Mechanism of terahertz emission from an asymmetric double quantum well

The Science and technology of ultrashort-pulse lasers have enjoyed much exciting progress. The past decade has seen the achievement of pulses shorter than 10 fs. Time-resolved optical spectroscopy with femtosecond-pulse lasers can be used to examine the electronic and phonon dynamic processes and their interaction in semiconductors. An asymmetric double quantum well is a very simple system, in which it is possible to study the interplay between quantum beating, tunneling and interactions with light and with the lattice. In this system it is possible to create a macroscopic coherent superposition of two different conduction sub-bands by femtosecond intersub-band photoexcitation of sufficient bandwidth. The two excited eigenstates form a wave packet, and each of these components evolves at different rates leading to an oscillating electric dipole moment. The dephasing time is mainly related to electron-electron interactions but elastic electron-LO-phonon scattering also gives an important contribution to the dephasing process. A microscopic study of the polaron lifetime in each of the three, previously designed, states gives valuable information on the behavior of electrons in the coherent stage, while in the thermalization stage it is the inelastic scattering coming from electronic transitions between the three states that is responsible for the decay of the terahertz radiation. The time evolution of populations is compared, within a classical phenomenological model, for scattering rates obtained under two different assumptions and it is found that the time evolution of the population is strongly dependent on these rates.