Polarization effects on the effective temperature of an ultracold electron source

TL;DR

This study investigates how the polarization of ionization lasers affects the effective temperature of ultracold electron sources, revealing sinusoidal dependencies and regimes for optimal electron coherence, supported by a classical trajectory model.

Contribution

It demonstrates the polarization dependence of electron temperature in ultracold sources and introduces a classical trajectory model to explain the underlying mechanisms.

Findings

Temperature varies sinusoidally with polarization angle.

Minimal temperature occurs when polarization aligns with acceleration, except in nanosecond ionization regimes.

Charge oscillations as a function of polarization were observed in nanosecond ionization.

Abstract

The influence has been studied of the ionization laser polarization on the effective temperature of an ultracold electron source, which is based on near-threshold photoionization. This source is capable of producing both high-intensity and high-coherence electron pulses, with applications in for example electron diffraction experiments. For both nanosecond and femtosecond photoionization, a sinusoidal dependence of the temperature on polarization angle has been found. For most experimental conditions, the temperature is minimal when the polarization coincides with the direction of acceleration. However, surprisingly, for nanosecond ionization a regime exists when the temperature is minimal when the polarization is perpendicular to the acceleration direction. This shows that in order to create electron bunches with the highest transverse coherence length, it is important to control the polarization of the ionization laser. The general trends and magnitudes of the temperature measurements are described by a model, based on the analysis of classical electron trajectories; this model further deepens our understanding of the internal mechanisms during the photoionization process. Furthermore, for nanosecond ionization, charge oscillations as a function of laser polarization have been observed; for most situations the oscillation amplitude is small.