Sub-wavelength nano-electrode structures to improve the performance of terahertz photomixers

Abstract

By utilizing the sub-wavelength metallic structures in the active region of the photomixer, the confinement and concentration of electric field from optical pump lasers on a photoconductive substrate can be efficiently achieved as these sub-wavelength metallic structures are exhibiting the nano-antenna effect over a high index photoconductive substrate. Designing the sub-wavelength metallic structures, branch-like nano-electrodes structures, a new strategy to improve carrier capture was developed in which more carrier collection points occupy across the area of the pumping laser source. These branch-like nano-electrode structures were found to improve THz emission intensity of a photomixer by approximately one order of magnitude and optical-to-THz conversion efficiency by 10 times higher than that of photomixer with one row of nano-electrodes separated by the same 100nm gap. The enhancement is attributed to a more efficient collection of generated carriers due to a more intense electric field under the branch-like nano-electrodes structures. This is coupled with increased effective areas where strong tip-to-tip THz field enhancements were observed. The more efficient THz photomixer will greatly benefit the development of continuous wave THz imaging and spectroscopy system.