H. Q. Le

Picosecond GaAs‐based photoconductive optoelectronic detectors

A novel material deposited by molecular beam epitaxy at low substrate temperatures using Ga and As4 beam fluxes has been used as the active layer for a high‐speed photoconductive optoelectronic switch. The high‐speed photoconductive performance of the material was assessed by fabricating two devices: an Auston switch and a photoconductive‐gap switch with a coplanar transmission line. In a coplanar transmission line configuration, the speed of response is 1.6 ps (full width at half maximum) and the response is 10 to 100 times greater than that of conventional photoconductive switches. Since the material is compatible with GaAs discrete device and integrated circuit technologies, this photoconductive switch may find extensive applications for high‐speed device and circuit testing.

Observation of millimeter‐wave oscillations from resonant tunneling diodes and some theoretical considerations of ultimate frequency limits

Recent observations of oscillation frequencies up to 56 GHz in resonant tunneling structures are discussed in relation to calculations by several authors of the ultimate frequency limits of these devices. We find that calculations relying on the Wentzel–Kramers–Brillouin (WKB) approximation give limits well below the observed oscillation frequencies. Two other techniques for calculating the upper frequency limit were found to give more reasonable results. In one method we use the solution of the time‐dependent Schrodinger equation obtained by Kundrotas and Dargys [Phys. Status Solidi B 134, 267 (1986)], while in the other we use the energy width of the transmission function for electrons through the double‐barrier structure. This last technique is believed to be the most accurate since it is based on general results for the lifetime of any resonant state. It gives frequency limits on the order of 1 THz for two recently fabricated structures. It appears that the primary limitation of the oscillation freque...

Large room‐temperature effects from resonant tunneling through AlAs barriers

At room temperature, we have observed negative differential resistance in AlAs double‐barrier structures and a large hysteresis in the current‐voltage characteristic of a stack of five AlAs double‐barrier structures. The peak‐to‐valley ratio of the current was as high as 3.5:1 in a double‐barrier structure. To the best of our knowledge, this is the largest room‐temperature peak‐to‐valley ratio observed to date in a double‐barrier structure and the first report of a room‐temperature hysteresis in a stacked structure. These structures were grown by molecular beam epitaxy using thin AlAs barriers in GaAs. Both the first and second resonances were observed, and are well explained by simple tunneling theory assuming a value of 1.0±0.1 eV for the GaAs‐AlAs conduction‐band discontinuity seen by the tunneling electrons. This value is very close to the difference in conduction‐band energy at the Γ points found by using the accepted values of GaAs and AlAs band gaps with 65% of the band‐gap difference appearing in ...

Stark effect in AlxGa1−xAs/GaAs coupled quantum wells

Optical spectra near the band edges of AlxGa1−xAs/GaAs coupled quantum well structures are found to exhibit rich structure. Under the Stark perturbation, these transitions have behavior remarkably different from those associated with single quantum wells. Positive energy shifts and high sensitivity to electric fields have been observed and interpreted as evidence of well coupling. Results of a simple numerical calculation support this interpretation.

High‐power diode‐laser‐pumped InAsSb/GaSb and GaInAsSb/GaSb lasers emitting from 3 to 4 μm

Diode‐array‐pumped GaInAsSb/GaSb and InAsSb/GaSb double heterostructure lasers operated at 85 K yielded 95 mW average and 1.5 W peak power per facet at 3 μm, and 50 mW average and 0.8 W peak power facet at 4 μm. The highest operational temperature was 210 K for the 3‐μm quaternary and 150 K for the 4‐μm ternary.

Optimization of four‐wave mixing conversion efficiency in the presence of nonlinear loss

We analyze the effect of nonlinear loss on four‐wave mixing (FWM) conversion efficiency. Maximum conversion efficiency is geometry independent and equal to e−2|χ(3)/Im{χ(3)}|2 when nonlinear loss dominates. Optimum device length and operating conditions are obtained and theoretical results are verified with a picosecond pulse FWM experiment.

Low-loss high-efficiency and high-power diode-pumped mid-infrared GaInSb/InAs quantum well lasers

A 4 μm GaInSb/InAs type-II quantum well (QW) laser has shown a substantial improvement in internal loss and quantum efficiency, which has been a problem for this type of laser. It yielded 0.9–1.5 W peak, 90–150 mW average single-ended output for 0.1–1 ms pulses at 71 K, with a net power efficiency of ∼3.5%–4%. The power and efficiency are among the highest long-pulse results reported for any semiconductor laser of comparable wavelength. Comparison with similar QW lasers suggests that the improvement is a result of better material growth.

InAsSb/InAlAs strained quantum‐well lasers emitting at 4.5 μm

Strained quantum‐well lasers emitting at 4.5 μm have been fabricated. The laser structure, grown on a GaSb substrate by molecular beam epitaxy, consists of compressively strained InAsSb active layers and tensile‐strained InAlAs barrier layers, surrounded by AlAsSb cladding layers. Under electrical injection, the laser exhibited pulsed operation up to 85 K, with threshold current density of 350 A/cm2 at 50 K. Under optical pumping, the laser operated pulsed up to 144 K, with peak power at 95 K of 0.54 W.

Scalable high‐power optically pumped GaAs laser

The use of disk geometry, optically pumped semiconductor gain elements for high‐power scalability and good transverse mode quality has been studied. A room‐temperature TEM00 transverse mode, external‐cavity GaAs disk laser has been demonstrated with 500 W peak‐power output and 40% slope efficiency, when pumped by a Ti:Al2O3 laser. The conditions for diode laser pumping are shown to be consistent with available power level.

Effects of internal loss on power efficiency of mid-infrared InAs-GaInSb-AlSb quantum-well lasers and comparison with InAsSb lasers

Experimental studies of the lasing efficiency of optically pumped 4-/spl mu/m GaInSb-InAs-AlSb multiple-quantum-well (MQW) lasers that emitted >1-W peak power/facet at 80 K indicated that internal loss is the main factor that limits the power output. The internal loss coefficient and internal quantum efficiency were determined by measuring the lasing efficiency versus temperature for devices of different facet reflectivities and lengths. The internal loss coefficient was found to increase from /spl sim/18 cm/sup -1/ near 70 K to /spl sim/60-100 cm/sup -1/ near 180 K, while the internal quantum efficiency remained constant at /spl sim/47% (or /spl sim/67% with the correction for the finite absorption of the active region) from 70 to 130 K. The increase of internal loss and the decrease of external quantum efficiency versus temperature were found very similar to those of double-heterostructure InAsSb-GaSb lasers and were similarly interpreted in terms of intervalence band carrier absorption. Extrapolation of power performance for improved devices with lower internal loss indicated that high-efficiency multi-watt quasi-CW output with a broad-area brightness of /spl sim/1 MW/cm/sup 2/.sterad is possible.