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Fakultät Physik
A detail shot of a laser in the lab in dark green colors. © Roland Baege​/​TU Dortmund

Ultrafast optoacoustics in semiconductor nanostructures

In semiconductors, we focus on the coherent optoacoustics effects. We exploit the deformation potential mechanism, i.e., the strain-induced change of the bandgap. Accordingly, the dynamical strain carried by the propagating acoustic wavepacket or by localized monochromatic acoustic wave shifts the spectral position of the exciton resonance. It affects the optical properties of the studied structure and modulates dynamic processes such as light reflection, emission, or absorption.

The effects we have demonstrated are:

  • Modulation of exciton and polariton resonances in a quantum well [1,2] and a microcavity [3]. We have achieved a non-adiabatic regime when the modulation rate exceeds the decoherence time of the resonance. This leads to the chirping of the corresponding optical transitions and the formation of optical frequency combs.
  • Ultrafast acoustic modulation, both pulsed and quasi-monochromatic, of laser emission from microcavities with active media [4-6].
  • Switching of a polariton condensate in a microcavity pumped to the bistable regime [7].
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Modulation of laser emission from a microcavity with a QDs ensemble. (a) Experimental scheme with the optical excitation of a picosecond strain pulse and cw- pumping of a microcavity above the QDs exciton resonance. (b) Schematic illustration of the energy shift of the exciton resonance by uniaxial strain. (c) Spectra of the light emission of the studied microcavity at the equilibrium conditions (in absence of the strain pulse) collected normal to the cavity from its front (black trace) or parallel to the cavity from the side (red trace). (d) Change of emission intensity from the quantum-dot microcavity by incident and reflected strain pulses due to the strain-induced modulation of the QDs exciton resonance relatively to the cavity mode spectral position

Polaritonic detection of acoustic pulses with ultimate sensitivity

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Polariton probing of picosecond acoustic pulses. (a) Experimental scheme in which we detect the acoustic pulse propagating through a set of 30 GaAs/(Ga,Al)As quantum wells (MQW) by measuring transient reflectivity. The probe pulse photon energy is tuned to the spectral position of the polariton resonance. (b) Dispersion (black curve) and its derivative on strain (red curve) in the spectral vicinity of the polariton resonance. (c) Two exemplary transient reflectivity signals. The oscillations are due to the interference of the probe pulse reflected from the front surface and from the propagating acoustic pulse

In the case of an extended polaritonic medium, such as a superlattice or multi-quantum wells structure, the strain-induced energy shift of the exciton resonance strongly affects the permittivity in the corresponding spectral range. This results in an enhanced photoelastic effect, thereby improving the sensitivity of acoustic pulse detection. We have demonstrated that by measuring the transient reflectivity from such a structure, we can detect the propagating acoustic pulse, which carries the strain of only 10-9 corresponding to the lattice displacement of 10-16 m (100 attometer). There is no alternative technique providing such sensitivity to the coherent lattice dynamics.

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