Absorption imaging of quantum gases near surfaces using incoherent light

Abstract

We introduce an absorption imaging technique for ultracold gases that suppresses interference fringes and coherence-induced artifacts by reducing the transverse spatial coherence of the imaging light. The method preserves the narrow spectral bandwidth required for resonant absorption imaging and is implemented as a modular extension to standard imaging setups using a rotating diffuser. We demonstrate tunability of the illumination light's coherence without modifying the imaging optics. Using this approach, we achieve reliable imaging of ultracold atomic clouds in micron-scale proximity to complex surfaces, where standing waves, edge diffraction, and speckle severely limit conventional absorption imaging.

Figures (5)

• Figure 1: Experimental scheme including the incoherent imaging module, the vacuum chamber with atoms below the planar trapping structure, to which a sample on a substrate is attached, and imaging system with the camera. The optional F-lens can be moved to tune the coherence of the light. Without the incoherent imaging module inserted, a large collimated beam provides coherent illumination to the atoms following the standard absorption imaging scheme. Red lines represent rays of particular interest for the image formation of atoms and their mirrored (dashed line) image.
• Figure 2: Raw images with an ultracold cloud placed at 430 distance from the surface. (a) Speckle pattern obtained with the incoherent imaging setup with a static diffuser and C-lens, without F-lens. (b) Image taken with the same setup, but the diffuser rotating at 90, as used throughout the paper. (c-e) Images demonstrating the tunability of coherence by moving the additional F-lens along the optical axis. Diffuser and C-lens are unchanged relative to (b). (f) Standard absorption imaging scheme with the incoherent imaging module fully removed.
• Figure 3: Raw images of atoms in the near vicinity of the surface, using (a) coherent and (b) incoherent light. A region centered on the surface location is indicated with yellow solid (dashed) line with vertical extent of 34 (270) . (c: inset) Atomic distribution recorded with incoherent light when atoms are placed at $d_{\textrm{as}} \approx \qty{62}{\um}$ height below the surface, appearing as two clouds separated vertically by $2d_{\textrm{as}}$ distance. Dashed lines indicate center-of-mass positions extracted from fits to the vertical and horizontal profiles. (c) Atom-surface distance $d_{\textrm{as}}$ calibration to trapping current, used for moving the atoms: current in the Helmholtz coil pair provides bias field for a Z-wire trap where the Z-wire current is kept constant at $85.0 \pm \qty{0.03}{\ampere}$. Each data point is obtained using Gaussian fits on the atomic clouds and their mirror images while the currents were recorded. Closer to the surface, the two clouds merge, and the multi-Gaussian fit becomes unreliable. (d-e) 34 height section of ACD images of atoms trapped within 6 from the surface. (d) is a single and (e) is average of 10 images taken with incoherent imaging. Dashed lines indicate the surface. (f) Linear density profiles calculated for data in (d-e), with single profiles in gray, mean profile in dark red.
• Figure 4: (a,e) Raw images, (b,f) single and (c,g) averaged ACD of 25 repeats, and (d,h) the corresponding linear density profiles, for coherent (a-d) and incoherent (e-h) light. The data were taken of a thermal cloud after 1 TOF.
• Figure 5: (a-g) ACD images and resulting linear densities for data taken with an imaging system comprising optical aberrations. (a-c) Atomic distributions taken in trap and after 0.85 and 1.4 TOF, respectively, using coherent light. Pronounced imaging aberration effects can be observed as fringes above the in-situ cloud. (d) Linear density profiles from images (a-c) indicate almost complementary distributions for the two distributions measured after a TOF. (e) Linear densities for incoherent and partially coherent light. Aberrations for data taken with incoherent light cause suboptimal resolution but no visible artifacts. (f-g) ACD images taken after 1.4 TOF with incoherent and partially coherent light. (h-j) Data taken with a near-diffraction-limited imaging system. (h-i) ACD images taken after 1.4 TOF for coherent and incoherent light. (j) Linear densities for various TOF using coherent and incoherent imaging.