Probing radial electron-hole wavepacket dynamics in quantum wells with terahertz pulses

Abstract

The routine availability of sub-ps laser pulses has led to the preparation and probing of electronic wavepackets in Rydberg atoms. [1] An atom – initially in the ground state – is excited optically by a short pulse to a superposition of states centered at a large principle quantum number n. Both radially [2] and angularly localized [3] electronic wavepackets have been created. The wavepacket at various time delays may then be probed by field ionization using a pulsed electric field. [2-4] In semiconductor quantum wells (QW) optically excited electron-hole (e-h) wavepackets have also been observed; [5,6] however, such wavepackets differ markedly from their Rydberg-atom counterparts. In the QW, an optical pulse centered near the QW bandedge E0 and with a bandwidth that exceeds the 1s-exciton binding energy coherently excites from the crystal ground state excitons with principle quantum numbers ranging from n = 1 through the continuum. Moreover, the dipole selection rule for the excitation of e-h pairs from the crystal ground state dictates that only s states are created since these are the only states with nonzero e-h overlap. In Ref. [5], two-pulse four-wave mixing (FWM) experiments were performed on compressively strained Ino.o8Gao.92As/GaAs multiple QW’s excited by 110-fs optical pulses. Compressive strain in such structures substantially increases the light-hole-heavy-hole splitting, thus allowing the excitation of e-h pairs involving only the heavy-hole. The time-integrated (TI-) FWM signal as a function of time delay between the two pulses shows distinct beating when the laser center frequency is near resonance with E0.