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Physical Review A

Cooling molecules in optical cavities

Weiping Lu, Yongkai Zhao, P. F. Barker

Published Jul 25, 2007• 28 citations
Publisher

Abstract

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 $100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ temperatures to submillikelvin temperatures in less than $1\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$. 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 ${10}^{9}\phantom{\rule{0.3em}{0ex}}\text{molecules}$ can be cooled in the cavity by use of a far-off-resonant and high-intensity pump source.

DOI
10.1103/physreva.76.013417

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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 100mK100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}100mK temperatures to submillikelvin temperatures in less than 1ms1\phantom{\rule{0.3em}{0ex}}\mathrm{ms}1ms. 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 109molecules{10}^{9}\phantom{\rule{0.3em}{0ex}}\text{molecules}109molecules can be cooled in the cavity by use of a far-off-resonant and high-intensity pump source.
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.
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
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.
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.
The efficiency of cavity sideband cooling of trapped molecules is theoretically investigated for the case in which the infrared transition between two rovibrational states is used as a cycling transition. The molecules are assumed to be trapped either by a radiofrequency or optical trapping potential, depending on whether they are charged or neutral, and confined inside a high-finesse optical resonator that enhances radiative emission into the cavity mode. Using realistic experimental parameters and COS as a representative molecular example, we show that in this setup, cooling to the trap ground state is feasible.