Distinctive Features of Mesa-Type Metal/Dielectric Surface Grating Structures Assisting Resonant Enhancement of Thermal Emission at Longitudinal-Optical Phonon Energy

Previously, a longitudinal optical (LO) phonon resonant emission (LORE) at 8.5 THz from Au-GaAs surface microstructures at 630 K is reported. This emission ascribed to thermally generated electric dipoles by the coherently vibrating charges at Au/GaAs/Au interfaces shows no dependence of the emission photon energy on the emission direction, which is different from the property of surface phonon polaritons. Herein, the advantage of surface grating structures with high mesa and narrow window width is shown. The experimental emission properties of samples with 1.4-6.0 mu m window width and mesa height up to 2.7 mu m and finite difference time domain simulations reveal the increase in mesa height increases the electric dipole moment, while it augments the reabsorption of LORE and the background radiation subject to Planck's law for a wide window width of 6 mu m. Reabsorption and the background emission can be reduced by adopting high mesa and narrow windows, which is due to the reduction of both the wire-grid polarizer function and the distribution of the electric field beyond the LO-phonon coherence inside the GaAs wafer. Theoretical simulation results suggest that a structure with a window width of 0.75 mu m and a mesa height of 2 mu m is an effective countermeasure.