Lawrence Berkeley National Laboratory

Excitons are of fundamental interest and of importance for opto-electronic applications of bulk and nano-structured semiconductors. This paper discusses the utilization of ultrafast terahertz (THz) pulses for the study of characteristic low-energy excitations of photoexcited quasi 2D electron-hole (e-h) gases. Optical-pump THz-probe spectroscopy at 250-kHz repetition rate is employed to detect characteristic THz signatures of excitons and unbound e-h pairs in GaAs quantum wells. Exciton and free-carrier densities are extracted from the data using a two-component model. We report the detailed THz response and pair densities for different photoexcitation energies resonant to heavy-hole excitons, light-hole excitons, or the continuum of unbound pairs. Such experiments can provide quantitative insights into wavelength, time, and temperature dependence of the low-energy response and composition of optically excited e-h gases in low-dimensional semiconductors.