Laser cooling of molecules by dynamically trapped states

Faraday Discussions, 1999

In the last several years we have discovered a variety of remarkable pulse strategies for manipulating molecular motion by employing a design strategy we call "" local optimization.ÏÏ Here we review the concept of local optimization and contrast it with optimal control theory. By way of background, we give highlights from two recent examples of the method : (1) a strategy for eliminating population transfer to one or many excited electronic states during strong Ðeld excitation, an e †ect we call " optical paralysis Ï ;

The Journal of Physical Chemistry A, 1999

Optimal control theory (OCT) is applied to the problem of cooling molecular rotations. The optimal field gives rise to a striking behavior, in which there is no noticeable increase in the lowest rotational state population until the last percent or so of the control interval, at which point the population jumps to 1. Further analysis of the intermediate time interval reveals that cooling is taking place all along, in the sense that the purity of the system, as measured by Tr(F 2), is increasing monotonically in time. Once the system becomes almost completely pure, the external control field can transfer the amplitude to the lowest rotational state by a completely Hamiltonian manipulation. This mechanism is interesting because it suggests a possible way of accelerating cooling, by exploiting the cooling induced by spontaneous emission to all the ground electronic state levels, not just the lowest rotational level. However, it also raises a major paradox: it may be shown that external control fields, no matter how complicated, cannot change the value of Tr(F 2); changing this quantity requires spontaneous emission which is inherently uncontrollable. What place is there then for control, let alone optimal control, using external fields? We discuss the resolution to this paradox with a detailed analysis of cooling in a two-level system.

Science, 2008

The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, Cs 2 molecules initially formed via photoassociation of cold Cs atoms are in several vibrational levels, v, of the electronic ground state. Here we apply a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation -spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the v = 0 level, preventing re-excitation from that state. We observe a fast and efficient accumulation, ∼ 70% of the initially detected molecules, in the lowest vibrational level, v = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.

Physical Review A

We propose a scheme that combines velocity-selective coherent population trapping (CPT) and Raman sideband cooling (RSC) for subrecoil cooling of optically trapped atoms outside the Lamb-Dicke regime. This scheme is based on an inverted Y configuration in an alkali-metal atom. It consists of a Λ formed by two Raman transitions between the ground hyperfine levels and the D transition, allowing RSC along two paths and formation of a CPT dark state. Using statedependent difference in vibration frequency of the atom in a circularly polarized trap, we can tune the Λ to make only the motional ground state a CPT dark state. We call this scheme motionselective coherent population trapping (MSCPT). We write the master equations for RSC and MSCPT and solve them numerically for a 87 Rb atom in a one-dimensional optical lattice when the Lamb-Dicke parameter is 1. Although MSCPT reaches the steady state slowly compared with RSC, the former consistently produces colder atoms than the latter. The numerical results also show that subrecoil cooling by MSCPT outside the Lamb-Dicke regime is possible under a favorable, yet experimentally feasible, condition. We explain this performance quantitatively by calculating the relative darkness of each motional state. Finally, we discuss on application of the MSCPT scheme to an optically trapped diatomic polar molecule whose Stark shift and vibration frequency exhibit large variations depending on the rotational quantum number.

Many aspects of intense-field molecular dynamics rely on the concept of resonances. The chapter gives a thorough review of these aspects, bringing out the specificity of laser-induced resonances, in particular those defined in the Floquet or dressed molecule picture. The role of these resonances in the timeresolved dynamics of molecules subjected to an intense, ultrafast laser pulse is discussed and basic mechanisms of molecular fragmentation and its control are reviewed. We discuss how a thorough interpretation of two-colour XUV + IR pump-probe experiments on the dissociative ionization of H 2 can be made in terms of adiabatic vs. non-adiabatic resonance transports (i.e. laser-induced time evolutions) and in terms of field-induced processes such as Bond-Softening (BS) and Vibrational Trapping (VT), associated with the Floquet representation or the Dynamical Dissociation Quenching (DDQ) effects associated with a time-dependent quasi-static representation. Another application of the concepts of laser-induced resonances, and of their adiabatic evolution, is devoted to laser control strategies based on Zero-Width Resonances (ZWR) and Exceptional Points (EP), the approach of which in laser parameter space corresponds to the coalescence of two laser-induced resonances. We illustrate how the concept of ZWR can be useful for the molecular cooling problem. We then show how advantage can be taken of resonance coalescence at an EP to devise new laser control strategies pertaining to vibrational energy transfer processes.

The Journal of Chemical Physics, 1997

Theoretical progress in the cooling of internal degrees of freedom of molecules using shaped laser pulses is reported. The emphasis is on general concepts and universal constraints. Several alternative definitions of cooling are considered, including reduction of the von Neumann entropy, Ϫtr͕ log͖ and increase of the Renyi entropy, tr͕ 2 ͖. A distinction between intensive and extensive considerations is used to analyse the cooling process in open systems. It is shown that the Renyi entropy increase is consistent with an increase in the system phase space density and an increase in the absolute population in the ground state. The limitations on cooling processes imposed by Hamiltonian generated unitary transformations are analyzed. For a single mode system with a ground and excited electronic surfaces driven by an external field it is shown that it is impossible to increase the ground state population beyond its initial value. A numerical example based on optimal control theory demonstrates this result. For this model only intensive cooling is possible which can be classified as evaporative cooling. To overcome this constraint, a single bath degree of freedom is added to the model. This allows a heat pump mechanism in which entropy is pumped by the radiation from the primary degree of freedom to the bath mode, resulting in extensive cooling.

Physical Review A, 2008

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 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.

We demonstrate slowing and longitudinal cooling of a supersonic beam of CaF molecules using counter-propagating laser light resonant with a closed rotational and almost closed vibrational transition. A group of molecules are decelerated by about 20 m/s by applying light of a fixed frequency for 1.8 ms. Their velocity spread is reduced, corresponding to a final temperature of about 85 mK. The velocity is further reduced by chirping the frequency of the light to keep it in resonance as the molecules slow down.

Physical Review A, 2002

We demonstrate the possibility of energy-selective removal of cold atoms from a tight optical trap by means of parametric excitation of the trap vibrational modes. Taking advantage of the anharmonicity of the trap potential, we selectively remove the most energetic trapped atoms or excite those at the bottom of the trap by tuning the parametric modulation frequency. This process, which had been previously identified as a possible source of heating, also appears to be a robust way for forcing evaporative cooling in anharmonic traps.

Physical Review Letters, 2000

We point out a laser cooling method for atoms, molecules, or ions at low saturation and large detuning from the particles' resonances. The moving particle modifies the field inside a cavity with a time delay characteristic of the cavity linewidth, while the field acts on the particle via the light shift. The dissipative mechanism can be interpreted as Doppler cooling based on preferential scattering rather than preferential absorption. It depends on particle properties only through the coherent scattering rate, opening new possibilities for optically cooling molecules or interacting atoms.