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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Cooling of Molecules in Optical Cavities

Weiping Lu, Y. Zhao, P. F. Barker

Published Jun 1, 2007• 1 citation
Publisher

Abstract

Inspired by the great success in laser cooling of atoms, scientists now want to create cold molecules to study their applications that have already realized for cold atoms. However, a simple extension of laser cooling techniques fails for molecules because complex energy structure of most molecules makes a closed multi-level system impossible. Buffer gas cooling is a general scheme that can produce cold paramagnetic molecules at temperatures of hundreds milikelvin (mK). The next requirement for this field to be successful is to develop new and complementary techniques that can break the mK bottleneck and produce a dense molecular samples in microkelvin (muK) region. In this report, we study a general optical cavity cooling scheme to cool molecular species from hundreds mK to muK temperature region.

DOI
10.1109/cleoe-iqec.2007.4386721

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We have studied theoretically and numerically the cooling of CN molecules in a high-finesse optical cavity and show that these molecules can be cooled from 100 mK temperatures to submillikelvin temperatures in less than 1 ms. We establish that the cooling time does not change significantly with molecular numbers and initial temperatures over a wide range. We have further studied the scaling of the system for extending the current results for hundreds of molecules to a very large molecular ensemble. The results indicate that a gas of 109 molecules can be cooled in the cavity by use of a far-off-resonant and high-intensity pump source. © 2007 The American Physical Society.

•Jan 1, 2007•1 citation
Inspired by the great success in laser cooling of atoms, scientists now want to create cold molecules to study their applications that have already realized for cold atoms. However, a simple extension of laser cooling techniques fails for molecules because complex energy structure of most molecules makes a closed multi-level system impossible. Buffer gas cooling is a general scheme that can produce cold paramagnetic molecules at temperatures of hundreds milikelvin (mK). The next requirement for this field to be successful is to develop new and complementary techniques that can break the mK bottleneck and produce a dense molecular samples in microkelvin (muK) region. In this report, we study a general optical cavity cooling scheme to cool molecular species from hundreds mK to muK temperature region.
The ability to cool and trap atoms has revolutionized atomic and ultra-cold physics. Molecular physics is currently undergoing a similar transformation. This thesis aims to research a general cooling method that will be applicable to wide range of molecular and atomic species and other particles. We studied the dynamics of molecules in optical fields, focusing in particular on exploring the molecular self-organisation phenomena in optical cavities to cool molecular ensembles to sub-mK temperatures. Firstly, the general model of cavity cooling from atoms to molecules and the dynamics of a particle in a single cavity mode were discussed. We extended the existing cooling scheme for two-level atoms to an ensemble of multi-level molecules. Then we studied the spatial dynamics of molecules in the new parameter conditions, focusing in particular on exploring the molecular self-organisation phenomena in optical cavities to cool molecular ensembles to sub-mK temperatures. The scheme complements well with our present experimental work on the deceleration and focusing of cold molecules and can extend our present capability to simultaneously cool and trap a large cold molecular ensemble. For simulation of a large ensemble of molecules, we proposed a new statistical model based on the Boltzmann equation beside the traditional discrete model and studied two solution methods. The comparison of the theory and numerical simulations between discrete model and statistical model showed a good agreement, which validated this new model. We then explored the scaling laws with a view to the self-organization and cooling of a large ensemble of species. We studied the cooling of a CN molecular cloud of the density 10/cm, with an initial temperature at 10 mK in an optical cavity. We found that more than a third of the molecules are stably trapped by the intracavity field and the final temperature is below 1mK. We discussed the scaling laws in the case when a large ensemble of species is involved. Finally we argue that cavity cooling using a far
Different from atoms, molecules have unique properties, and play an important role in the research of atomic, molecular and optical physics. Cold molecules have important applications in science and have been studied for more than 20 years. But traditional methods, such as the Stark decelerator, have hit a bottleneck: it is hard to increase the phase space density of molecules. Extending the direct laser-cooling technique to new molecular species has recently been a hot topic and also a big challenge. In this review paper, on one hand, we make a brief review to recent progresses on the direct laser cooling of polar molecules. On the other hand, a demonstration on the feasibility of laser cooling BaF molecule has been experimentally illustrated, including the analysis on the molecular energy levels, measurements of the high-resolution spectroscopy, efficient pre-cooling and state preparation via buffer-gas cooling and detailed investigations on the molcule-light interactions. All these results not only pave the way for future laser-cooling and -trapping experiments, but also serve as a reference for the laser-cooling explorations on new molecular species.
Abstract The extension of laser cooling and trapping techniques from atoms to molecules is currently the subject of intense and growing interest. Molecular laser cooling and trapping offers a general technique applicable to a variety of molecules with different properties and internal structures and is well-suited to the growing list of proposed applications for ultracold molecules. The recent development of optical cycling schemes for specific molecular species now allow each molecule to scatter up to ∼10 6 photons. This has proven sufficient for multiple groups to laser cool and slow molecular beams and, notably, capture molecules in magneto-optical traps (MOTs) and cool them to ultracold temperatures. MOTs are the workhorse technique for atomic physics in the ultracold regime and today’s molecular MOTs represent a powerful starting point for the study and manipulation of ultracold molecular gases. This Topical Review presents a summary of the progress made in this new and emerging field.
Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be
Cooling a range of molecules to ultracold temperatures (<1 mK) is a difficult but important challenge in molecular physics and chemistry. Collective cavity cooling of molecules is a promising method that does not rely on molecular energy level and thus can be applied to all molecules in principle. However, the initial lack of cold molecules leads to the difficulty in its experimental implementation. We show that efficient collective sympathetic cooling of molecules to sub-mK temperatures using a large ensemble of atoms within a cavity is feasible. This approach is a new type of sympathetic cooling which does not rely on direct collisions between atoms and molecules, but utilizes thermalization via their mutual interaction with a cavity field. Two important mechanisms are identified. This include: (1) giant enhancement of cavity optical field from the efficient scattering of the pump light by the atoms; (2) cavity-mediated collective interaction between the atoms and the molecules. We show an optimal cavity detuning for maximizing cooling, which is dependent on the atom and molecule numbers. We determine a threshold for the molecular pump strength and show that it is independent of molecule number when the number of atoms is much greater than the molecules. This can be reduced by orders of magnitude when compared to cavity cooling of single molecular species only. Using this new sympathetic cavity cooling technique, cooling molecules to sub-mK within a high-Q cavity could be within reach of experimental demonstration.
We have studied theoretically and numerically the cooling of CN molecules in a high-finesse optical cavity and show that these molecules can be cooled from 100 mK temperatures to submillikelvin temperatures in less than 1 ms. We establish that the cooling time does not change significantly with molecular numbers and initial temperatures over a wide range. We have further studied the scaling of the system for extending the current results for hundreds of molecules to a very large molecular ensemble. The results indicate that a gas of 109 molecules can be cooled in the cavity by use of a far-off-resonant and high-intensity pump source. © 2007 The American Physical Society.