Laser pulse energy localization in a photoconductive THz emitter via sapphire fibers

We report a seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via a focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at a certain ratio between the fiber diameter and antenna gap size ($d/g\sim$ 22.5), one can attain a $\sim$35-fold enhancement of laser irradiation in the vicinity of the PCA electrodes. This provides the formation of subwavelength electromagnetic wave caustics located at the edges of the PCA electrodes, which potentially promotes an increase in the optical-to-terahertz conversion efficiency.