  Even though the terahertz spectrum is well suited for chemical
identification, material characterization, biological sensing and medical
imaging, practical development of these applications has been hindered by the
attributes of terahertz sources, namely low output power and poor efficiency.
Here, we demonstrate that use of plasmonic contact electrodes can significantly
enhance the optical-to-terahertz conversion efficiency in a photoconductive
terahertz emitter. The use of plasmonic contact electrodes offers nanoscale
carrier transport path lengths for the photocarriers, enabling efficient
collection of the majority of carriers in a sub-picosecond time-scale. It also
allows increasing photoconductor active area without a considerable increase in
the capacitive loading to the antenna, boosting the maximum terahertz radiation
power by preventing the carrier screening effect and thermal breakdown at high
optical pump powers. We experimentally demonstrated 50 times higher terahertz
radiation power from a plasmonic photoconductive emitter in comparison with a
similar photoconductive emitter with non-plasmonic contact electrodes.
