A focused laser diode (20 mW, 658 nm) causes an increase of 5% in the dc field emission current from a single-walled or multiwalled metallic carbon nanotube (MCNT), and the author's hypothesis that this effect is due to optical rectification is confirmed by analysis. Measurements with the MCNT at audio frequencies, and with microwave prototypes, as well as analyses, suggest that photomixing (optical heterodyning) with two such lasers would cause a terahertz mixer current with a peak value equal to 10% of the dc field emission current. Single-walled MCNT (SWMCNT) act as terahertz transmission lines with a characteristic impedance of 40 k Omega. Consider a cluster of 800 SWMCNT having a common junction at a 50 Omega load, with two lasers causing photomixing at the free end of each SWMCNT. The mixer current would cause a surface wave to propagate on each SWMCNT with a broadband impedance match to the load (50 Omega= 40 k Omega/800), so the mixer currents would add coherently at the load. Thus, a dc field emission current of 50 mu A causing a peak mixer current of 5 mu A at each of the 800 SWMCNT would provide a wideband-tunable terahertz output at a power of 400 mu W. (c) 2008 American Vacuum Society.