Ultrafast optoelectronic devices using laser-assisted field emission

Abstract

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 s. 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 10 10 tips/cm 2 are used in flat panel displays.

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“…These solutions show that the radiation increases the DC current, and also causes the current to have time-dependent terms with the frequencies n 1 f 1 +n 2 f 2 , where f 1 and f 2 are the frequencies of the radiation and the integers n 1 and n 2 may each be positive, zero, or negative for harmonics and mixing terms. Furthermore, to aid in understanding and applying these phenomena, closed-form expressions have been derived for the response of the field emission current to a pulsed sinusoidal source, 24 as well as the mixer current produced by two CW sinusoidal sources. 17 An adiabatic approximation is required in deriving the closed-form expressions, in that the imposed oscillatory fields must have low frequencies where the photon energy may be neglected.…”