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Review of Scientific Instruments

Phase-contrast imaging of a dense atomic cloud

Marcia Frómeta Fernández, Pablo Gabriel Santos Dias, P. H. N. Magnani, M. do Amaral Martins, Mathilde Hugbart, A. Cipris, Ph. W. Courteille, R Teixeira

Published Aug 1, 2025• 1 citation
Publisher

Abstract

We demonstrate that phase-contrast imaging (PCI) can reliably reconstruct the in situ density profile even for highly spatially and optically dense samples. In our experiment, we achieve high spatial densities of up to 7.9 × 1013 atoms/cm3 and optical depths up to 64 in a dense cold atomic cloud of 88Sr atoms. 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 quantify the conditions under which the standard single-atom polarizability model remains accurate for PCI in a density regime where collective effects should modify the atomic response of the system. We experimentally validate our statements by showing excellent agreement with time-of-flight measurements even at the highest densities. Our results establish PCI as a reliable and versatile method for characterizing spatially dense atomic clouds.

DOI
10.1063/5.0256884

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We demonstrate that phase-contrast imaging (PCI) can reliably reconstruct the in situ density profile even for highly spatially and optically dense samples. In our experiment, we achieve high spatial densities of up to 7.9 × 1013 atoms/cm3 and optical depths up to 64 in a dense cold atomic cloud of 88Sr atoms. 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 quantify the conditions under which the standard single-atom polarizability model remains accurate for PCI in a density regime where collective effects should modify the atomic response of the system. We experimentally validate our statements by showing excellent agreement with time-of-flight measurements even at the highest densities. Our results establish PCI as a reliable and versatile method for characterizing spatially dense atomic clouds.
We present the experimental production and characterization of a dense cold atomic cloud of 88Sr^{88}\text{Sr}88Sr 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 7.9×1013 atoms/cm37.9 \times 10^{13} \, \text{atoms/cm}^37.9×1013atoms/cm3 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.
We report on a far above saturation absorption imaging technique to investigate the characteristics of dense packets of ultracold atoms. The transparency of the cloud is controlled by the incident light intensity as a result of the nonlinear response of the atoms to the probe beam. We detail our experimental procedure to calibrate the imaging system for reliable quantitative measurements and demonstrate the use of this technique to extract the profile and its spatial extent of an optically thick atomic cloud.
The main objective of this research is to study a particular case of light-matter interaction: the scattering of light in dense atomic samples.The regime of high atomic densities is represented by samples where the distance between neighboring atoms is on the order of the wavelength associated with the atomic transition.In this regime, short-range atomic interactions cannot be neglected, and in these systems, we expect to observe significant modifications in collective effects such as subradiance or superradiance.Overall, the propagation of light through this type of system is not well-known to the scientific community, and the existing models work to describe very specific situations.To pave the way for achieving our overall goal, this work proposes an experimental setup to obtain a dense cloud of 88 Sr, which is the chemical species we work with in our laboratory.For this purpose, atoms previously trapped using a Magneto-Optical Trap are transferred to a Crossed Optical Dipole Trap.During the optimization and characterization of the optical trap, problems were encountered in producing an image with quantitative information of the atomic ensemble and in transferring a large number of atoms in a very small trapping volume.These problems were resolved with the implementation of a Phase Contrast Imaging system and the implementation of an Optical Molasses as an additional cooling step.Under these conditions, we obtained a cloud that can be suitable for conducting studies on collective effects, transitioning from the dilute to the dense regime.The initial experiments, once our atomic ensemble was prepared, involve measuring the coherent transmission of a low-intensity beam through the cloud.For this purpose, incident light near the resonance of a dipolar transition of the type J = 0 ↔ J = 1 of 88 Sr is used, interacting with a homogeneous region of the sample.Simulations of this type of process can be performed using the Coupled Dipole Model, which describes the overall response of light scattering by the sample considering dipole-dipole interactions mediated by light.With this work, we aim to advance the understanding of density effects by comparing the coherent optical response of the dense cloud with the theory based on the Coupled Dipole Model.
Phase contrast imaging is used to observe Bose-Einstein condensates at finite temperature in situ. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two-relevant-parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and the thermal atoms is taken into account to various degrees.
Phase contrast imaging is used to observe Bose-Einstein condensates (BECs) at finite temperature in situ. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two relevant parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and thermal atoms is taken into account to various degrees.
Abstract The accurate determination of atom numbers is an ubiquitous problem in the field of ultracold atoms. For modest atom numbers, absolute calibration techniques are available, however, for large numbers and high densities, the available techniques neglect many-body scattering processes. Here, a spatial calibration technique for time-of-flight absorption images of ultracold atomic clouds is presented. The calibration is obtained from radially averaged absorption images and we provide a practical guide to the calibration process. It is shown that the calibration coefficient scales linearly with optical density and depends on the absorbed photon number for the experimental conditions explored here. This allows for the direct inclusion of a spatially dependent calibration in the image analysis. For typical ultracold atom clouds the spatial calibration technique leads to corrections in the detected atom number up to ≈ 12 % and temperature up to ≈ 14 % in comparison to previous calibration techniques. The technique presented here addresses a major difficulty in absorption imaging of ultracold atomic clouds and prompts further theoretical work to understand the scattering processes in ultracold dense clouds of atoms for accurate atom number calibration.