8 September 2004 Pulsed and widely tunable terahertz sources for security: imaging and spectroscopy
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Abstract
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 surface potential follows each cycle of the incident radiation. Electron emission responds to the electric field with a delay τ< 2 fs, and the current-voltage characteristics of field emission are highly nonlinear. Thus, photomixing in laser-assisted field emission can cause current oscillations that may be tuned from DC to 500 THz (1/delay). A field emission current density of 1012 A/m2 can be generated using a 20 pJ 70 fs pulse from a Ti:sapphire laser, to provide 200 W THz pulses. Microwave prototypes for 1-10 GHz are now being tested.
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Mark J. Hagmann, Mark J. Hagmann, B. A. McBride, B. A. McBride, Zulaima S. Hagmann, Zulaima S. Hagmann, } "Pulsed and widely tunable terahertz sources for security: imaging and spectroscopy", Proc. SPIE 5411, Terahertz for Military and Security Applications II, (8 September 2004); doi: 10.1117/12.540808; https://doi.org/10.1117/12.540808
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