Resonant generation of biexcitons in a GaAs quantum well

Abstract

Generation of cold excitons in GaAs quantum wells is shown to lead to enhanced condensation to the biexciton state under cw illumination. Resonant two-photon absorption to the biexciton state in the same structure is found to account for a quantum beating phenomenon involving the biexciton and exciton states in a more complex three-level system than hitherto investigated.PACS numbers: 71.35. +Z, 42.50.Md, 78.47.+p, 78.55.Cr Excitons in three-dimensional materials have been known for some time to be an important paradigm for the interacting boson system, where a precursor to BoseEinstein condensation is the formation of excitonic molecules, or biexcitons [1]. Only recently has it become clear that biexcitons also play an important role in the nonlinear optical response of GaAs quantum-well structures [2]. So far, however, there has been no systematic study of the behavior of biexcitons in GaAs under the conditions of resonant excitation which are implicit in the investigation of the excitonic resonances in the third-and higher-order optical nonlinearities which mediate fourwave mixing. In order to observe biexcitonic effects in GaAs it is necessary to work with quantum wells, since in two dimensions the binding energy of the excitonic molecule is increased over the bulk value by nearly an order of magnitude, to ^ 1 meV. For material of appropriate high quality the free exciton linewidth may be reduced to 0.5 meV thus permitting the spectral resolution of the biexciton [3][4][5][6]. This narrow, largely homogeneous, linewidth also ensures that the dephasing time for the excitonic system is of the order of picoseconds, so coherent transient effects may be resolved in the time domain if light pulses of duration ~ 1 ps interact with the excitons, Recent work has indicated that biexcitons play an important role in such coherent interactions, but there has been no clear corroboration by combined time-and frequency-domain measurements. In this Letter we report optical processes dominated by biexcitons generated in both two-photon and single-photon transitions in a single GaAs quantum well, and show that the biexcitons are involved in coherent processes over a range of biexciton densities.The simplest model of an exciton in GaAs treats it as a two-level quantum system. The excitons interact little at very low densities, but as the density is increased, pairs of excitons may bind weakly together to form excitonic molecules. These biexcitons add an extra level to the twolevel scheme, at an energy 2E^ ~ Eh, where E^ is the exciton energy and Eh the binding energy of the molecule. Three-level systems recently investigated by quantumbeating spectroscopy using self-diffraction include lightand heavy-hole excitons [7], exciton transitions split by spatial variations in well thickness [8], and free and bound excitons [9]. For each of these systems the optical emission process ends with the crystal in its ground state. In the case of the system of excitons and biexcitons, the biexciton state may be reached by two-photon absorpt...

Quantum beats due to excitonic ground-state splitting in colloidal quantum dots

“…The period of the oscillations increases as the time delay τ increases 32,33 . However, biexcitons in the presence of strong inhomogeneous broadening are shown to generate beats in the time integrated FWM signal [34][35][36][37] . In fact, the time integrated FWM technique allows one to measure precisely the biexciton binding energy in an inhomogeneously broadened system, where the inhomogeneous broadening exceeds the biexciton binding energy.…”

Binding of biexcitons in GaAs/AlxGa1−Mizeikis1, Birkedal2, Langbein3 et al. 1997Phys. Rev. B18180View full textAdd to dashboardCite

“…With resonant excitation, biexcitons can be created via two-photon transitions with a single exciton as an intermediate state. 12 E b xx in this case can be determined from the energy difference between the biexciton and exciton resonance enhancements in the TFWM spectrum, representing the third-order nonlinear susceptibility, or from the period of quantum beats in the TFWM signal decay. 7 The spectra of the linear optical density of all samples are shown in Fig.…”

“…The biexciton binding is enhanced in MQWs and E xx b varies within the range 1.0-2.6 meV, depending on the well width and degree of localization. 2,7,12,19 When the quantum well width varies from 80 to 160 Å, the ratio between the biexciton and exciton binding energies has been found to be constant 2 as described by Eq. ͑1͒.…”

Polarization-dependent quantum beats of four-wave mixing in the Luttinger model for bulk semiconductors

“…Polarization-dependent four-wave mixing ͑FWM͒ quantum beats [1][2][3][4][5][6][7][8][9][10][11] have been observed after simultaneous excitations of two optical transitions, associated with heavy hole and light hole. The signal magnitude and its beat phase were found to depend on the relative linear polarization of the pump and test pulses.…”

“…For instance, they predicted identical FWM intensities for the two cases: pump and probe having either parallel or perpendicular linear polarizations. Subsequently, a successful explanation was claimed to be given by the biexciton theory, 3,[5][6][7][8][9][10][11] invoking a phenomenological exciton-exciton coupling attributed to Coulombic interactions. However, it has remained obscure why the SBE would fail for the phenomenon in a semiconductor.…”