase epitaxy in the bandgap range of EG = 0.5 to 0.6 eV. InGaAsSb TPV diodes, utilizing front-surface spectral control filters, are measured with thermal-to-electric conversion efficiency and power density (PD) of nTPV = 19.7% and PD = 0.58 W/cm2, respectively, for a radiator temperature of Tradiator = 950 degC, diode temperature of Tdiode = 27 degC, and diode bandgap of EG = 0.53 eV. Practical limits to TPV energy conversion efficiency are established using measured recombination coefficients and optical properties of front surface spectral control filters which for 0.53-eV InGaAsSb TPV energy conversion are nTPV = 28% and PD = 0.85 W/cm2 at the above operating temperatures. The most severe performance limits are imposed by 1) diode open-circuit voltage (VOC) limits due to intrinsic Auger recombination and 2) parasitic photon absorption in the inactive regions of the module. Experimentally, the diode VOC is 15% below the practical limit imposed by intrinsic Auger recombination processes. Analysis of InGaAsSb diode electrical performance versus diode architecture indicates that VOC and thus efficiency are limited by extrinsic recombination processes such as through bulk defects

InGaAsSb thermophotovoltaic diode: Physics evaluation

The hotside operating temperatures for many projected thermophotovoltaic (TPV) conversion system applications are approximately 1000 °C, which sets an upper limit on the TPV diode band gap of 0.6 eV from efficiency and power density considerations. This band gap requirement has necessitated the development of new diode material systems never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice matched on GaSb substrates have achieved the highest performance. In this article we relate observed diode performance to electro-optical properties such as minority carrier lifetime, diffusion length, and mobility and provide initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.53 eV InGaAsSb diodes used in this analysis (active layer doping is 2×1017 cm−3) the dark current density measured is 2×10−5 A/cm2 versus a potential Auger and/or a radiative limit of 2×10−6 A/cm2 (no photon recycling), and an abso...