A new mechanism for observation of THz acoustic waves: Coherent THz radiation emission
EJ Reed and MR Armstrong and KY Kim and JH Glownia and WM Howard and EL Piner and JC Roberts, ULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XIII, 7214, 72140P (2009).
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.
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