C. Y. Sung

High-carrier-density electron dynamics in low-temperature-grown GaAs

Pump-probe differential transmission measurements examine high-carrier-density phenomena in as-grown and annealed GaAs samples grown at temperatures from 210 to 270 °C. We observe trap saturation and Auger recombination, and accurately model the measurements on annealed samples with a simple two level rate equation, allowing us to extract the trapped-electron lifetimes.

Femtosecond intersubband relaxation and population inversion in stepped quantum well

We have investigated intersubband relaxation rates in a stepped quantum well at room temperature using differential transmission spectroscopy with subpicosecond time resolution. The dynamics of the subband populations are derived from the experimentally observed reduction of oscillator strength of the corresponding exciton transitions. In the stepped quantum well the relaxation through longitudinal optical‐phonon emission from n=3 to 1 (25 ps) is slower than that from 2 to 1 (220 fs), due to the reduced wave function overlap and larger wave vector required for intersubband scattering. When the n=3 state is pumped, a population inversion between n3 and n2 (which are separated by 7 THz) is observed.

Studies of carrier relaxation in low dimensional structures

Abstract We have made theoretical and experimental studies of the carrier relaxation process in quantum wells and quantum wires. Monte Carlo studies of the relaxation process in wells and wires indicate that the relaxation times can be very large in narrow structures. Femtosecond differential transmission spectra measurements on similar structures grown by molecular beam epitaxy give relaxation times in good agreement with the theoretically calculated values.

Time-Resolved Femtosecond Intersubband Relaxations and Population Inversion in Stepped Quantum Wells

The generation of coherent far-infrared radiation in superlattices or multiple-quantum-well (MQW) structures has been a goal for many years. The use of intersubband transitions to generate coherent mid-infrared radiation was first successfully demonstrated by J. Faist et al. [1]. However, new structures are needed to generate radiation in the 30–300 mm regime (1–10 THz). In this paper, we demonstrate how a stepped quantum well structure can modify the intersubband relaxation rates, allowing a population inversion between subbands to be achieved. We measured the intersubband relaxation rates in the stepped quantum well with femtosecond differential transmission spectroscopy and experimentally observed the population inversion.