Terahertz emission from nanophononic heterostructures: the nexus between scale and origin

Newly found mechanisms of terahertz (THz) radiation via acoustic standing waves and piezoelectric fields in GaN-based strained heterostructures are discussed in terms of the relevant layer thickness. Two sample structures, with quantum well widths smaller (S1) or larger (S2) than the quasi-two-dimensional exciton Bohr radius, were investigated with increasing excitation energy. For comparative purposes, the differential reflectivity spectra (DRS) were also measured, and the signal strengths were proportional to the phase-space filling effect of quantum well carriers. The signal trace versus excitation energy in THz radiation compared to the DRS reveals that the radiation is closely related with the AlGaN/GaN interface (S1) and quantum wells (S2).