We report on the biaxial response of photoluminescence to electric fields in GaAs quantum-well structures. For fields along [001], emission due to nominally forbidden excitons exhibits substantial differences between [110] and [1 TO] polarizations. Transitions which are allowed within the envelopefunction formalism show no noticeable anisotropy. In terms of symmetry, our observations relate to the linear electro-optic (Pockels) effect. The field dependence of the anisotropy is unusual in that it decreases with increasing field. Results can be qualitatively accounted for by perturbative analyses.PACS numbers: 78.20.Jq, 73.20.Dx, 73.40.Kp, 78.55.Cr Electric fields normal to the layers modify the optical properties of semiconductor quantum-well (QW) structures and superlattices in ways which depart appreciably from those affecting bulk materials [1][2][3][4][5][6][7][8][9][10]. This refers in particular to the quantum-confined Stark effect [1-4] and features such as Wannier-Stark localization and ladders [6-9] bearing on the problem of Bloch oscillations [10]. In this Letter, we report on a novel phenomenon relying on quantum confinement which relates to Pockels linear electrorefraction. Unlike previously considered phenomena where the optical uniaxial symmetry of the QW is maintained [1-9], the new effect manifests itself in a giant field-induced biaxial anisotropy (electropleochroism). A further associated distinction is that the relevant dependence on the field is odd, as opposed to even [11]. Enhanced Pockels-like anisotropy is observed in photoluminescence (PL), primarily from excitons such as eih\ showing weak coupling to photons at zero field [12] {eihj denotes states with one electron and one heavy hole in the ith and jth subbands, respectively). Consistent with recent near-gap measurements of linear electro-optic coefficients in GaAs-Al x Gaix As QW structures [13], we find no enhancement at the dominant e\h\ PL.In zinc-blende (point group Tj) semiconductors, the presence of [001] electric fields leads to biaxial behavior [14] where the anisotropy in the plane perpendicular to the field distinguishes [110] from [llO]. Important as it is for some applications, however, this (Pockels) effect remains very small [15] up to the breakdown field ( < 10 6 Vcm -1 ). Below the band gap, the field derivative of the refractive index n at F=0 is given by dn/dF
We report on time-resolved studies of electric-field domains in weakly coupled GaAs quantum-well structures. Photoluminescence and photocurrent experiments probe the motion of charged domain walls triggered by steplike changes in the illumination. Results reveal complex oscillations with frequencies in the range 4 -8 MHz. These findings are discussed in terms of a discrete drift model relying on negative-differential conductance due to resonant tunneling. Calculations give a global phase diagram and time behavior consistent with experiments.
The actual Beld distribution in superlattices under electric field domain formation is investigated by photoluminescence and Raman spectroscopy. Prom the measured subband spacings, we determine the magnitude of the Beld that corresponds to resonant alignment of subbands in adjacent wells.The electron occupation of higher subbands is probed by photoluminescence (PL) measurements. Comparing the results of higher subband PL and the current-voltage characteristics, it is shown that the high-Beld domain is always nonresonantly coupled with a field strength below the resonance value. The sudden increase in the current when the high-field domain extends over the entire superlattice is explained. Calculations of the field distribution based on a microscopic model support our experimental observations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.