Optical excitations in a non-ideal Bose gas

TL;DR

This paper investigates how optical excitations in a non-ideal Bose gas are highly sensitive to many-body effects, revealing phenomena like spectral line splitting and narrowing due to collective excitations and BEC physics.

Contribution

It provides a theoretical framework explaining line doubling and calculates oscillator strengths and linewidths for optical excitations in a Bose gas.

Findings

Spectral lines narrow dramatically in the Boltzmann gas due to collective effects.

In the BEC state, spectral lines split into two components, akin to second sound.

The theory quantitatively predicts oscillator strengths and linewidths for these excitations.

Abstract

Optical excitations in a Bose gas are demonstrated to be very sensitive to many-body effects. At low temperature the momentum relaxation is provided by momentum exchange collisions, rather than by elastic collisions. A collective excitation mode forms, which in a Boltzmann gas is manifest in a collision shift and dramatic narrowing of spectral lines. In the BEC state, each spectral line splits into two components. The doubling of the optical excitations results from the physics analogous to that of the second sound. We present a theory of the line doubling, and calculate the oscillator strengths and linewidth.