Giant electropleochroism in GaAs-(Al,Ga)As heterostructures: The quantum-well Pockels effect

Abstract

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<xn 3 r4\. Here, F is the z component of the field and r*\ is the electro-optic Pockels coefficient. Experiments on bulk GaAs in the vicinity of the fundamental EQ gap give A-41-1.5X10" 10 V-'cm, i.e., (dn/dF) F = 0 -10" 8 V" 1 cm, with no significant wavelength dependence [16]. The corresponding value for the imaginary part of the index k is not well known. An estimate using opticalsusceptibility x\V data at the EQ resonance [17] gives \dk/&F\f-o< 10~9 V _1 cm. It will become evident later that these values are too small to account for our QW findings.To discuss the mechanism for enhanced biaxial anisotropy, it is appropriate to focus first on the symmetry arg...

Interface-related in-plane optical anisotropy inGaAs/AlxChen1, Ye2, Wang3 et al. 2002Phys. Rev. B360452View full textAdd to dashboardCite

“…Kwok et al reported optical anisotropy of GaAs/Al x Ga 1Ϫx As multiple QW's under an electric field, which was known as the quantum-confined Pockels effect. 5,6 The unusual electric-field dependence of a forbidden transition could be well explained by the C 2v symmetry of interfaces. 8 The apparent effects of C 2v -interface symmetry on the optical anisotropy in In x Ga 1Ϫx As/GaAs QW's were evidenced by reflectance difference spectroscopy ͑RDS͒.…”

“…[5][6][7][8] For an ͑001͒-oriented SQW with perfectly abrupt interfaces at zϭϮw/2, the hole-mixing induced by an electric field and the two interfaces can be included in the frame work of the classical envelope function theory by a perturbation Hamiltonian, …”

“…A practical QW, however, always shows asymmetry in the growth direction to some extent, thus, the symmetry is reduced to C 2v . [5][6][7][8][9][10] An interface-related contribution to optical anisotropy is therefore expected, due to the broken balance of the anisotropy of the two interfaces.…”

“…Bulk asymmetry can be caused by an electric field or compositional variation across the QW, 5,6 while the difference in interface bonds, interface composition profile ͑segre-gation effect͒, the anisotropic-interface structures, etc., may cause the interface asymmetry. [7][8][9][10][11] We are interested in the interface-related optical anisotropy of the QW due to the unbalance of the two interfaces.…”

Quantum Coherence of Continuum States in the Valence Band of GaAs Quantum Wells

“…The HH-LH beats are by far more pronounced in the EOTC signal, which is dominated by the oscillatory contribution for resonant and OR excitation. The amplitude of the oscillations decreases by a factor of 4 for OR excitation, which is attributed to resonant contributions to the electro-optic effect in quantum wells [12], resonant anisotropic contributions to a coherent interband polarization [11], and the difference in the excitation density. The dephasing time of the beats decreases by less than a factor of 2 from resonant (t 700 fs) to OR excitation (t 360 fs).…”