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We studied GHz hypersound in heterostructures of ${\mathrm{SrRuO}}_{3}$ (SRO) and ${\mathrm{SrTiO}}_{3}$ (STO) by using femtosecond pulses with fluence as low as several tens of $\ensuremath{\mu}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$. We found that the efficiency of generating hypersound in SRO, through thermal expansion, is linear up to optical fluence of a few $\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$. We further investigated the propagation of acoustic pulses in the STO substrate. The acoustic properties of STO dramatically changed around the temperature of structural transition (105 K) because the generated hypersound was coupled to the softening mode. Nevertheless, our experimental results revealed that the quadratic frequency dependence of acoustic attenuation in STO, predicted by the mode-coupling theory, breaks down at tens of GHz. Finally, the refractive index of STO was experimentally obtained between 365 and 400 nm, for which there was disagreement in previous results.

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature ${T}_{c}$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1--2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

We present a theoretical model for the photogeneration of coherent acoustic phonons in a piezoelectric p-n junction. In our model, the transport of photoexcited carriers is governed by the drift-diffusion equation, whereas the dynamics of acoustic phonons obeys a loaded string equation. Among various mechanisms, the piezoelectric coupling is found to dominate the acoustic-phonon generation process. The waveform of the photogenerated acoustic pulse is strongly influenced by the various dynamics of the photoexcited carriers, especially the picosecond hole drifting. Our calculation also confirms the crucial role of the built-in electric field in the formation of coherent acoustic phonons under optical excitations.

We observed ultrafast electron distribution modulation following the strain oscillation of coherent acoustic nanowaves up to a THz frequency range. Through piezoelectric effects, the acoustic nanowave modifies the electronic subband structure, and further modulates the energy distributions of electrons and holes. With amplitude-fixed acoustic waves, a pronounced enhancement of transmission modulation due to acoustic modulation of electron distribution was observed when the optically probed energy position is close to the Fermi level.

Multiple harmonic components have been observed in the Fourier spectra of the coherent acoustic phonon oscillations in InGaN/GaN multiple quantum wells (MQW's). The dependence of the relative magnitudes of the first two Fourier components on the sample geometry was analyzed by conducting time-resolved pump-probe measurements on MQW samples with different well-to-barrier ratios. The results were also compared with the recently proposed loaded-string model [Phys. Rev. B 64, 235316 (2001)], and an overall agreement is obtained which validates the picture of string oscillation with inhomogeneous loadings. We also demonstrated manipulation of the spectral components using the coherent optical control technique. Our demonstration can be regarded as a first step to the realization of wave-form synthesis of nanoacoustic waves.