br Acknowledgements This work was supported as part of the

Acknowledgements
This work was supported as part of the S3TEC Energy Frontier Research Center funded by the US. Department of Energy, Office of Basic Energy Sciences under Award No. DE-SC0001299/DE-FG02-09ER46577. The authors would like to thank Clivia M. Sotomayor and Gang Chen for stimulating discussions. C.M. Sotomayor and Timothy Kehoe are also thanked for their help in fabricating the in-house samples, which were fabricated at the Catalan Institute for Nanoscience and Nanotechnology using facilities of the ICTS ‘‘Integrated Nano and Microfabrication Clean Room’’ (CSIC-CNM).

Introduction
Nowadays picosecond acoustics, pioneered by Thomsen et al. [1] in 1984, has become a powerful tool for studying the properties of various solid objects. The technique operates with acoustic wave packets with spectra spreading up to several Terahertz (THz) and corresponding wavelengths down to several nanometers. Such high frequencies and small wavelengths of the elastic waves allow applying ultrasonic methods, like imaging and defecting probing, on the nanometer scale [2,3]. Besides achievements related to extending the traditional MHz ultrasonic techniques to the THz range, picosecond acoustics resulted in studies of qualitatively new phenomena, like observation of acoustic solitons [4–6].
Since the end of the 1990s picosecond acoustic techniques are also used to probe semiconductor nanostructures containing quantum wells (QWs) [7–9] and quantum dots (QDs) [10]. These studies were aimed mainly on obtaining information about the electron–phonon interaction in the case of electrons (holes) being quantum-confined. In this case the dynamic strain η(t) produced by the acoustic wave in the semiconductor nanostructure may be considered as perturbation, which changes in time. The dynamic strain modulates the energies of the a01 (hole) states (e.g. the band gap) in the nanostructure, which results in modulation of the optical outputs governed by these quantum states. In particular, the dynamic strain modulates the optical frequency ω of the electron–hole (exciton) transition. This effect may be called ultrafast piezospectroscopic effect in analogy with the stationary piezospectroscopic effect, which governs the dependence of the electron energy on uniaxial strain [11,12].
Traditional MHz and GHz acoustics allow efficient modulation of ω such that the modulation amplitude ΔΩ is large enough to be seen in the spectrum of the optical signal [13,14]. In this case the modulation occurs on a timescale (the time that it takes the optical frequency to shift by ΔΩ) that is much longer than any other transient times (coherence, relaxation etc.). Therefore, the modulation occurs adiabatically and it is easily possible to follow the time dependence of the modulated optical frequency ω(t) from the light intensity and the optical spectrum. In picosecond acoustics, which operates in the sub-THz and THz frequency ranges, this simple adiabatic approximation may be not valid anymore because becomes comparable or even shorter than the transient time, which governs the corresponding optical phenomenon. The typical example is the chirping of the optical transition in reflectivity spectra of QWs, when is comparable to the coherence time of the excitons [15].
In the present paper, we describe three recent experiments, where picosecond acoustic techniques were applied to semiconductor optoelectronic nanostructures [16–18]. In these experiments it is essential that the modulation of the electron energies occurs on a picosecond time scale and the amplitude of this modulation is higher than the spectral width of the corresponding optical transition. The GaAs-based nanostructures used in the experiments contain a single semiconductor layer (QW or QD) where the electrons and holes possess quantum confinement, and this layer is embedded into an optical microcavity (MC) with high finesse. The MC photonic resonance overlaps spectrally with the electron–hole (exciton) optical transition in the semiconductor layer. The effects studied in the three presented experiments have a common basis through the nonadiabatic modulation of ω. However, there is a significant difference between the three experiments: in the first one (Section 2) the nonadiabatic character is related to the long coherence time > of the modulated optical resonance; in the second experiment (Section 3) the amplitude of the picosecond strain pulses is so high that the exciton state in the QW becomes destroyed which results in the ultrafast transition from the strong to the weak coupling regime for a QW-microcavity; in the last experiment (Section 4) we show how nonadiabatic processes in an active optical microcavity result in a giant modulation of the emission output for a vertical cavity surface emitting laser (VCSEL) with QDs.