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Frontiers in Optics

Laser Cooling of Molecules by Zero Velocity Selection

C. H. Raymond Ooi

Published Jan 1, 2010• 0 citations
Publisher

Abstract

We present laser cooling scheme for molecules using repeated Raman zero velocity selection, STIRAP deceleration and accumulation by single spontaneous emission which circumvents the multilevels in molecules. Smulations with OH show practicality of the scheme.
DOI
10.1364/ls.2010.lthh3

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Laser cooling of molecules by zero-velocity selection and single spontaneous emission

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A laser-cooling scheme for molecules is presented based on repeated cycle of zero-velocity selection, deceleration, and irreversible accumulation. Although this scheme also employs a single spontaneous emission as in [Raymond Ooi, Marzlin, and Audretsch, Eur. Phys. J. D 22, 259 (2003)], in order to circumvent the difficulty of maintaining closed pumping cycles in molecules, there are two distinct features which make the cooling process of this scheme faster and more practical. First, the zero-velocity selection creates a narrow velocity-width population with zero mean velocity, such that no further deceleration (with many stimulated Raman adiabatic passage (STIRAP) pulses) is required. Second, only two STIRAP processes are required to decelerate the remaining hot molecular ensemble to create a finite population around zero velocity for the next cycle. We present a setup to realize the cooling process in one dimension with trapping in the other two dimensions using a Stark barrel. Numerical estimates of the cooling parameters and simulations with density matrix equations using OH molecules show the applicability of the cooling scheme. For a gas at temperature $T=1$ K, the estimated cooling time is only $2$ ms, with phase-space density increased by about 30 times. The possibility of extension to three-dimensional cooling via thermalization is also discussed.

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A laser-cooling scheme for molecules is presented based on repeated cycle of zero-velocity selection, deceleration, and irreversible accumulation. Although this scheme also employs a single spontaneous emission as in [Raymond Ooi, Marzlin, and Audretsch, Eur. Phys. J. D 22, 259 (2003)], in order to circumvent the difficulty of maintaining closed pumping cycles in molecules, there are two distinct features which make the cooling process of this scheme faster and more practical. First, the zero-velocity selection creates a narrow velocity-width population with zero mean velocity, such that no further deceleration (with many stimulated Raman adiabatic passage (STIRAP) pulses) is required. Second, only two STIRAP processes are required to decelerate the remaining hot molecular ensemble to create a finite population around zero velocity for the next cycle. We present a setup to realize the cooling process in one dimension with trapping in the other two dimensions using a Stark barrel. Numerical estimates of the cooling parameters and simulations with density matrix equations using OH molecules show the applicability of the cooling scheme. For a gas at temperature T=1T=1T=1 K, the estimated cooling time is only 222 ms, with phase-space density increased by about 30 times. The possibility of extension to three-dimensional cooling via thermalization is also discussed.
We propose a laser cooling concept for the translational motion of molecules which does not require repeated spontaneous emission by each molecule. The cooling works by repetition of three main steps; velocity selection of a narrow momentum width, deceleration of velocity selected molecules by repeated STIRAP and accumulation of decelerated molecules via one time spontaneous emission. We use single spontaneous emission for irreversible accumulation of narrow velocity selected and decelerated populations. We develop an analytical model to describe the evolution of the molecular gas system, internal subsystem and external (center of mass) subsystem density matrices and entropies. Simulation results show cooling from an arbitrary momentum width towards final width of about hbark\\hbar khbark. The external entropy is reduced while the internal entropy is increased. The external entropy is shown to decrease during coherent laser interactions, a possibility of translational cooling via unitary evolution. The system entropy increases in consistency with the Araki-Lieb inequality.
A novel scheme is proposed for sequential cooling of rotation, translation, and vibration of molecules. More generally, this scheme manipulates and controls the states and energies of molecules. The scheme, while somewhat complex, is simpler and more feasible than simply providing a large number of synchronously but independently tunable lasers. The key component is a multiple single frequency laser (MSFL) in which a single narrow band pump laser generates an ensemble of resonant ‘‘stimulated Raman’’ (RSR) sidebands (subsequently amplified and selected) in a sample of the molecules to be cooled. Starting with a relatively cold molecular sample (e.g., a supersonic beam of Cs2), the rotation of molecules is cooled by sequential application of P branch electronic transition frequencies transverse to the molecular beam beginning at higher rotational angular momentum J. Then translation of molecules is cooled by application of multiple low J, P, and R branch transition frequencies which counterpropagate with the molecular beam and are synchronously chirped over their Doppler profiles. Finally, vibration of molecules is cooled by blocking the R(0) line of the 0–0 band. Only this specific order of rotation–translation–vibration appears feasible (using molecules produced by photoassociation of ultracold atoms avoids the requirement for translational cooling). Each step employs true dissipative cooling (i.e., reduction of system entropy in three degrees of freedom) by spontaneous emission and should yield a large translationally cold sample of molecules in the lowest (v=0, J=0) level of the ground electronic state, suitable for studies such as molecule trapping, ‘‘molecule optics,’’ or long range intermolecular states.
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.
We describe a possible method for laser cooling OH molecules. The method is based on exciting the Δ J = 2 stimulated Raman transition in the ground electronic and vibronic state. The molecules are assumed to already be trapped with a temperature of ∼10 mK or less in a spatially varying magnetic field. By exciting the molecule in a region of high magnetic field, the subsequent spontaneous photon emission at lower magnetic field removes centre-of-mass energy from the molecule. The vacuum decay rate from the J = 7/2, F = 4, M F = 4 to J = 5/2, F = 3, M F = 3 is roughly 0.31 Hz and the next cascade to J = 3/2, F = 2, M F = 2 is roughly 0.15 Hz. A cavity tuned to stimulate both transitions in a small region of space can enhance the decay. Simulations of the radiative transitions indicate that up to 20% of a population of OH molecules with an average energy of 10 mK will finish with energies less than 200 µK.
Cooling of molecules via free-space dissipative scattering of photons is thought not to be practicable due to the inherently large number of Raman loss channels available to molecules and the prohibitive expense of building multiple-repumping laser systems. The use of an optical cavity to enhance coherent Rayleigh scattering into a decaying cavity mode has been suggested as a potential method to mitigate Raman loss, thereby enabling the laser cooling of molecules to ultracold temperatures. We discuss the possibility of cavity-assisted laser cooling of particles without closed transitions, identify conditions necessary to achieve efficient cooling, and suggest solutions given experimental constraints. Specifically, it is shown that cooperativities much greater than unity are required for cooling without loss, and that this could be achieved via the superradiant scattering associated with intracavity self-localization of the molecules. Particular emphasis is given to the polar hydroxyl radical (OH), cold samples of which are readily obtained from Stark deceleration.