Direct absorption imaging of ultracold polar molecules

We demonstrate a scheme for direct absorption imaging of an ultracold ground-state polar molecular gas near quantum degeneracy. Imaging molecules without closed optical cycling transitions is challenging. Our technique relies on photon-shot-noise-limited absorption imaging on a strong but open bound-bound molecular transition. We present a systematic characterization of this imaging technique. Using this technique combined with time-of-flight expansion, we demonstrate the capability to determine momentum and spatial distributions for the molecular gas. With its capability of imaging molecules in arbitrary external fields, we anticipate that this technique will find many applications in the study of molecular quantum gases.

Figure 1

(a) Ab initio electronic potentials for KRb, showing the direct absorption detection scheme for $X^1{\Sigma }^{+}$ molecules in the $v^{″}=0$, $N^{″}=0$ level. The probe light drives a transition to the $v^{\prime }=3$ level of the 1${}^1\Pi$ state. Inset: Schematic of the open two-level model described in the text. (b) An example population depletion line shape for this transition taken with a 10-$\mu$s probe pulse duration. (c) Decay of the ground-state population as a function of the probe pulse duration. The probe intensity is $~$0.7 mW/cm${}^2$. Solid curves are fits based on the open two-level model.

Figure 2

(a) An absorption image ($228\times 171$ $\mu$m) of 39 000 ground-state molecules after a 2-ms TOF. The false color indicates CCD counts, taken for the best SNR, with $~$75% of the molecules detected. (b) Peak SNR as a function of $n_{\mathrm{photon}}$. Filled circles, experimentally measured peak SNR; solid curve, fit to the model described in the text. The SNR here corresponds to a measured $N_{\mathrm{molecule}}=950$ per pixel at the cloud center.

Figure 3

(a) Measurement of the molecular gas temperature by TOF expansion followed by direct absorption imaging. The trap frequency along the $x$ and $z$ directions is 32 and 195 Hz, respectively. Filled circles and open triangles are the rms cloud widths in the $x$ and $z$ directions, respectively. Solid curves are fits to a ballistic expansion. (b) Measurement of the molecular two-body loss rate $\beta$, for an initial temperature of 320 nK, using the direct imaging (open circles) and Feshbach imaging (filled squares) methods. Solid curves are fits to a two-body loss model. The extracted values of $\beta$ agree within the experimental uncertainty.

Figure 4

Measurement of the Zeeman (a; filled circles) and Stark (b; filled circles) shifts of the optical transition used for direct absorption imaging. Open triangles correspond to excited-state Stark shifts obtained after subtracting the known Stark shift of the ground state [8]; solid curves are quadratic fits.