Ultrafast spectroscopy of semiconductors has provided extensive new information about dynamics of coherent processes, relaxation processes and transport processes in semiconductors during the past 30 years. The field continues to thrive with emphasis on accessing new physics and on new experimental techniques. We provide a brief overview of some recent developments and discuss how one new technique, femtosecond spectral interferometry, has provided new insights into the physics of resonant Rayleigh scattering.
In 1928 Bloch proposed that electrons in a solid subject to an electric field will undergo periodic oscillations in the momentum and real space. These Bloch oscillations have generated considerable interest and controversy since the original proposal. Semiconductor superlattices provide an ideal system for investigating Bloch oscillations. We review recent observations of Bloch oscillations in semiconductor superlattices excited by ultrashort light pulses and observed through Four-Wave-Mixing techniques and the coherent sub-millimeter-wave radiation generated by these oscillations. Negative differential resistance has also been observed in electrically biased superlattices. We also briefly discuss the relation between these two kinds of experiments.
We show that the initial dynamics of resonantly excited excitons in quantum wells is controlled by several processes, such as radiative recombination, spin-relaxation of excitons, electrons and holes, and scattering between different momentum states of excitons. We present results of experiments designed to simultaneously probe these processes. By a unified analysis of the results we extract quantitative information about radiative, spin-relaxation and momentum- relaxation rates, obtaining a good physical understanding of the initial dynamics of non- equilibrium excitons.
We present a combined experimental and theoretical study of the ultrafast internal thermalization of high energy carriers created by laser excitation. Luminescence up-conversion is used to monitor the spectral and temporal evolution of the photoexcited carrier distributions with a time resolution of about 100 fs. A Monte Carlo simulation joined with a molecular dynamics approach is then used to interpret the experimental results. We show that the coulomb interaction among carriers is responsible for the initial ultrafast thermalization. The simulation allow us to distinguish between binary carrier-carrier collisions and plasmon losses and reconcile the results obtained with time resolved vs. c.w. hot (e, angstroms) luminescence.
We present the results of a comprehensive investigation of spin-relaxation processes of electrons, holes and excitons in quantum wells using subpicosecond spectroscopy of luminescence polarization. Spin relaxation rates of electrons and holes are measured directly in modulation-doped quantum wells and give a good understanding of spin-relaxation processes of electrons and holes. We show that spin-relaxation dynamics of excitons, on the other hand, is quite complicated and is strongly influenced by their formation dynamics, many-body effects and localization dynamics. Although we have made good progress towards understanding exciton spin relaxation processes, some other outstanding issues will require further attention. We compare our results to those in bulk GaAs, and those in quantum wells obtained by other techniques.
We study resonant and nonresonant hole tunneling in an asymmetric double quantum well structure by picosecond timeresolved
photoluminescence. The tunneling times are directly determined by studying the luminescence decay time in one
of the wells. Various hole levels in the two quantum wells are brought in resonance by applying an electric field to the
doped layers which clad the inirinsic region containing the quantum well structures. The luminescence decay shows a sharp
resonance due to tunneling of carriers when two heavy-hole levels are brought in resonance. The tunneling time at
resonance, however, is much longer than expected from a simple theoretical model assuming a coherent tunneling process.
We develop a quantitative theory of resonant tunneling under the influence of scattering and relaxation processes. The
results predict large increases in the tunneling times in good agreement with the experimental observations.
We have investigated the formation of intrinsic excitons following excitation of electron-hole pairs
close to the bandgap by a subpicosecond laser pulse. We show that excitons form very rapidly
('c≤2Ops) and that they are initially in large wavevector states because of energy and momentum
conservation requirements. These non-thermal excitons then interact with other excitons and
acoustic phonons and relax very slowly (400ps) to the KO states which couple directly to light. This
leads to an extremely slow rise of exciton luminescence and unusual dependence of this risetime on
temperature, excitation density and excitation energy. These studies raise a number of fundamental
issues related to excitons in semiconductors.