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 describe experiments demonstrating the generation of ultrafast, high strain rate acoustic waves in a precompressed
transparent medium at static pressure up to 24 GPa. We also observe shock waves in precompressed aluminum with
transient pressures above 40 GPa under precompression. Using ultrafast interferometry, we determine parameters such
as the shock pressure and acoustic wave velocity using multiple and single shot methods. These methods form the basis
for material experiments under extreme conditions which are challenging to access using other techniques.
We present examples of two-dimensional photonic crystal band structures which defy the conventional wisdom that a photonic crystal provides optimal confinement for frequencies at the middle of the photonic band gap. This is due to the presence of an ultra-flat photonic band that leads to enhanced confinement in an adjacent band gap at frequencies near the band gap edge and far from midgap.
We have predicted that reversed and anomalous non-relativisitic Doppler shifts can be observed under some circumstances when light reflects from a shock wave front propagating through a photonic crystal. This theoretical prediction is generalizable and applies to wave-like excitations in a variety of periodic media. The first experimental observation of a non-relativistic reversed Doppler effect has recently been made by Seddon and Bearpark in a creative experiment involving the propagation of an electromagnetic shock through a periodic electrical transmission line. We show how our theory quantitatively describes this experiment and how the theory is fundamentally different from the theoretical description proposed by Seddon and Bearpark.
Unexpected and novel new physical phenomena result when light interacts with a shock wave or shock-like dielectric modulation propagating through a photonic crystal. These theoretically predicted new phenomena include the capture of light at the shock wave front and re-emission at a tunable pulse rate and carrier frequency across the photonic crystal bandgap, and bandwidth narrowing as opposed to the ubiquitous bandwidth broadening. To our knowledge, these effects do not occur in any other physical system.
Reversed Doppler shifts are also predicted to be observable. The
generality of these effects make them amenable to observation in a variety of time-dependent photonic crystal systems, which may have interesting technological implications.