Self-ordering and cavity cooling using a train of ultrashort pulses

TL;DR

This paper investigates how ultrashort pulse trains induce self-ordering and cavity cooling in atomic gases within optical resonators, revealing phase transition behaviors, enhanced cooling, and novel cavity output structures.

Contribution

It introduces a generalized pumping scheme with femtosecond pulses, demonstrating altered phase transition orders, improved cooling, and new cavity output phenomena compared to traditional methods.

Findings

Transition from second to first order phase transition with increasing bandwidth.

Formation of a double pulse traveling within the cavity, creating a time crystal.

Multi-mode operation enhances cooling efficiency and reduces atomic kinetic temperatures.

Abstract

A thin atomic gas in an optical resonator exhibits a phase transition from a homogeneous density to crystalline order when laser illuminated orthogonal to the resonator axis. We study this well-known self-organization phenomenon for a generalized pumping scheme using a femtosecond pulse train with a frequency spectrum spanning a large bandwidth covering many cavity modes. We show that due to simultaneous scattering into adjacent longitudinal cavity modes the induced light forces and the atomic dynamics becomes nearly translation-invariant along the cavity axis. In addition the laser bandwidth introduces a new correlation length scale within which clustering of the atoms is energetically favorable. Numerical simulations allow us to determine the self-consistent ordering threshold power as function of bandwidth and atomic cloud size. We find strong evidence for a change from a second order to a first order self-ordering phase transition with growing laser bandwidth when the size of the atomic cloud gets bigger than the clustering length. An analysis of the cavity output reveals a corresponding transition from a single to a double pulse traveling within the cavity. This doubles the output pulse repetition rate and creates a new time crystal structure in the cavity output. Simulations also show that multi-mode operation significantly improves cavity cooling generating lower kinetic temperatures at a much faster cooling rate.