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arXiv:1903.01627 · Physical Review Letters

Suppressed Spontaneous Emission for Coherent Momentum Transfer

Xueping Long, Scarlett S. Yu, Andrew M. Jayich, Wesley C. Campbell

Published Jul 19, 2019• 18 citations
arXivHTMLPDF

Abstract

Strong optical forces with minimal spontaneous emission are desired for molecular deceleration and atom interferometry applications. We report experimental benchmarking of such a stimulated optical force driven by ultrafast laser pulses. We apply this technique to accelerate atoms, demonstrating up to an average of 19ℏk momentum transfers per spontaneous emission event. This represents more than an order of magnitude improvement in suppression of spontaneous emission compared to radiative scattering forces. For molecular beam slowing, this technique is capable of delivering a many-fold increase in the achievable time-averaged force to significantly reduce both the slowing distance and detrimental losses to dark vibrational states.

DOI
10.1103/physrevlett.123.033603

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Strong optical forces with minimal spontaneous emission are desired for molecular deceleration and atom interferometry applications. We report experimental benchmarking of such a stimulated optical force driven by ultrafast laser pulses. We apply this technique to accelerate atoms, demonstrating up to an average of 19ℏk momentum transfers per spontaneous emission event. This represents more than an order of magnitude improvement in suppression of spontaneous emission compared to radiative scattering forces. For molecular beam slowing, this technique is capable of delivering a many-fold increase in the achievable time-averaged force to significantly reduce both the slowing distance and detrimental losses to dark vibrational states.
The spontaneous decay rate $\ensuremath{\gamma}$ of excited atomic states limits the radiative force on atoms to $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2.$ This bottleneck in the dissipative exchange of momentum between atoms and light can be overcome by coherent control of the momentum exchange interactions. The bichromatic force provides such control by using stimulated instead of spontaneous emission to produce the force, while at the same time exploiting spontaneous emission to enable a dissipative interaction. It is implemented with two counterpropagating light beams, each containing two frequencies, whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.
Over the past 30 years, optical manipulation of neutral atoms has been primarily performed with a monochromatic laser beam. The simplest tool for the control of atomic motion is the radiative force exerted by a monochromatic laser on a twolevel atom. The radiative force arises from absorption followed by spontaneous emission, and its magnitude is limited by the atom's excited state lifetime. The coherent momentum exchanges between light elds and atoms can be exploited to produce long-range optical forces much greater than the radiative force through the use of absorption-stimulated emission processes. Adiabatic Rapid Passage (ARP) is a long-existing method to invert the population of a two-level nuclear spin system. Its extension to the optical domain necessitates a frequency chirped light pulse to interact with a two-level atom via the dipole interaction. I will first present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple ARP sequences. This will be followed by experimental observations of long-range ARP forces much larger than the radiative force in metastable helium. Sequences of properly timed laser pulses may be used for rapid deceleration of neutral atomic (or molecular) beams. | 171 pages
Ultracold molecular gases are promising as an avenue to rich many-body physics, quantum chemistry, quantum information, and precision measurements. This richness, which flows from the complex internal structure of molecules, makes the creation of ultracold molecular gases using traditional methods (laser plus evaporative cooling) a challenge, in particular due to the spontaneous decay of molecules into dark states. We propose a way to circumvent this key bottleneck using an all-optical method for decelerating molecules using stimulated absorption and emission with a single ultrafast laser. We further describe single-photon cooling of the decelerating molecules that exploits their high dark state pumping rates, turning the principal obstacle to molecular laser cooling into an advantage. Cooling and deceleration may be applied simultaneously and continuously to load molecules into a trap. We discuss implementation details including multilevel numerical simulations of strontium monohydride. These techniques are applicable to a large number of molecular species and atoms with the only requirement being an electric dipole transition that can be accessed with an ultrafast laser.
Heavy atoms play a critical role in scientific research and applications by providing a better understanding of fundamental physical phenomena. The use of a slowed atomic beam to extend the interaction time is expected to enhance measurement sensitivity, resolution, and precision. Here, we theoretically demonstrate a rapid and short-distance deceleration of heavy atoms by using a continuous stimulated force with optical phase compensation and frequency chirping. The scheme effectively overcomes the spatial inhomogeneity of the stimulated force on atoms during deceleration. Our approach employs the numerical solutions of the optical Bloch equations for two-level atoms driven by time-dependent fields, and simulates atom trajectories using a three-dimensional Monte Carlo method. Concerning a buffer-gas-cooled radium atom beam with an initial velocity of 130 m/s, from our simulations it can be decelerated to 5 m/s within 5 cm distance using 40 mW laser power, achieving a deceleration force approximately 35 times stronger than the radiation force and covering an effective velocity range over eight times that of the radiation force. In addition, the deceleration of other heavy atoms, such as francium and thallium, also are applicable. This approach offers an efficient tool for rapidly slowing and cooling complex atoms and molecules, facilitating future precision measurements.
The bichromatic force allows laser cooling using stimulated processes only. We provide details of our simulations of the bichromatic force when the effects of spontaneous emission are suppressed by restricting the atom-light interaction time to be short enough that there cannot be a significant number of spontaneous emission events. This short interaction time requires that the simulation include dynamics of the atomic motion through the light field that is coupled to the internal state dynamics that determine the force on the atom. The simulation is first tested with several standard optical field configurations to confirm its accuracy. Then the results, using conditions that match our experiment, are presented and compared with our measurements. The simulation shows that the resolution of the experiment greatly obscures the degree of cooling. Our simulation predicts an observed velocity distribution reduced by up to a factor of 4 in width over a time comparable to the excited state lifetime. This technique can allow the direct laser cooling of atoms and molecules without closed cycling transitions.
The optical scattering force, behind Doppler cooling and magneto-optical trapping, may be amplified without incurring additional spontaneous emission by the state-dependent coherent deflection produced by a pulsed or chirped laser field. At some cost in experimental complexity, amplified forces allow efficient cooling on narrow transitions and permit the compact deceleration of beams with reduced transverse heating, and will be of interest for molecules and atoms with open level schemes where losses following spontaneous emission would otherwise prevail. We present a general analysis of the amplification scheme, and propose an optimized, dynamic cooling scheme that allows the temperature of a sample to be reduced by around a factor of two per excited state lifetime.