Free-space coupling to symmetric high-Q terahertz whispering-gallery mode resonators.

We report on the coupling of a free-space Gaussian beam to symmetric high-quality (Q) whispering-gallery mode resonators (WGMRs) for terahertz (THz) radiation. We achieve very high excitation efficiencies up to 50% to THz WGMs with a Q-factor of 1.5×104 at 0.7 THz. The high coupling efficiencies have been realized by leveraging a Gaussian beam with a nearly diffraction-limited focal spot, as well as readily available low-loss, high-index silicon spheres with diameters comparable to the wavelength. The results convincingly underline the viability of free-space coupling in the THz frequency range.