Phase-contrast imaging of a dense atomic cloud

TL;DR

This paper demonstrates a method to produce and accurately image dense cold atomic clouds of strontium using phase-contrast imaging, enabling studies of light transport in dense atomic media.

Contribution

It introduces a novel combination of narrow-line molasses and optical dipole trapping for high-density atomic clouds and validates phase-contrast imaging as a reliable technique in dense regimes.

Findings

Achieved spatial densities of 7.9×10^{13} atoms/cm^3

Demonstrated phase-contrast imaging accurately reconstructs density profiles in dense samples

Validated PCI method against time-of-flight measurements at high densities

Abstract

We present the experimental production and characterization of a dense cold atomic cloud of \(^{88}\text{Sr}\) atoms, optimized for the future studies of light transport in highly dense regimes. Using narrow-line molasses on the 689 nm transition, combined with a far off-resonant optical dipole trap, we achieve spatial densities as high as \(7.9 \times 10^{13} \, \text{atoms/cm}^3\) and optical depths up to 64. This approach stands out from previous methods by integrating narrow-line molasses with an optical dipole trap, enabling high-density samples without relying on evaporative cooling. Unlike traditional absorption imaging, which becomes inaccurate in such dense regimes, we demonstrate that phase-contrast imaging (PCI) can reliably reconstruct the in-situ density profile even for highly spatially and optically dense samples. The use of a spatial light modulator instead of a fixed phase plate in the PCI setup provides enhanced flexibility and control of imaging parameters, making this imaging technique robust against imaging artifacts and adaptable to varying experimental conditions. Moreover, we derive theoretical conditions for reliable PCI operation in dense regimes and validate these experimentally, showing excellent agreement with time-of-flight measurements even at the highest densities. Our results establish a robust method for producing and characterizing dense atomic clouds.