Transition from ultrafast laser photo-electron emission to space charge limited current in a 1D gap

TL;DR

This paper models the transition from ultrafast laser photo-electron emission to space charge limited current in a 1D gap, considering non-equilibrium heating effects and validating results with PIC simulations.

Contribution

It introduces a 1D model that captures the transition to space charge limited emission, including non-equilibrium heating effects, and compares predictions with PIC simulations.

Findings

Space charge effects lower the emission current threshold.

Materials with lower work function reach SCL regime at lower laser fields.

Both tungsten and gold reach SCL at laser fields > 5 GV/m.

Abstract

A one-dimensional (1D) model has been constructed to study the transition of the time-dependent ultrafast laser photo-electron emission from a flat metallic surface to the space charge limited (SCL) current, including the effect of non-equilibrium laser heating on metals at the ultrafast time scale. At a high laser field, it is found that the space charge effect cannot be ignored and the SCL current emission is reached at a lower value predicted by a short pulse SCL current model that assumed a time-independent emission process. The threshold of the laser field to reach the SCL regime is determined over a wide range of operating parameters. The calculated results agree well with particle-in-cell (PIC) simulation. It is found that the space charge effect is more important for materials with lower work function like tungsten (4.4 eV) as compared to gold (5.4 eV). However for a flat surface, both materials will reach the space charge limited regime at the sufficiently high laser field such as $>$ 5 GV/m with a laser pulse length of tens to one hundred femtoseconds.