Dressed excitons within an incoherent electron gas: Observation of a Mollow triplet and an Autler-Townes doublet

Abstract

We demonstrate that the interaction between excitons and a sea of incoherent electrons does not preclude excitons dressing by light. We investigate if exciton-electron scattering plays some inhibiting role in the coherent light-exciton coupling by measuring the dynamical absorption spectrum of a modulation-doped CdTe quantum well that shows clear evidence for significant electron scattering of the excitonic states. We show the occurrence of dressed and correlated excitons by detecting quantum coherent interferences through excitonic Autler-Townes doublet and ac Stark splitting that evolves into a Mollow triplet with gain. We also evidence the partial inhibition of the electron-exciton scattering by exciton-light coupling. DOI: 10.1103/PhysRevB.77.121301 PACS number͑s͒: 78.67.De, 71.35.Cc, 71.35.Ee, 78.47.Ϫp The absorption spectrum of semiconductor quantum wells is dominated by a strong exciton resonance. Excitons are bound, conduction electron valence-hole pairs and are quite often compared to a two level system as a hydrogen atom. The role of many body interactions and exciton correlations in the exciton dynamical absorption spectrum has attracted much interest for several decades. The response of the exciton absorption to an incoherent and thermalized electronhole population obtained through nonresonant pumping in the continuum corresponds mainly to a broadening of the exciton resonance that is dominated by collision broadening ͑excitation-induced dephasing͒ rather than by a bleaching due to the saturation of the oscillator strength caused by phase space filling.1 The effect increases with electron-hole concentration. In the regime of coherent pumping at the exciton resonance, the light-field dresses the electron-hole gas and dynamical Stark splitting has been clearly observed in GaAs quantum wells embedded 2 or not 3,4 in a microcavity. Actually, in this coherent regime, the excitation-induced dephasing is found to be reduced as a result of the significant decrease of the Coulomb collision rates.5 As a consequence, the light-induced exciton dressing renders this many-body excitonic system comparable to a noninteracting atom system. 6 This issue is very important if one tries to extend coherent manipulations as those performed in atomic systems 7 to semiconductors in view of possible applications. In modulation-doped semiconductor quantum wells, excess carriers coalesce with excitons to form charged excitons ͑trions͒. The absorption spectrum is then modified: a trion resonance appears below the exciton resonance.8 However, many-body interactions also tend to become more complex. Electrons, for instance, are known to strongly screen the exciton oscillator strength 8,9 and to modify their linewidth and binding energy through exchange interactions.10 Thus, the shape of the exciton line, as well as that of the trion line, depends strongly on the strength of electron scattering with excitons and trions. Indeed, we have observed, as others before us, that a high-energy tail develops at both exciton and trion res...

Spectral features of the tunneling-induced transparency and the Autler-Townes doublet and triplet in a triple quantum dot

“…For the purpose of inhibiting the hole tunneling between valence bands, we here consider that the central dot is identical to the left dot but structural asymmetry with the right dot. This structural asymmetry is similar to the case shown in the double quantum dots system 36 , 37 , 42 – 44 . With a gate (bias) voltage being applied along the growth direction, as shown in Fig.…”

Ultrafast tristable spin memory of a coherent polariton gas

“…The ultimate limitation of the spin flip time is the time scale of the nonlinear spinor polaritonpolariton interactions, which is expected to be much faster. [34][35] [36] In the tristable regime, whereas the decay of a given spin population is as fast as in the bistable case, the rise time of the majority spin population is around 300 ps. Such a long time is absolutely unexpected in purely polaritonic systems, which usually feature dynamics in the range of a few ps.…”

Tunneling induced transparency and slow light in quantum dot molecules

“…2,3 Some of these effects were partially observed in single quantum dots (QDs). For example, it has been confirmed that under strong optical excitation, the interference between the exciton-biexciton transition produces a dip in the optical absorption line.…”