Deflection of a molecular beam using the bichromatic stimulated force

Abstract

We demonstrate that a bichromatic standing-wave laser field can exert a significantly larger force on a molecule than ordinary radiation pressure. Our experiment measures the deflection of a pulsed supersonic beam of CaF molecules by a two-frequency laser field detuned symmetrically about resonance with the nearly closed X(v = 0) → B(v ′ = 0) transition. The inferred force as a function of relative phase between the two counterpropagating beams is in reasonable agreement with numerical simulations of the bichromatic force in this multilevel system. The large magnitude of the force, coupled with the reduced rate of spontaneous emission, indicates its potential utility in the production and manipulation of ultracold molecules.Radiative forces on atoms have been the major tool enabling laser cooling and trapping [1] and the myriad of applications which have resulted, including precision spectroscopy, quantum degenerate gases, ultracold collisions, and quantum simulations. There are two general types of radiative force: spontaneous force, also known as radiation pressure, and stimulated force, also known as the dipole force. Radiation pressure is the result of absorption/spontaneous emission cycles, while the stimulated force arises from absorption followed by stimulated emission. The latter requires an intensity gradient and can be thought of as a coherent redistribution of photons between various propagation directions. Laser manipulation of atoms is a well-developed field, but recently, these techniques have been increasingly applied to molecules [2]. This extension is nontrivial due to the complicated internal structure of molecules caused by their vibrational and rotational degrees of freedom [3]. Radiation pressure has been used to slow, cool, and trap molecules that fortuitously have near-cycling transitions [4][5][6][7][8][9][10][11][12][13]. Stimulated forces have also been used to manipulate molecules [14-17], but on a more limited scale. Compared to radiation pressure, stimulated forces have two significant advantages for molecules: (1) radiation pressure is limited by the spontaneous emission rate, while stimulated forces can greatly exceed this saturated value; and (2) radiation pressure relies on spontaneous emission which can optically pump the molecules into "dark" states which no longer interact with the laser field.A specific type of stimulated force, the bichromatic force (BCF) [18,19], is particularly promising for manipulating molecules [20]. The BCF involves two frequencies which are tuned symmetrically above and below a resonant frequency ω by ±δ. The two frequencies are both present in oppositely-directed beams, which gives rise to counterpropagating trains of beat notes with a fixed relative phase, χ. In a simplified picture of BCF, if each beat is considered an effective π-pulse which inverts the population, a molecule can be excited by a beat from one direction and rapidly returned to the ground state by a beat from the other direction. Each absorption/stimulated emission cycle imparts an i...

Prospects for laser cooling of polyatomic molecules with increasing complexity

“…There is also the potential of stimulated optical forces with these large molecules. For example, the bichromatic force has been used to control SrOH [66] and CaF [67]. A related technique, using a mode-locked laser, has recently been demonstrated for Rb atoms [68].…”

Spectroscopic characterization of aluminum monofluoride with relevance to laser cooling and trapping

“…Ultracold molecular samples produced this way typically have * truppe@fhi-berlin.mpg.de † meijer@fhi-berlin.mpg.de a density many orders of magnitude lower compared to the association methods [44]. New methods are steadily being developed to deliver more molecules at low speeds to the trapping region [45][46][47][48][49].…”

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].…”