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When optical radiation is focused on a nanoscale field emitter tip, the potential and emitted current follows each cycle of the radiation field. Nonlinearity causes mixing of the optical radiation from two lasers so that current oscillations from dc to over 500 THz can be created at the tip. Several different methods are considered for coupling these signals directly from the tip, while avoiding losses from shunting by the anode capacitance, and prototypes are described that use two of these methods. Analyses, which are consistent with measurements of the current oscillations in field emission that are caused by a transistor-transistor logic amplitude-modulated laser diode, show that this technique offers promise as a new method for wide-band-tunable terahertz sources.