Impact of Metallization on the Performance of Plasmonic Photoconductive Terahertz Emitters

The use of plasmonic contact electrodes in photoconductive emitters is very effective for generating high terahertz powers. This is because plasmonic electrodes concentrate a major portion of photo-generated carriers in their close proximity when excited by an incident optical pump beam. As a result, a large number of photocarriers drift to the terahertz radiating elements of the emitter within a sub-picosecond time-scale to efficiently contribute to terahertz generation. Au is a desired choice of metal for plasmonic contact electrodes due to its strong plasmonic enhancement factors at near-infrared wavelengths. However, it requires an adhesion layer to stick well to device substrates. We show that optical and electrical characteristics of the Au adhesion layer have significant impact on performance of plasmonic photoconductive terahertz emitters. We demonstrate that use of Cr adhesion layer instead of Ti, which is used in most existing plasmonic terahertz emitters, offers 50% enhancement in the generated terahertz powers.