Magneto-optical trapping of aluminum monofluoride

Abstract

Magneto-optical trapping of molecules has thus far been restricted to molecules with $^2Σ$ electronic ground states. These species are chemically reactive and only support a simple laser cooling scheme from their first excited rotational level. Here, we demonstrate a magneto-optical trap (MOT) of aluminum monofluoride (AlF), a deeply bound and intrinsically stable diatomic molecule with a $^1Σ^+$ electronic ground state. The MOT operates on the strong A$^1Π\leftarrow{}$X$^1Σ^+$ transition near 227.5~nm, whose Q$(J)$ lines are all rotationally closed. We demonstrate a MOT of about $6\times 10^4$ molecules for the $J=1$ level of AlF, more than $10^4$ molecules for $J=2$ and $3$, and with no fundamental limit in going to higher rotational levels. Laser cooling and trapping of AlF is conceptually similar to the introduction of alkaline-earth atoms into cold atom physics, and is key to leveraging its spin-forbidden a$^3Π\leftarrow{}$X$^1Σ^+$ transition for precision spectroscopy and narrow-line cooling.

Figures (3)

• Figure 1: a) Schematic view of the experimental setup, showing the molecular beam source, magneto-optical trap and the laser configuration. The laser cooling scheme is shown as an inset on the left, where $L_{v'v"}$ labels the corresponding A$^1\Pi, v' \leftarrow{}$ X$^1\Sigma^+ , v"$ band excited in AlF. PBC = polarising beam splitter cube; AOM = acousto-optic modulator. b) Rotational and hyperfine structure of the lowest X$^1\Sigma^+,v"=0$ and A$^1\Pi, v'=0$ levels, showing the Q($J$) rotational lines excited by the $L_{00}$ lasers. Zeeman shifts are plotted on the right for the A$^1\Pi, v'=0$, positive parity levels with $J'=1$ and $J'=3$. Zoom-ins show the region $0<B<50$ G, and label the hyperfine levels by their total angular momentum quantum number $F$Truppe2019. Blue (orange) lines indicate levels with magnetic $g$-factor $g_F \geq 0$ ($< 0$). The natural linewidth of the transition, $\Gamma/(2\pi) = 84$ MHz, is indicated in the top left of each plot, with the $L_{00}^{\mathrm{MOT}}$ laser detuning for the MOT shown in the same color by shaded horizontal bars.
• Figure 2: Magneto-optical trapping of AlF. a) Laser-induced fluorescence (LIF) traces demonstrating loading of the Q(1) (upper), Q(2) (middle) and Q(3) (lower) MOTs, for trapping (red) and anti-trapping (blue) configurations. Time-of-flight traces for the molecular beam arriving at the centre of the MOT chamber are shown in light grey. Insets: camera images taken using the four circular polarisation configurations of the $L_{00}^{\mathrm{MOT}}$ light; orange bars along the $x$-axis of the main panel show the camera exposure duration. (b) LIF spectra of the molecular beam (upper sub-panel) and each of the three MOTs (lower sub-panel) versus the $L_{00}^{\mathrm{MOT}}$ laser frequency. MOT spectra are vertically offset, and shown for trapping (red) and anti-trapping (blue) polarisation configurations. Black horizontal arrows show the $L_{00}^\mathrm{S}$ frequency chirp used to load each MOT. Simulations (grey) pointing downwards show the Q$(J)$ lineshapes in zero magnetic field, including hyperfine structure of the A$^1\Pi,v'=0$ levels. Insets show camera images taken at the indicated laser frequencies.
• Figure 3: (a) Rapid switch-off of the $L_{12}$ light results in an increase in loss rate from the Q(1) MOT. Grey line: no switch-off of $L_{12}$. Transparent blue line: switch-off at $t=30$ ms. Red, dashed lines are fits to exponential decays from $t=30$ ms. Insets: camera images with identical imaging parameters for (i) Rayleigh scattering in nitrogen (1 bar) using a single $z$-axis $L_{00}^{\mathrm{MOT}}$ beam and (ii) Fluorescence from the Q(1) MOT. (b) Q(1) MOT fluorescence spectrum scanning the $L_{12}$ laser detuning.