Tunability of the THz space-charge modulation in a vacuum microdiode

TL;DR

This study demonstrates that the THz frequency modulation in vacuum microdiodes can be precisely tuned by adjusting the applied electric potential and emitter area, with frequency primarily dependent on the electric field and emitter size.

Contribution

It provides a computational analysis showing how to control and tune THz space-charge oscillations in microdiodes through simple parameter variations.

Findings

Frequency depends on the electric field via a power law.

Reducing emitter size increases modulation frequency.

Spectral quality is affected by gap spacing and initial electron velocity.

Abstract

Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the Terahertz range. In this computational study it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependent on the vacuum electric field in the diode, described by a simple power law. It is also found that, for a fixed value of the electric field, the frequency increases with diminishing size of the emitting spot on the cathode. Some observations are made on the spectral quality, and how it is affected by the gap spacing in the diode and the initial velocity of the electrons.