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Frontiers of Physics

Laser cooling with adiabatic passage for type-II transitions

Qian Liang, Tao Chen, Wenhao Bu, Yuhe Zhang, Bo Yan

Published Dec 4, 2020• 12 citations
Publisher

Abstract

We extend the idea of laser cooling with adiabatic passage to multi-level type-II transitions. We find the cooling force can be significantly enhanced when a proper magnetic field is applied. That is because the magnetic field decomposes the multi-level system into several two-level sub-systems, hence the stimulated absorption and stimulated emission can occur in order, allowing for the multiple photon momentum transfer. We show that this scheme also works on the laser-coolable molecules with a better cooling effect compared to the conventional Doppler cooling. A reduced dependence on spontaneous emission based on our scheme is observed as well. Our results suggest this scheme is very feasible for laser cooling of polar molecules.

DOI
10.1007/s11467-020-1019-8

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We extend the idea of laser cooling with adiabatic passage to multi-level type-II transitions. We find the cooling force can be significantly enhanced when a proper magnetic field is applied. That is because the magnetic field decomposes the multi-level system into several two-level sub-systems, hence the stimulated absorption and stimulated emission can occur in order, allowing for the multiple photon momentum transfer. We show that this scheme also works on the laser-coolable molecules with a better cooling effect compared to the conventional Doppler cooling. A reduced dependence on spontaneous emission based on our scheme is observed as well. Our results suggest this scheme is very feasible for laser cooling of polar molecules.
We present a magnetically enhanced laser cooling scheme applicable to multi-level type-II transitions and further diatomic molecules with adiabatic transfer. An angled magnetic field is introduced to not only remix the dark states, but also decompose the multi-level system into several two-level sub-systems in time-ordering, hence allowing multiple photon momentum transfer. For complex multi-level diatomic molecules, although the enhancement gets weakened, our simulations still predict a ∼4×\sim 4\times∼4× larger value of the maximum achievable cooling force and a wider coolable velocity range compared to the conventional Doppler cooling. A reduced dependence on spontaneous emission of this scheme makes laser cooling a molecule with leakage channels become a feasibility.
We provide a theoretical analysis for a recently demonstrated cooling method. Two-level particles undergo successive adiabatic transfers upon interaction with counterpropagating laser beams that are repeatedly swept over the transition frequency. We show that particles with narrow linewidth transitions can be cooled to near the recoil limit. This cooling mechanism has a reduced reliance on spontaneous emission compared to Doppler cooling, and hence shows promise for application to systems lacking closed cycling transitions, such as molecules.
We have observed a new type of sub-Doppler laser cooling with neither polarization gradients nor magnetic fields. All known sub-Doppler laser cooling methods require one of these two to resdistribute the atomic population among the energy levels. This allows the irreversible processes of optical pumping vels. This allows the irreversible processes of optical pumping (OP) to manipulate the conservative force of light shifts (e.g. resonant exchange between the two beams comprising a standing wave) into a damping force. Our scheme employs stimulated Raman transitions between ground state hyperfine levels of Rb with a bichromatic standing wave, accompanied by a momentum change of {Delta}p = 2{h_bar}k. OP, with {Delta}p = 1{h_bar}k, creates a population imbalance allowing repetition of the Raman process. When the detuning of the light from Raman resonance {delta} is properly chosen, the net momentum exchange damps the atomic motion. Changing {delta} by only {gamma}/6(1 MHz) replaces cooling by heating. This process separates the velocity selective Raman resonance and OP processes differently from others, and helps clarify the role of these different mechanisms.
Laser cooling refers to the techniques that cool atomic and molecular samples down to near absolute zero through the interaction with laser fields. Atoms absorb a photon and the transferred momentum generates force to lower the atom's velocity. The Doppler shift is associated with the motion of the atom, in which a moving atom always observes a slightly different frequency than the laser at rest. Based on the two-level system theory, this thesis studied the process of multiple sequences of adiabatic rapid passage by simulating the process of the interaction between optical forces and helium atoms. By studying the force map in parameter space, trying to understand the relationship between the average force and relevant laser parameters Ω0\Omega_0Ω0​ and δ0\delta_0δ0​ in the Rabi frequency. Furthermore, we studied the relationship between the average force and velocity by comparing the dragged atom model and moving atom model. We also studied the process of multiple sequences of adiabatic rapid passage by changing spontaneous emission, sweep range, pulse number for the interaction, exploring the distance between peaks that appear in the velocity distribution. The observational results will be presented in the thesis as well as the explanation. | 86 pages
The theory of laser cooling assisted by transverse magnetic field Bt{\mathbf{B}}_{\mathbf{t}}Bt​ based on the (1+3)(1+3)(1+3)-level atomic system is presented. With the additional Bt{\mathbf{B}}_{\mathbf{t}}Bt​, atoms could be redistributed among the upper Zeeman sublevels via Larmor precession, and results in large stimulated radiation force and low temperature. Here we consider this cooling model both for broad transition line and narrow line. A Fokker-Planck-type kinetic equation is derived for a broad line. Numerical result shows that the final temperature could be lower than the Doppler limit. For a narrow line, the Monte Carlo approach is applied to simulate the distribution of atoms in the momentum regime and the result shows that most atoms are distributed around zero velocity with width about several $\ensuremath{\hbar}k∕m$.
This paper extends the calculation of laser cooling forces on atoms with magnetic degeneracy to the case where the excited state is composed by a multiplet of levels with arbitrary energy separation. The sub-Doppler force that arises in the ${\ensuremath{\sigma}}_{+},{\ensuremath{\sigma}}_{\ensuremath{-}}$ field configuration is found to be strongly affected if other atomic levels lie close to the excited level of the cooling transition. The paper examines in detail the case in which the excited multiplet comes from spin-orbit coupling of angular momentum L=1L=1L=1 and spin S=1,S=1,S=1, forming the Je=0,{J}_{e}=0,Je​=0, Je=1,{J}_{e}=1,Je​=1, and Je=2{J}_{e}=2Je​=2 energy levels. The cooling transition ${J}_{g}=1\ensuremath{\leftrightarrow}{J}_{e}=2$ shows the sub-Doppler structure due to magnetic degeneracy. The force is modified by the two other transitions (if sufficiently close), even if these do not provide any mechanical effect on the atom when acting alone. The paper presents a detailed scheme for the construction of the relevant optical Bloch equations for the present case, which can be generalized to treat more complex atomic structures. The solutions to these equations are then discussed; several cases of energy separation are worked out and the ensuing graphs of the radiative force are shown. The calculations were carried out in both low- and strong-field regimes.