  Photoconductive antennas and photomixers are very promising sources of
terahertz radiation that is widely used for spectroscopy, characterisation and
imaging of biological objects, deep space studies, scanning of surfaces and
detection of potentially hazardous substances. These antennas are compact and
allow generation of both ultrabroadband pulse and tunable continuous wave
terahertz signal at room temperatures, without a need of high-power optical
sources. However, such antennas have relatively low energy conversion
efficiency of femtosecond laser pulses or two close pump wavelengths
(photomixer) into the pulsed and continuous terahertz radiation,
correspondingly. Recently, an approach to solving this problem has been
proposed, that involves known methods of nanophotonics applied to terahertz
photoconductive antennas and photomixers. This approach comprises the use of
optical nanoantennas for enhancing the efficiency of pump laser radiation
absorption in the antenna gap, reducing the lifetime of photoexcited carriers,
and improving antenna thermal efficiency. This Review is intended to
systematise all the results obtained by researchers in this promising field of
hybrid optical-to-terahertz photoconductive antennas and photomixers.
