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.
Field emission, quantum tunneling from the clean surface of a nanoscale conductor tip in vacuum, is an extremely fast process, where the instantaneous value of the current responds to the intense applied electric field with a delay of less than 2 fs. The cause for this intrinsic delay is shown to be the traversal time for quantum tunneling. Because the tip is much smaller than optical wavelengths, quasistatic conditions require that the potential of the tip must follow the instantaneous electric field in the imposed optical radiation. There is a resonance that is caused by virtual photon processes in which the electrons absorb single quanta from the radiation field while they are tunneling, to be promoted to energies where the wave function is reinforced by reflections at the classical turning points. Numerical solutions of the time-dependent Schrödinger equation show that the transient response to pulsed radiation consists of beating of the radiation with this resonance, and is intensified by the resonance. Experiments show that when a field emission tube is used as a two-terminal device, by placing the load in the external bias circuit, the response to a pulsed laser is delayed by a time constant equal to the product of the load resistance and the electrode capacitance, typically 10-100 μs. Thus, other means for coupling are recommended, including propagation as surface waves on an extended tip and radiation from antennas formed on the tip, and these methods have been tested with microwave prototypes. Ultimately miniature multifunction devices could be built to implement this new technology because nanoscale field emission tubes are now available, and field emitter arrays with 1010 tips/cm2 are used in flat panel displays.
Quantum simulations, supported by experiments, show that photomixing in laser-assisted field emission offers promise as a new mechanism for wide-band tunable sources at terahertz frequencies. In this technique the bandwidth is only limited by the methods for coupling power from the current oscillations that are generated in photomixing, and not by the fundamental processes that generate the mixer current. Photomixing is simulated as a stationary stochastic process in which the frequencies and phases of the incident optical radiation are random variables. The waveform of the current is determined by solving the time-independent Schroedinger equation at discrete time steps for which the potential barrier is a superposition of the instantaneous value of the radiation field and the static barrier. These samples satisfy the criteria of a Poisson process to allow for the discrete emission of electrons at the specified total current. The one-sided power spectral density is calculated with the FFT to produce periodogram estimates. The simulations show that the signal-to-noise ratio may be increased by (1) raising the power flux density of each laser, (2) raising the DC static current, (3) reducing the linewidth of each laser, and (4) using a static current densityof no more than 1010 A/m2.
Photomixing in laser-assisted field emission offers promise as a new mechanism for generating terahertz radiation. A nanoscale field emission tip is much smaller than optical wavelengths, so the potential of the tip follows each cycle of the incident radiation. Electron emission responds to the total electric field (DC + radiation) in τ < 2 fs, and the current-voltage characteristics of field emission are highly nonlinear. Thus, two lasers cause current oscillations by photomixing, which can be tuned from DC to 500 THz (1/τ) by shifting the offset of the lasers. Photomixing is simulated as a stationary stochastic process in which the frequencies and phases of the incident optical radiation are random variables. The waveform of the current is determined by solving the time-independent Schroedinger equation at discrete time steps for which the potential barrier is a superposition of the incident radiation field and the static barrier. These samples satisfy the criteria of a Poisson process to allow for the discrete emission of electrons at the specified total current. The one-sided power spectral density is calculated with the FFT to produce periodogram estimates.
Quantum theory shows that tunneling electrons have a resonant interaction with a radiation field, and because of this resonance a highly-focused amplitude-modulated laser diode (670 nm, 30 mW) changes field emission current enough to be seen with an oscilloscope. The emitting tip is much smaller than the optical wavelength, so the potential of the tip follows each cycle of the radiation field. Electron emission responds to the total electric field (DC + radiation) with a delay τ < 2 fs, and the current-voltage characteristics of field emission are highly nonlinear. Thus, quantum simulations show that two lasers can cause current oscillations by photomixing, which can be tuned from DC to 500 THz (1/τ) by shifting the frequency offset of the lasers. Microwave prototypes for 1-10 GHz are being tested. The output power is
proportional to the resistive part of the impedance that is seen by the current oscillations. However, the electric field at the mixer frequency, which is proportional to this impedance, causes negative feedback to reduce the current oscillations, so there is an optimum impedance for maximum output power. Analyses with equivalent circuits are used to optimize the design. Simulations suggest that 1 μW may be obtained in CW operation, or 10 mW in pulsed operation, using 10 mW
laser diodes as the pump sources.
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