We demonstrate the destruction and similarly fast recurrence of excitonic states in a semiconductor Micro Cavity (MC) with embedded quantum well by application of ps acoustic shock waves. We prove the exciton destruction by an ultrafast breakdown of the strong coupling regime of light matter interaction by sound.
Embedding a semiconductor quantum well (QW) in a MC enforces the light matter interaction in a characteristic way depending on the finesse of the cavity and the oscillator strength of the exciton. If the finesse is high enough, the interaction of the excitonic states of the QW and the photon field of the MC can no longer be treated perturbatively, but mixed states of exciton and photon called polaritons emerge. This is called the strong coupling regime.
In the experiment we study a 8-nm-wide In0.04Ga0.96As quantum well sandwiched between GaAs barriers and embedded in a Λ-cavity of GaAs/AlAs quarter wave stacks that form the Bragg mirrors. The finesse of the MC is about 10000 and the exciton linewidth is 0.6mV at 2K. The Rabi-splitting is 4meV with equal excitonic and photonic contribution to the polaritonic state, resulting in reflectivity dips of equal depth (fig 1a). The MC is grown on a GaAs substrate and on the backside of the substrate a 100nm aluminum film is evaporated that serves as a thermo elastic coupler for the generation of an acoustic shock wave by a high intensity fs-laser pulse (800nm, 100fs, 100 kHz repetition rate). The balancing of the non-linearity and dispersion for the acoustic wave travelling through the GaAs substrate towards the MC leads to the formation of acoustic solitons with an amplitude exceeding 10-3 containing frequencies up to 2THz (fig 1b, 1c). We measure the changes in reflectivity introduced by the acoustic shock wave using a pump-probe scheme. The time resolution is achieved by the variation of the time delay between the optical probe and acoustic pump pulse.