Narrow-line cooling of $^{87}$Rb using 5S$_{1/2} \rightarrow$ 6P$_{3/2}$ open transition at 420 nm

TL;DR

The paper demonstrates narrow-line cooling of $^{87}$Rb by driving an open blue transition $5S_{1/2} \rightarrow 6P_{3/2}$ at 420 nm, loading a blue MOT from a conventional IR MOT on the D2 line. With a blue-line linewidth of $\Gamma = 2\pi\times1.35$ MHz (≈1.4 MHz), the authors achieve about $1.1\times10^8$ atoms at $54\ \mu$K, after optimizing detuning, power, and hold times, while studying lifetime as a function of dispenser current. The setup uses a single 780 nm repumper and a 420 nm trap beam, jointly addressing an overlapping MOT geometry with fast magnetic-field switching. The results indicate that narrow-line cooling on an open transition is an effective route to high atom numbers at low temperatures, with potential improvements by employing a dedicated 420 nm repumper to further enhance loading and cooling efficiency for subsequent ODT-assisted evaporation.

Abstract

Magneto-optical trap (MOT) at narrow (weak) transition offers lower temperature and hence is the key for production of high phase density atomic cloud and subsequently quantum degeneracy with high number of atoms for many elements. In this paper, we describe loading of $^{87}$Rb atoms in the MOT using a narrow open transition at 420 nm from the routinely implemented MOT using broad cyclic transition at 780 nm (IR). The total linewidth of the blue transition, 5S$_{1/2} \rightarrow $ 6P$_{3/2}$ is 1.4 MHz, which is around 4 times narrower than the standard 5S$_{1/2} \rightarrow$ 5P$_{3/2}$ cyclic transition. Using this narrow transition, we have trapped around $10^{8}$ atoms in the MOT with a typical temperature of around $54~μ$K. We have also studied the behavior of the blue MOT with various parameters such as hold time, detuning and power of trapping and repumper beams.

Figures (8)

• Figure 1: (Color online) The relevant energy levels of $^{87}$Rb with the hyperfine splittings and various decay paths of the 6P$_{3/2}$ state. Decay rates, linewidth of the excited state and the hyperfine splittings are shown in MHz unit.
• Figure 2: (Color online) (a) Polarization spectroscopy scheme for 780 nm MOT and repumper lasers. (b) Double resonance spectroscopy scheme for 420 nm laser. Figure abbreviations: DM: dichroic mirror, M: mirror, PBS: polarizing beam splitter, PD: photo-detector. 780 nm and 420 nm beams are shown in red and blue color respectively.
• Figure 3: (Color online) (a) Mixing scheme of the IR MOT, repumper and blue MOT beams. (b) Top view of the MOT set-up. Figure abbreviations: A: anti-Helmholtz coil, D: Rb dispenser, DM: dichroic mirror, F: electric feedthrough, FWC: 4-way cross, G: glass chamber, GV: all metal gate valve, HWP: $\lambda/2$ wave-plate, L$_{1}$: plano-concave lens, L$_{2}$: plano-convex lens, M: mirror, PBS: polarizing beam splitter, S: Quartz to metal seal, QWP: dual $\lambda/4$ wave-plate, $\sigma_{+}$: co-propagating MOT beams in $\sigma_{+}$ configuration. 780 nm and 420 nm beams are shown in red and blue color respectively.
• Figure 4: (Color online) Time sequence along with the corresponding experimental parameters for loading the IR and blue MOT and characterization of the MOT cloud. The coil current of 1 A corresponds to the magnetic field gradient of around 18 Gauss/cm. The ON (OFF) state of the camera trigger and 780 nm imaging beam is referred as 1 (0). Three images captured at each cycle (i) image with MOT cloud, (ii) reference beam, and (iii) background are shown as I, Ref, and BG respectively. The axes are not to scale.
• Figure 5: (Color online) Number of atoms is shown as a function of (a) 420 nm MOT laser detuning in loading phase, (b) total 420 nm MOT laser power and (c) 780 nm repumper power. In (a) and (b) repumper laser is kept at its maximum available power, 10.2 mW. In (b) and (c) detuning of the 420 nm trapping beam in loading phase is kept at -4 MHz. In (a) and (c) power of the 420 nm MOT beam is kept at its maximum available power, 32 mW.