Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

Abstract

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of ∼ 4 meV to zero, at a field of ∼35 kV/cm. This decrease, due to the fieldinduced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.PACS numbers: 78.47.+p, 71.70.Gm, 78.66.Fd Spin dependent exciton-exciton interactions in low dimensionality semiconductors have been studied extensively during this decade [1][2][3][4][5][6][7][8]. The first observation of an energy splitting between the two spin polarized components of an exciton gas without a magnetic field was reported by Damen et al. [1]. Since then this effect has been considered both experimentally [5,8] and theoretically [6,7]. Fernández-Rossier et al. attributed that splitting to a density-and spin-dependent modification of the two dimensional (2D) exciton binding energy (E 2D ) by two competing processes [6,7]. One of these many-body processes is the exchange interaction (I EC ) between electrons (and holes) of different excitons, which always increases E 2D in proportion to the spin-polarized exciton density, so that the change, ∆E ± 2D , is ∆E ± 2D ∝ n ± X I EC , where n ± X refers to the spin +1 and -1 exciton populations, respectively [9]. The second process is the vertex correction (I V C ) to the Coulomb interaction between electrons and holes of different excitons. This correction effectively reduces the electron-hole attraction due to the occupation of final states in the electron-hole scattering processes and lowers E 2D proportionally to n ± X I V C . Hence, neglecting the small coupling between +1 and -1 states due to valence band mixing, the total change of the exciton binding energy is ∆E ± 2D ∝ n ± X (I EC − I V C ).When the overlap between the electron and hole wavefunctions is large the vertex correction outweighs exchange effects (that is, (I EC − I V C ) < 0) and the exciton binding energy is reduced (with respect to that of a single exciton). Moreover, when n + X > n − X , the reduction is such that |∆E + 2D | > |∆E − 2D |, leading to a smaller binding energy of the +1 excitons than that of the -1. As a result an energy splitting δǫ between the components of a dense spin-polarized quasi 2D exciton gas is expected. Such a splitting has been observed in time-resolved photoluminescence (PL) experiments in quantum wells, in which a circularly polarized light pulse (σ + ) creates excitons with spin +1 that gradually relax into the spin -1 state, until both populations are equal. The PL spectrum corresponding to the spin +1 exciton has been found to be at higher energy than that of the -1 exciton,...

Exciton-exciton interactions in quantum wells: Optical properties and energy and spin relaxation

“…In a double well structure Aichmayr et al [14] detected an energy spin splitting dependence on the gate voltage that separated electrons and holes. As the voltage increased, the energy splitting decreased from 4 meV to zero.…”

“…A spontaneous energy splitting between the two different components of spin-polarized exciton gas in zero magnetic field was observed in multiple quantum wells [10][11][12][13] and in coupled quantum wells [14]. The splitting increased with the exciton density and decreased with the separation of the electron and hole induced by external electric field in coupled quantum wells.…”

Long-lived spin coherence of indirect excitons in GaAs coupled quantum wells

“…It has revealed electronic spin flips through a multi-component decay in polarisation-resolved spectroscopy (characteristic timescales are in the range of tens to hundreds of picoseconds [1][2][3]5]). In coupled quantum wells (CQWs) where electrons and holes are spatially separated, the spin relaxation of indirect excitons has been found very different [11][12][13]. Experiments have shown that when prepared in a pure state (| ± 1 ), indirect excitons exhibit a very long-lived polarisation [14] with a time decay that can even exceed the radiative lifetime (≈ 100 ns), i.e.…”

Nonlinear optical spectroscopy of indirect excitons in coupled quantum wells

“…The hole spin relaxation time τ h s should satisfy 1/τ h s < δ exc /h, where δ exc is the electron-hole exchange energy. Application of the electrical bias reduces the exchange energy 22 , and the condition is no longer fulfilled. Indeed, at V g > 0.2 V the Kerr rotation decay becomes monotonous.…”