All-optical generation of coherent in-plane charge oscillations in GaAs quantum wells

Abstract

We have induced ultrafast charge oscillations in the plane of an unbiased and undoped (110)-oriented GaAs quantum well by coherent optical excitation of heavy-hole and light-hole exciton transitions. The oscillations arise from an in-plane charge displacement between heavy-hole and light-hole states, which results from the periodic parts of the Bloch wave functions and the (110) orientation. The observations are evidence for the existence of a substantial far-infrared transition-dipole moment between heavy-and light-hole subbands for in-plane wave vectors, which we estimate to be ∼0.5 eÅ for the quantum well under study. Our findings might prove important for designs of far-infrared detectors and emitters.Excitation of semiconductors with ultrashort optical pulses leads to a polarization that coherently couples to the electric field of the excitation pulse. This coherent excitation is responsible for the initial, ultrafast nonlinear response of optically excited semiconductors and has been studied in great detail for both free-carrier and exciton excitations for two decades. 1 One of the fascinating phenomena linked to this coherent regime is the occurrence of quantum beats and the accompanying charge oscillations, the study of which has provided fundamental insight into ultrafast light-matter interactions. [2][3][4][5] Quantum beats occur in a three-level system in which two excited states are coupled via a common ground state. 2,3,6-8 If a transition-dipole moment exists between the two excited states, the quantum beats will be accompanied by charge oscillations with a frequency corresponding to the energy difference between the two excited levels. 4,5,9,10 Thus, in semiconductor heterostructures, a measurement of the simultaneously emitted electromagnetic radiation, whose frequency is usually within the THz range, is direct evidence of an intersubband transition-dipole moment which cannot be studied by all-optical techniques such as four-wave mixing and transmission measurements. 4,6 So far, charge oscillations have been measured in a coupled double or in a single asymmetric quantum well (QW) employing beating between two electron subbands 5,9 and in a single QW employing beating between heavy-and light-hole subbands. 4 Specifically, the charge oscillations studied in Refs. 4, 5, and 9 resulted from a displacement of the slowly varying envelope function along the growth direction of the QW.In this paper we report the observation of charge oscillations between heavy-and light-hole exciton states occurring in the plane of an undoped GaAs QW. We take advantage of the (110) orientation of the QW, in which optically excited heavyand light-hole states lead to oppositely oriented, macroscopic polarizations in the plane of the QW. 11,12 This in-plane charge displacement results from the periodic parts of the Bloch wave functions and, thus, is of a different physical nature than the one linked to previously observed out-ofplane oscillations. 4,5,9 Our observations prove the existence of a substantial transition-dipole ...

Effects of non-exciton components excited by broadband pulses on quantum beats in a GaAs/AlAs multiple quantum well

“…Coherent exciton oscillations, such as quantum beats and Bloch oscillations, have been extensively studied to understand the ultrafast exciton dynamics 1 2 3 4 5 6 7 8 9 , the interaction of excitons with phonons 10 11 12 13 14 15 16 17 18 , the determination of the splitting energies in inhomogeneous systems 19 20 21 , and applications of ultrafast devices 22 23 24 . However, one problem has been the difficulty of generating a clear and long-lived oscillation.…”

“…However, one problem has been the difficulty of generating a clear and long-lived oscillation. In general, to generate a strong oscillation, the pump pulse needs to be tuned to the central energy of the two exciton states 6 12 . For example, when an ultrashort pulse laser with a broad spectral width is tuned to the central energy of heavy-hole (HH) and light-hole (LH) excitons split by a quantum size effect, an HH-LH exciton quantum beat with the maximum amplitude is obtained.…”

All-Optically Induced Ultrafast Photocurrents: Beyond the Instantaneous Coherent Response

“…Electronic motion now can be controlled with a femtosecond or even subfemtosecond precision using ultrashort laser pulses. Among other applications, such coherent control schemes have been shown to result in all-optically induced currents which can be linked to nonlinear optical processes [4][5][6][7][8][9][10][11][12][13]. For the lowest-order nonlinear process, i.e., second order in electric field, the currents resulting from above-bandgap excitation can be classified into two types: (i) injection current or circular photogalvanic current [4,8,14,15] and (ii) shift current or linear photogalvanic current [4,5,9].…”

Photocurrents in GaAs quantum wires including excitonic effects

“…Depending on the formation mechanism, photocurrents can be classified into different types: ballistic, shift, and rectification currents. These photocurrents have been investigated in bulk materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and quantum wells (QW) [15][16][17][18][19][20][21] but have not received much attention in one-dimensional (1D) systems such as quantum wires (QWR). Here, we present a theoretical study of the photocurrents in semiconductor QWRs.…”