Our simulations and experiments demonstrate a new physical mechanism for detecting acoustic waves of THz
frequencies. We find that strain waves of THz frequencies can coherently generate radiation when they propagate
past an interface between materials with different piezoelectric coefficients. By considering AlN/GaN
heterostructures, we show that the radiation is of detectable amplitude and contains sufficient information to
determine the time-dependence of the strain wave with potentially sub-picosecond, nearly atomic time and space
resolution. This mechanism is distinct from optical approaches to strain wave measurement. We demonstrate
this phenomenon within the context of high amplitude THz frequency strain waves that spontaneously form at
the front of shock waves in GaN crystals. We also show how the mechanism can be utilized to determine the
layer thicknesses in thin film GaN/AlN heterostructures.
We investigate the phase transitions of intense ultrashort laser-heated solids, from the cold solid to the hot dense plasma state, by measuring the complex electrical conductivity (or refractive index) transients at terahertz (1 THz = 1012 Hz) frequencies. Using optical-pump, terahertz-probe spectroscopy, we measured the phase shifts and absorption of terahertz probe pulses that were reflected from the warm dense plasma. To characterize the THz field, we developed and used a single-shot, high-temporal-resolution THz diagnostic capable of measuring free-space electromagnetic pulse fields in time and space. Due to relatively large focal spot sizes of the THz probe (~mm), mainly limited by the diffraction properties of THz radiation, the optical pump pulse was weakly focused onto the target in order to overfill the THz probe spot size with a peak intensity of ~1013 W/cm2. In contrast to the previous measurements of conductivities at optical frequencies, our THz non-contact probe method can directly measure quasi-DC electrical conductivities, providing insight into the transport nature of warm dense matter and any present discrepancies with the Drude model. In case of warm dense aluminum, we observe a noticeable deviation from the Drude model even in the ~1013 W/cm2 laser intensity regime. In addition, we observe strong coherent THz emission produced by a current surge in the laser-produced plasma.