Pulsed and widely-tunable terahertz sources for security: Imaging and spectroscopy

Terahertz imaging and spectroscopy are being studied for inspecting packages and personnel, but advanced THz sources with much greater power are needed to increase the signal-to-noise ratio, and much greater frequency bandwidth to obtain more information about the target. Photomixing in resonant laser-assisted field emission is a new method that shows potential for increasing both the output power and frequency bandwidth by more than an order of magnitude. Tunneling electrons have a resonance with a radiation field, so a highly focused laser diode (670 nm, 30 mW) increases the emitted current enough to be seen with an oscilloscope, in good agreement with simulations. The electron-emitting tip is much smaller than optical wavelengths, so the potential follows each cycle of the incident radiation field. Electron emission responds to the electric field with a delay tau < 2 fs, and the current-voltage characteristics of field emission are highly nonlinear. Thus, photomixing in laser-assisted field emission can cause CW current oscillations that may be tuned from DC to 500 THz (1/tau). A field emission current density of 10(12) A/m(2) can be generated using a 20 pJ 70 fs pulse from a Ti:sapphire laser, to provide 185 W THz pulses. Microwave prototypes for 1-10 GHz are now being tested.