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arXiv:2102.05492 · Journal of Physics B Atomic Molecular and Optical Physics

Optical control of the density and spin spatial profiles of a planar Bose gas

Y.-Q. Zou, É. Le Cerf, Brice Bakkali-Hassani, C. Maury, G Chauveau, P. C. M. Castilho, R. Saint-Jalm, Sylvain Nascimbène, Jean Dalibard, J. Beugnon

Published Mar 26, 2021• 20 citations
arXivHTMLPDF

Abstract

Abstract We demonstrate the arbitrary control of the density profile of a two-dimensional Bose gas by shaping the optical potential applied to the atoms. We use a digital micromirror device (DMD) directly imaged onto the atomic cloud through a high resolution imaging system. Our approach relies on averaging the response of many pixels of the DMD over the diffraction spot of the imaging system, which allows us to create an optical potential with an arbitrary intensity profile and with micron-scale resolution. The obtained density distribution is optimized with a feedback loop based on the measured absorption images of the cloud. Using the same device, we also engineer arbitrary spin distributions thanks to a two-photon Raman transfer between internal ground states.

DOI
10.1088/1361-6455/abf298

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Abstract We demonstrate the arbitrary control of the density profile of a two-dimensional Bose gas by shaping the optical potential applied to the atoms. We use a digital micromirror device (DMD) directly imaged onto the atomic cloud through a high resolution imaging system. Our approach relies on averaging the response of many pixels of the DMD over the diffraction spot of the imaging system, which allows us to create an optical potential with an arbitrary intensity profile and with micron-scale resolution. The obtained density distribution is optimized with a feedback loop based on the measured absorption images of the cloud. Using the same device, we also engineer arbitrary spin distributions thanks to a two-photon Raman transfer between internal ground states.
The development of novel trapping potentials for degenerate quantum gases has been an important factor driving experimental progress in the field. The introduction of spatial light modulators (SLMs) into quantum gas laboratories means that a range of configurable geometries are now possible. Here we demonstrate the production of arbitrary and dynamic high-resolution traps for Bose-Einstein condensates (BECs) using a (1200 x 1920) pixel digital-micromirror device (DMD) and commercially available microscope objectives. Direct imaging of the DMD provides significant advantages over phase-based SLMs, while still providing high resolution projected patterns. We find that atoms confined in a hybrid optical/magnetic or all-optical potential can be patterned using repulsive blue- detuned (532 nm) light with 630(10) nm full-width at half-maximum (FWHM) resolution, while imaging at 780 nm similarly achieves 960(80) nm resolution FWHM, within 5% and 8% of the diffraction limit respectively. This performance is > 7 times improved over previously reported DMD trapping. The result is near arbitrary control of the atomic distribution over the spatial extent of the BEC trapping volume. Furthermore, we produce time-averaged potentials with the DMD, demonstrating the ability to produce multiple grayscale levels with minimal heating of the atomic cloud. These techniques, along with the high level of dynamic control afforded by DMDs, will enable the realization and control of diverse optical potentials for superfluid dynamics and atomtronics applications with quantum gases.
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The implementation of spatial light modulators (SLMs) in quantum gas experiments has allowed the realization of ever more complex trapping geometries. As ultracold atoms may be sensitive to perturbations of the trapping potential at the 1% level, the high contrast ratios of digital micromirror devices are proving advantageous for use in optical trapping. Our laboratory pursues configurable traps based on the direct (nearly diffraction limited) imaging of a digital micromirror device (DMD). We achieve highly flexible potentials using commercially available microscope objectives external to our vacuum chamber that directly project the DMD to the atom plane, producing optical traps over an area of 130 μm × 200 μm, with a resolution of 630(10) nm full width at half maximum (FWHM) at 532 nm illumination. We combine these potentials with a horizontally propagating TEM00 or TEM01 Hermite-Gaussian optical sheet that provides vertical confinement. With the dynamic control enabled by the maximum full-frame rate of 20 KHz and on-board storage of <13,000 frames of the DMD, we study the transport of atoms and superfluid dynamics in configurable trapping geometries. Using the fast frame rate of the DMD we also produce intermediate grey levels that complement half-toning techniques for producing optimized grayscale patterns.
This thesis focusses on the use of a Digital Micromirror Device (DMD) to produce spatially and temporally varying optical potentials for the manipulation of ultracold atoms. I discuss the apparatus used to generate ultracold atomic samples as well as that used to manipulate them, including a comparison of the various light sources used with the DMD. Here I demonstrate the advantages in image quality that can be achieved using a superluminescent diode. I also report on work which uses the capability to produce spatially and temporally controlled potentials to engineer a synthetic dimension of harmonic oscillator sites, forming a lattice consisting of 10s of lattice sites on which Bloch Oscillations are demonstrated. Finally I describe simulations which demonstrate the clustering behaviour of quantised vortices in a Bose-Einstein condensate when arranged in linear structures, as could be done by the method of phase imprinting using a DMD. I demonstrate the use of a cluster detection algorithm which is novel in the field of quantised vortices, and show that in many cases, vortex clusters can be modelled by methods similar to those used for single vortices.
We present a method for producing a flat, large-area Fermi gas of 6Li^{6}\mathrm{Li}6Li with a uniform area density. The method uses a programmable optical potential within a feedback loop to flatten the in-plane trapping potential for atoms. The optical potential is generated using a laser beam, whose intensity profile is adjusted by a spatial light modulator and optimized through measurements of the density distribution of the sample. The resulting planar sample exhibits a uniform area density within a region of about 480 $\textmu{}\mathrm{m}$ in diameter, and the standard deviation of the trap bottom potential is estimated to be $\ensuremath{\approx}{k}_{B}\ifmmode\times\else\texttimes\fi{}6.1$ nK, which is less than 20% of the transverse confinement energy. We discuss a dimensional crossover toward the two-dimensional regime by reducing the number of atoms in the planar trap, including the effect of the spatial variation of the transverse trapping frequency in the large-area sample.
We present two spatial-shaping approaches - phase and amplitude - for creating two-dimensional optical dipole potentials for ultracold neutral atoms. When combined with an attractive or repulsive Gaussian sheet formed by an astigmatically focused beam, atoms are trapped in three dimensions resulting in planar confinement with an arbitrary network of potentials - a free-space atom chip. The first approach utilizes an adaptation of the generalized phase-contrast technique to convert a phase structure embedded in a beam after traversing a phase mask, to an identical intensity profile in the image plane. Phase masks, and a requisite phase-contrast filter, can be chemically etched into optical material (e.g., fused silica) or implemented with spatial light modulators; etching provides the highest quality while spatial light modulators enable prototyping and realtime structure modification. This approach was demonstrated on an ensemble of thermal atoms. Amplitude shaping is possible when the potential structure is made as an opaque mask in the path of a dipole trap beam, followed by imaging the shadow onto the plane of the atoms. While much more lossy, this very simple and inexpensive approach can produce dipole potentials suitable for containing degenerate gases. High-quality amplitude masks can be produced with standard photolithography techniques. Amplitude shaping was demonstrated on a Bose-Einstein condensate.