|Title:||Ultrafast piezospectroscopy of semiconductor micro- and nanostructures|
|Abstract:||In this work experiments on the interaction of picosecond strain wave packets with semiconductor quantum wells and optical microcavities are presented. The focus is on the transient change of the semiconductors optical properties due to the applied dynamical strain. The strain pulses are injected into the sample structures using laser heating of a thin aluminum film, which was deposited on the backside of each sample. After propagating through a GaAs substrate, these strain pulses impact on the structured semiconductors at the front side of the sample. The induced changes in the optical properties are monitored by optical reflectance and photoluminescense spectroscopy. At first optical reflectance spectra of the cavity mode in a II-VI semiconductor based planar microcavity are presented. The modulation occurs when the strain pulse passes interfaces of the layered cavity structure at which the electric field has an antinode. Maximum modulation is reached when the pulse enters or leaves the central cavity layer. The mode energy shifts in the cavity with a finesse of about 2000 are comparable to its mode linewidth, which shows that the proposed technique is prospective for ultrafast optical switching. Further the effect of high amplitude strain waves on a II-VI quantum well is investigated. Acoustic solitons formed during the propagation of the picosecond strain wave in the GaAs substrate lead to exciton resonance energy shifts of up to 10 meV in the quantum well as well as to ultrafast frequency modulation, i.e., chirping, of the exciton transition. Both kind of semiconductor structures have been combined in a III-V semiconductor quantum well microcavity in the strong-coupling regime. A domain can be obtained in which large variations in the optical frequency are induced on time scales shorter than the polariton decoherence. Under these conditions characteristic sidebands which are spectral fingerprints of the terahertz modulation process appear in the reflectance spectra near the polariton resonance. In extreme cases of the polariton resonance energy modulation the strong coupling regime effectively is left. Thereby antiphase oscillations are observed concerning the intensity of the polariton modes and the photonic cavity mode. These oscillations occur on a timescale corresponding to the minimum normal mode splitting of the coupled system. Additionally ultrafast modulation of the photoluminescence intensity from the lower polariton branch is observed.|
|Appears in Collections:||Experimentelle Physik II|
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