Yuanjian Xu

Finite-difference time-domain calculation of spontaneous emission lifetime in a microcavity

We developed a general numerical method to calculate the spontaneous emission lifetime in an arbitrary microcavity, using a finite-difference time-domain algorithm. For structures with rotational symmetry we also developed a more efficient but less general algorithm. To simulate an open radiation problem, we use absorbing boundaries to truncate the computational domain. The accuracy of this method is limited only by numerical error and finite reflection at the absorbing boundaries. We compare our result with cases that can be solved analytically and find excellent agreement. Finally, we apply the method to calculate the spontaneous emission lifetime in a slab waveguide and in a dielectric microdisk, respectively.

Control of Electric Field Domain Formation in Multiquantum Well Structures

The formation, expansion, and readjustment of electric field domains in multiquantum well stacks is described and explained in terms of sequential resonant tunneling. These effects are used to control the multiband spectral response in IR detector applications of these structures.

High‐power operation of buried‐heterostructure strained‐layer InGaAs/GaAs single quantum well lasers

Very high power levels (330 mW) were obtained from single waveguide, single quantum well InGaAs/GaAs lasers. The lasers are shown to possess superior current threshold, temperature and optical damage characteristics.

Quantum interference effect and electric field domain formation in quantum well infrared photodetectors

An observation of quantum interference effect in photocurrent spectra of a weakly coupled bound‐to‐continuum multiple quantum well photodetector is reported. This effect persists even at high biases where the Kronig–Penney minibands of periodic superlattice potential in the continuum are destroyed. Our results show that electrons remain coherent over a distance of 40–50 nm. The observation was used to investigate electric field domain formation induced by sequential resonant tunneling in the superlattice. A large off‐resonant energy level alignment between two neighboring wells in the high field domain was observed.

Second quantized state oscillation and wavelength switching in strained‐layer multiquantum‐well lasers

The dependence of lasing wavelength on cavity length has been systemically studied in strained‐layer single, double, and triple quantum‐well InGaAs lasers. Lasing from the second quantized state has been observed for the first time in double and triple quantum‐well lasers. A wide range (∼500 A) wavelength switching between the first and second quantized states has been demonstrated by controlling the injection current and/or the operation temperature.

1.5 μm InGaAsP/InP buried crescent superluminescent diode on a p‐InP substrate

An InGaAsP/InP superluminescent diode (SLD) emitting at 1.54 μm has been fabricated. The device uses a buried crescent structure on a p‐InP substrate. The parameters were optimized for high output power, small spectral modulation, and smooth far‐field operation. The coherence function of the SLD emission was studied systematically. An output power of 5 mW, a coherence length of 41 μm, and a second coherence peak suppression ratio of 22 dB were obtained.

Monolithic integration of quantum well infrared photodetector and modulator

A modulation depth of 40% (0.7 dB/μm) was obtained with an infrared (10.6 μm) modulator consisting of a stack of 50 pairs of weakly coupled asymmetric quantum wells monolithically integrated with a quantum well infrared photodetector. The monolithic integration is shown to be a promising technique for the ‘‘ac’’ coupling of infrared focal‐plane arrays as well as for the direct study of the effects of electric fields and charge density variations on intersubband transitions.

Direct Measurement of Population-Induced Broadening of Quantum Well Intersubband Transitions

The dependence of the absorption spectral linewidth of quantum well intersubband transitions on the electron population in the well is experimentally demonstrated. We show that the dependence of the spectral linewidth on the population is substantial and accounts for some of the broadening previously attributed to donor scattering.

The effect of temperature on the resonant tunneling and electric field domain formation in multiple quantum well superlattices

Analyzing the photocurrent spectra and the I–V characteristics of weakly coupled GaAs/AlGaAs multiquantum well structures, different transport regimes are distinguished. At low temperatures (below ∼50 K), due to the electron coherence over a few periods of the superlattice, electron transport is dominated by sequential resonant tunneling. At higher temperatures, evidences for the increased contribution of nonresonant transport processes, and the subsequent modification in the electric field distribution in the device, are presented.

Structural and compositional control of the output wavelength of very high power 0.98 mu m GaInAs lasers for pumping fiber amplifiers

The authors report on the design, fabrication, and characterization of high-power strained-layer (SL) InGaAs single-quantum-well (SQW) lasers. The lasers emit a 0.980+or-0.002 mu m. They deliver over 100 mW CW optical power at room temperature in a stable single lateral mode with a beam divergence of 15 approximately=20 degrees . The maximum CW output power measured is 265 mW. The lasers have been successfully used as pump sources for an erbium-doped fiber amplifier.<<ETX>>