Coherent Bichromatic Force Deflection of Molecules

Abstract

We demonstrate the coherent optical bichromatic force on a molecule, the polar free radical strontium monohydroxide (SrOH). A dual-frequency retro-reflected laser beam addressing theX 2 Σ + ↔Ã 2 Π 1/2 electronic transition coherently imparts momentum onto a cryogenic beam of SrOH. This directional photon exchange creates a bichromatic force that transversely deflects the molecules. By adjusting the relative phase between the forward and counter propagating laser beams we reverse the direction of the applied force. A momentum transfer of 70 k is achieved with minimal loss of molecules to dark states. Modeling of the bichromatic force is performed via direct numerical solution of the time-dependent density matrix and is compared with experimental observations. Our results open the door to further coherent manipulation of molecular motion, including the efficient optical deceleration of diatomic and polyatomic molecules with complex level structures.Laser manipulation of atomic motion has revolutionized atomic, molecular and optical (AMO) physics [1,2]. The widely-used techniques of laser cooling and trapping made possible the creation of ultracold degenerate quantum gases [3], simulation of important condensed matter models [4] and development of new quantum sensors [5,6] and clocks [7]. Laser deceleration and cooling of atomic beams -a necessary part of the trap loading process -typically requires scattering tens of thousands of photons in order to bring room (or oven) temperature atoms to velocities where they can be confined by electromagnetic traps for further studies [8]. While beam deceleration employing the spontaneous radiation pressure force has been a standard for atomic experiments, its application to slowing molecular beams has been limited by the small change in kinetic energy per scattered photon and the myriad of internal molecular states, which inhibits photon cycling. At the same time, there is extreme interest in creating ultracold molecules for new physics applications [9].Neutral diatomic molecules are predicted to play an important role in diverse research areas of modern physics such as quantum simulation [10] and computation [11], as well as searches for new particles and fields beyond the Standard Model [12]. Larger polyatomic molecules will provide additional opportunities in physics and chemistry [13][14][15]. For example, exploring the origin of biomolecular homochirality [16] and understanding primordial chemistry leading to the development of organic life requires the use of large molecules [17]. However, these molecules' complexity presents significant challenges for direct laser slowing and cooling. Yet these are the key ingredients that allow for optical trapping, which, in turn, realizes long moleculelaser coherence times and high levels of quantum state control. Previously, the external motion of gas-phase polyatomic molecules has been manipulated with off-resonant laser fields [18] as well as electric [19], magnetic [20], and mechanical techniques [21]. Inspired by the success of...

Ultracold collisions of polyatomic molecules: CaOH

“…The newest members on the list of laser-coolable species are polyatomic species [7][8][9][10]. The linear triatomic species SrOH is a good candidate for laser cooling [11], and has been deflected by optical forces [12], opening the way for similar species such as CaOH and CaOCH 3 [13], and more besides. These species should expand opportunities for quantum information, sensing, and fundamental physics [9].…”

Laser cooling with adiabatic transfer on a Raman transition

“…After each photon absorption event, a molecule needs to return to the initial state via spontaneous emission for Doppler cooling to work, but it may be lost instead due to the abundance of accessible vibrational and rotational degrees of freedom [20,21]. This challenge has motivated the development of alternative techniques that rely on reducing the role of spontaneous emission relative to Doppler cooling, including Sisyphus cooling of molecules [18,23], bichromatic force slowing and cooling [16,24], and interferometric cooling [19].…”

A new route for laser cooling and trapping of cold molecules: Intensity-gradient cooling of MgF molecules using localized hollow beams

“…temperature was achieved by Λ-enhanced gray molasses and deep cooling with a single laser, both of which can cool CaF to several mK [27,28]. Besides, laser cooling and deceleration of molecules with stimulated force has also progressed a lot both theoretically and experimentally [29][30][31].As the common cooling method before molecules loaded into a type-II MOT, laser sweep slowing or whitelight slowing, both of which are used to compensate Doppler shift, are not a very efficient way for larger number of molecules. This causes the problem of the small number and the low density of trapped molecules in the type-II MOT.…”

“…temperature was achieved by Λ-enhanced gray molasses and deep cooling with a single laser, both of which can cool CaF to several mK [27,28]. Besides, laser cooling and deceleration of molecules with stimulated force has also progressed a lot both theoretically and experimentally [29][30][31].…”