Radiative Force from Optical Cycling on a Diatomic Molecule

Abstract

We demonstrate a scheme for optical cycling in the polar, diatomic molecule strontium monofluoride (SrF) using the X 2 Σ + → A 2 Π 1/2 electronic transition. SrF's highly diagonal Franck-Condon factors suppress vibrational branching. We eliminate rotational branching by employing a quasicycling N = 1 → N ′ = 0 type transition in conjunction with magnetic field remixing of dark Zeeman sublevels. We observe cycling fluorescence and deflection through radiative force of an SrF molecular beam using this scheme. With straightforward improvements our scheme promises to allow more than 10 5 photon scatters, possibly enabling the direct laser cooling of SrF. [5,6,7], as qubits in a quantum computer [8], and for studying ultracold chemistry [9,10]. Currently the best technique for creating ultracold molecules relies on their assembly from preexisting ultracold atoms [11,12], and recent experiments have produced ground state polar molecules near quantum degeneracy [12]. Unfortunately only bialkali molecules have been produced this way and the number of molecules created is fairly small (∼ 10 4 ) [12]. There is substantial interest in developing techniques for direct cooling of molecules, enabling use of species with different structures (e.g. unpaired electron spins, which are important for several proposed applications [2, 4, 6, 13]), and possibly also much larger samples. Molecules have been directly cooled using a cryogenic buffer gas [14] or via supersonic expansion followed by beam deceleration [15], but so far the temperature reached with these techniques is limited to ∼ 10 mK.The prospect of direct laser cooling of molecules is attractive because large numbers could in principle be obtained for new types of species at ultracold temperatures. Unfortunately, molecules possess rotational and vibrational degrees of freedom resulting in decays to unwanted sublevels, so finding the closed cycling transitions required for laser cooling is a challenge. However, recent proposals have shown that certain molecules may be amenable to laser cooling because their Franck-Condon factors (FCFs) suppress decays to excited vibrational states [16,17]. Furthermore Stuhl et al. pointed out that a F = 1 → F ′ = 0 type transition eliminates decays to all but the initial F = 1 state [17]. (We use N , v, I, S, F , M , and P as the rotational, vibrational, nuclear spin, electronic spin, total angular momentum, Zeeman, and parity quantum numbers respectively.) In this scheme dark Zeeman levels in the ground state must be remixed to obtain optical cycling [17,18]. Here we propose and demonstrate a scheme for optical cycling that uses the X 2 Σ + 1/2 → A 2 Π 1/2 electronic transition in the molecule SrF. This transition has the required highly diagonal FCFs. Use of a P branch N = 1 → N ′ = 0 type rotational transition eliminates rotational branching, while a magnetic field remixes the dark Zeeman sublevels. The X(N = 1) state of SrF has four hyperfine structure (HFS) levels, so we outline a technique for simultaneously addressing all of these ...

A cold and slow molecular beam

“…[17][18][19] Molecules produced purely from filtering techniques, however, are not necessarily Buffer-gas cooling is another direct cooling method. 20,21 Buffer-gas cooled beams are applicable to nearly any small molecule 13,18,22,23 because only elastic collisions with cold buffer gases are required to translationally and rotationally cool molecules. 22,24 When a buffer-gas beam is operated in the "hydrodynamic" enhancement regime, where the diffusion time of molecules is longer than the characteristic time the buffer gas spends in the production cell, molecules can be efficiently extracted into a beam, resulting in high molecular flux.…”

Sensitive imaging of electromagnetic fields with paramagnetic polar molecules

“…Alternatively, molecules can be confined in an electric or optical trap and transferred to state |2 by a sequence of microwave pulses [24]. Molecules can be cooled by a variety of recently developed experimental techniques such as buffer-gas cooling, Stark deceleration, or laser cooling [19][20][21][22][23].…”

“…is a number of photons scattered by each molecule before it goes to a dark state, and η is a branching factor equal to Franck-Condon factor [22,23] multiplied by the Hönl-London factor. To our knowledge, the Hönl-London factor for SrF molecules is not available in the literature, therefore for our estimates we use the value 2/3 that was determined for KRb molecules in an experiment reporting the absorption imaging of ultracold molecules [28].…”

Light Gives Molecules the Chills

“…Here SrF offers another pleasant surprise, as all frequencies needed for cooling and vibrational repumping fall into the 660-685 nm range which is easily covered by diode lasers. The group first demonstrated optical cycling of about 150 photons by using the minimum of two diode lasers [9,10], and then proceeded to adding a third laser, which significantly increases the number of scattered photons to 10 3 and thus enables Doppler cooling. Scattering of about 10 5 photons is expected to become feasible in the near future.…”