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arXiv:1603.04089 · Journal of Physics B Atomic Molecular and Optical Physics

Radiation pressure force from optical cycling on a polyatomic molecule

Ivan Kozyryev, Louis Baum, Kyle Matsuda, Boerge Hemmerling, John M. Doyle

Published Jun 1, 2016• 33 citations
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Abstract

Abstract We demonstrate multiple photon cycling and radiative force deflection on the triatomic free radical strontium monohydroxide (SrOH). Optical cycling is achieved on SrOH in a cryogenic buffer-gas beam by employing the rotationally closed P ( N ″ = 1 ) branch of the vibronic transition X ˜ 2 Σ + ( 000 ) ↔ A ˜ 2 Π 1 / 2 ( 000 ) . A single repumping laser excites the Sr–O stretching vibrational mode, and photon cycling of the molecule deflects the SrOH beam by an angle of 0.2 ° via scattering of ∼100 photons per molecule. This approach can be used for direct laser cooling of SrOH and more complex, isoelectronic species.

DOI
10.1088/0953-4075/49/13/134002

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Abstract We demonstrate multiple photon cycling and radiative force deflection on the triatomic free radical strontium monohydroxide (SrOH). Optical cycling is achieved on SrOH in a cryogenic buffer-gas beam by employing the rotationally closed P ( N ″ = 1 ) branch of the vibronic transition X ˜ 2 Σ + ( 000 ) ↔ A ˜ 2 Π 1 / 2 ( 000 ) . A single repumping laser excites the Sr–O stretching vibrational mode, and photon cycling of the molecule deflects the SrOH beam by an angle of 0.2 ° via scattering of ∼100 photons per molecule. This approach can be used for direct laser cooling of SrOH and more complex, isoelectronic species.
We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH). This is achieved with principal optical cycling in the rotationally closed P(N^{''}=1) branch of either the X[over ˜]^{2}Σ^{+}(000)↔A[over ˜]^{2}Π_{1/2}(000) or the X[over ˜]^{2}Σ^{+}(000)↔B[over ˜]^{2}Σ^{+}(000) vibronic transitions. Molecules lost into the excited vibrational states during the cooling process are repumped back through the B[over ˜](000) state for both the (100) level of the Sr-O stretching mode and the (02^{0}0) level of the bending mode. The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK. This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.
We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH).This is achieved with principal optical cycling in the rotationally closed PðN 00 ¼ 1Þ branch of either the X2 Σ þ ð000Þ ↔ Ã2 Π 1=2 ð000Þ or the X2 Σ þ ð000Þ ↔ B2 Σ þ ð000Þ vibronic transitions.Molecules lost into the excited vibrational states during the cooling process are repumped back through the Bð000Þ state for both the (100) level of the Sr-O stretching mode and the ð02 0 0Þ level of the bending mode.The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK.This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.
Polyatomic molecules, while ubiquitous in nature, were generally perceived as too complicated and "unruly" for the majority of quantum physics experiments. However, recent theoretical analyses have pinpointed the multiple advantages of using complex molecules for diverse applications such as the creation of a universal quantum computer, quantum simulation of non-trivial many-body states, and tests of fundamental physical laws. Throughout my dissertation research I focused on extending experimental techniques from atomic physics to complex molecules. The goal of my work was to not only demonstrate new ways to control molecules, but also develop a novel experimental platform for quantum science - complex polyatomic molecules - and provide tools for physicists to conquer new frontiers in physics and chemistry.\nThe main theme of this thesis is the use of laser radiation to control internal and external degrees of freedom for the triatomic radical strontium monohydroxide (SrOH). The research achievements presented here can be primarily divided into the following areas:\ni) out-of-equilibrium studies of vibrationally inelastic collisions at cryogenic temperatures,\nii) demonstration of the radiation pressure force and the direct laser cooling of polyatomic molecules, and\niii) the development of concepts for laser cooling of molecules with six and more atoms.\nFor the collisional studies, off-diagonal optical pumping was used to populate excited Sr-O stretching state and the ratio of elastic to vibrational quenching collisions was determined to be ~700. In the laser cooling work, using only laser light, the transverse temperature of the cryogenic buffer-gas beam of SrOH was reduced from 50 mK to below 1 mK, leading to an order of magnitude increase in phase space density of the molecular beam. Additionally, the prospects of using a coherent bichromatic force for optical deceleration of polyatomics were analyzed and experimentally characterized with the transverse deflection of the SrOH beam. Our experimental results with SrOH and theoretical analysis of more complex strontium alkoxides not only open a wide range of future directions for laser manipulation and trapping of polyatomic molecules but also help overturn long held assumptions about the difficulties of performing "traditional" atomic physics experiments with complex molecules.
We demonstrate the effect of the coherent optical bichromatic force on a molecule, the polar free radical strontium monohydroxide (SrOH). A dual-frequency retroreflected laser beam addressing the X[over ˜]^{2}Σ^{+}↔A[over ˜]^{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 counterpropagating laser beams we reverse the direction of the applied force. A momentum transfer of 70ℏk 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.
We demonstrate a scheme for optical cycling in the polar, diatomic molecule strontium monofluoride (SrF) using the X2Sigma+ --> A2Pi(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.
We present the first experimental demonstration of radiation pressure force deflection and direct laser cooling for barium monohydride (BaH) molecules resulting from multiple photon scattering. Despite a small recoil velocity (2.7 mm s −1 ) and a long excited state lifetime (137 ns), we use 1060 nm laser light exciting the X → A electronic transition of BaH to deflect a cryogenic buffer-gas beam and reduce its transverse velocity spread. Multiple experimental methods are employed to characterize the optical cycling dynamics and benchmark theoretical estimates based on rate equation models as well as solutions of the Lindblad master equation for the complete multilevel system. Broader implications for laser cooling and magneto-optical trapping of heavy-metal-containing molecules with narrow transition linewidths are presented. Our results pave the way for producing a new class of ultracold molecules—alkaline earth monohydrides—via direct laser cooling and trapping, opening the door to realizing a new method for delivering ultracold hydrogen atoms (Lane 2015 Phys . Rev . A 92 , 022511).