THz radiation and acoustic phonon pulse wave from GaN-based light emitting diode structures generated by ultra short pulse lasers

We discuss the generation mechanism of THz radiation and acoustic phonon pulse wave under ultra short pulse excitations in GaN-based light emitting diode (LED) structures containing InGaN/GaN multiple quantum wells. In order to understand the role of piezoelectricity in the THz radiation and acoustic phonon pulse wave generations, an external field was applied in these structures so that the piezoelectric field in the quantum wells was compensated under an external reverse bias. Coherent acoustic phonon pulse wave was found to be independent of the applied voltage, although the strain of the InGaN layers was crucial for the generation of the signals. The THz emission from these structures was found to increase with increasing reverse voltage and excitation energy, similar to the trend of the photocurrents in these structures. The bias and wavelength dependence of the THz generation suggests the carriers associated with the photocurrents are responsible for the THz radiation.