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.
Configurable trapping potentials are of great interest in cold atom physics, as they enable production of dynamical highly flexible fields that exhibit unprecedented stability and diverse geometries. Direct imaging can be used to create large area trapping potentials but is often overlooked due to its inability to correct for wavefront aberrations of the optical system . This need not be a major disadvantage for a well-corrected optical system and brings advantages including the simplicity and speed of direct imaging. This is in contrast to the Fourier plane method which requires complex calculations to generate proper holograms and suffers from phase defects and speckle. For applications in cold atom trapping, these effects are especially detrimental as the atoms are sensitive to perturbations at the ~1% level of the optical potential.
Our approach uses off-the-shelf lenses and microscope objectives and is able to achieve 630(10) nm full width half maximum (FWHM) patterning resolution using a 0.45 NA objective, within 5% of the diffraction limit of the system, while imaging through 1.25 mm of glass. The light field patterning is done using a digital micromirror device (DMD) which allows for dynamic trapping potentials due to its ability to store 13,889 frames and its 22 kHz full frame refresh rate. We use this method to pattern planar potentials for the purpose of cold atom experiments and have found that for atoms, which tend to respond relatively slowly to perturbations, it is possible to combine half-toning and time averaging to produce grey scale patterns, additionally allowing for pattern correction .
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