Emission control of longitudinal optical phonon resonant thermal terahertz radiation from two-dimensional surface metal micro-structures on GaAs

Thermal emission of radiation resonant with the longitudinal optical (LO) phonon (LORE) has been studied using surface metal line-and-space structures on semiconductors. The emission perpendicular to the surface and the independence of the peak photon energy on emission direction are significant features, unlike those of surface phonon polaritons (SPhP). However, this emission is restricted to the optical polarization perpendicular to the stripes. In this research, two-dimensional (2-D) square-lattice and ring-type surface structures are fabricated and almost unpolarized LORE spectra are observed at 8.5 THz at 630 K, persisting to 323 K. Although the local spectrum minima due to optical absorption via SPhP show dependence on structural periods, the LORE peak energy is found to be robust. The LORE intensity averaged over polarizations is nearly twice that of the one-dimensional structure. In contrast, when fabricating a rectangular emission window with the side length short enough for the LO-phonon coherence, no LORE is observed. A finite-difference time-domain calculation suggests that this phenomenon is due to the disappearance of electric dipoles. Thus, two-dimensional structures enable control over the allowance and prohibition of THz wave emission, as well as its polarization.