Fermi-edge singularities and enhanced magnetoexcitons in the optical spectra of GaAs/(Ga,Al)As single quantum wells

Abstract

Pronounced many-body exciton effects have been observed through photoluminescence in modulation-doped n-type GaAs quantum wells where a near-resonance coincidence exists between a transition involving two-dimensional electrons at the Fermi level in the first conduction subband and an exciton transition from the second conduction subband. Under these conditions, strong and distinct Fermi-edge singularities can be observed near k=0. A magnetic field induces further enhancement of the effect, but also leads to very large, 8 -periodic amplitude variations in the photoluminescence, with excursions over 3 orders of magnitude.There has been a surge of interest in the study of a two-dimensional (2D) electron gas in quantizing magnetic fields by optical methods, motivated in part by attempts to develop spectroscopic probes of the quantum Hall effect ' For example, we have reported recently on results in (In, Ga)As single quantum wells (SQW) where the connection to the integer QHE was argued to have been realized for a particular sample design in terms of optimizing its optical properties as a probe of the 2D electron gas. This design calls for a one-sided modulation-doped quantum well where the Fermi level (EF) lies very near a particular unoccupied conduction subband (typically n, =2). Under such circumstances, the dominant features in the recombination spectrum for such a nearly degenerate circumstance can show pronounced many-body exciton effects, which are closely related to the "Fermi-edge singularities" (ES) observed through photoluminescence or absorption in modulation-doped quantum wells (MDQW), albeit now under specific conditions.In terms of an optical probe, apart from the near energetic degeneracy between the second conduction subband (n, =2) and EF, the details of the heterostructure design also include careful consideration of the electron-hole (eh) wave-function overlap. 'In the asymmetric heterostructures considered here, this overlap can, in principle, be adjusted over a large range by constraining the carriers to opposite sides of the SQW. Hence, the e hCoulomb-(excitonic) energy is also "tunable. "The principal aim of this paper is to show how, within a relatively narrow range in the electron-sheet density (n, ) and the e-h separation, pronounced many-body contributions to interband excitonlike transitions are present in ntype GaAs MDQW's, as viewed through photoluminescence (PL). The application of magnetic fields produces striking effects for hybridized "magnetoexciton" resonances both in terms of amplitude and spectra. Our data suggest that the ES effects are, if anything, enhanced in magnetic fields in ways which suggest their use as an optical probe of the 2D electron gas. At the same time, the experiments also raise questions about the photoholes in terms of their perturbation of the 2D system. This is, needless to say, a critical issue in optical studies of collective 2D electron phenomena such as the fractional QHE and the Wigner crystallization. Figure 1 shows a portion of the photoluminescen...

Magnetophotoluminescence study of theGa0.5In0.5P/GYamashita1, Matsuura2, Wada3 et al. 2002Phys. Rev. B5250View full textAdd to dashboardCite

“…Figure 8 shows the PL intensities for peaks C ͑closed triangles͒, E 0 ͑closed circles͒, and F ͑open circles͒ as a function of the perpendicular magnetic field. The intensity of the peak F is resonantly enhanced 34,35 at the magnetic field indicated by dashed lines in contrast to the intensities of peaks C and E 0 showing local minima. 35,36 As shown in the inset, the inverted values of B corresponding to the local maxima for peak F linearly depends on integer numbers.…”

“…The intensity of the peak F is resonantly enhanced 34,35 at the magnetic field indicated by dashed lines in contrast to the intensities of peaks C and E 0 showing local minima. 35,36 As shown in the inset, the inverted values of B corresponding to the local maxima for peak F linearly depends on integer numbers. Furthermore, we have confirmed that these oscillatory behaviors disappear at a lattice temperature of у30 K, in which peak F corresponding to the FES vanishes.…”

Many-body correlation effects in the ultrafast non-linear optical response of confined Fermi seas

“…In this section, the above results are extended to the multisubband case in order to investigate the ultrafast PP dynamics of the FES-exciton hybrid formed in asymmetric QW's with partially occupied subbands [169,170,37,171,172,173,174]. In such one-sided MDQW structures, interband optical transitions from the valence band to several conduction subbands are allowed due to the finite overlap between the hole and electron envelope wave-functions.…”

“…In such one-sided MDQW structures, interband optical transitions from the valence band to several conduction subbands are allowed due to the finite overlap between the hole and electron envelope wave-functions. When the exciton Fano resonance from a higher empty conduction subband is nearly resonant (within a few meV) to the Fermi level, the FES coming from the lowest occupied subband is enhanced by over two orders of magnitude [37,169,170]. Such many-body effects on the linear spectrum have been described by using the simple two-subband Hamiltonian [171,172,173]…”

“…With decreasing ∆ − E F − E B , the FES and the exciton become hybridized due to the intersubband scattering arising from the Coulomb interaction. This results in the transfer of oscillator strength from the exciton to the FES and a strong enhancement of the absorption peak near the Fermi level due to the resonant scattering of the photoexcited electron by the exciton level [169,170,37,171,172,173]. Since the PP signal is linear in the probe field, the essential physics can be captured by assuming a δ-function probe pulse, E τ (t) = E τ e iωpτ δ(t − τ ), and a Gaussian pump pulse.…”