Ultrafast electron cooling in an expanding ultracold plasma

Tobias Kroker,Juliette Simonet,Philipp Wessels-Staarmann,Klaus Sengstock,Markus Drescher,Mario Großmann

doi:10.1038/s41467-020-20815-8

Abstract

Plasma dynamics critically depends on density and temperature, thus well-controlled experimental realizations are essential benchmarks for theoretical models. The formation of an ultracold plasma can be triggered by ionizing a tunable number of atoms in a micrometer-sized volume of a 87Rb Bose-Einstein condensate (BEC) by a single femtosecond laser pulse. The large density combined with the low temperature of the BEC give rise to an initially strongly coupled plasma in a so far unexplored regime bridging ultracold neutral plasma and ionized nanoclusters. Here, we report on ultrafast cooling of electrons, trapped on orbital trajectories in the long-range Coulomb potential of the dense ionic core, with a cooling rate of 400 K ps−1. Furthermore, our experimental setup grants direct access to the electron temperature that relaxes from 5250 K to below 10 K in less than 500 ns.

Highlights

Plasma dynamics critically depends on density and temperature, well-controlled experimental realizations are essential benchmarks for theoretical models
We experimentally investigate the dynamics of an ultracold microplasma by combining ultracold quantum gases with the ultrashort timescales of femtosecond laser pulses
Whereas earlier photoionization studies in 87Rb Bose-Einstein condensate (BEC) applied nanosecond laser pulses[20], here, the pulse duration is significantly shorter than the timescale for the electron dynamics given by the inverse electron plasma frequency ωÀp;1e

Summary

Introduction

Plasma dynamics critically depends on density and temperature, well-controlled experimental realizations are essential benchmarks for theoretical models. Compared to macroscopic ultracold neutral plasmas (UNPs) at MOT densities, these initial parameters allow for creating a micrometer-sized plasma with large charge imbalance and high plasma frequencies where the Coulomb energies initially exceed the ionic thermal energies by three orders of magnitude. Such an initially strongly coupled microplasma with a few hundred to thousands of particles bridges the dynamics and energy transfer studied in photoionized nanoclusters and UNP. This considerably simplifies theoretical models of the dynamics for benchmark comparisons

Methods

Results

Conclusion