Laser cooling with adiabatic transfer on a Raman transition

Abstract

Sawtooth Wave Adiabatic Passage (SWAP) laser cooling was recently demonstrated using a narrowlinewidth single-photon optical transition in atomic strontium and may prove useful for cooling other atoms and molecules. However, many atoms and molecules lack the appropriate narrow optical transition. Here we use such an atom, 87 Rb, to demonstrate that two-photon Raman transitions with arbitrarily-tunable linewidths can be used to achieve 1D SWAP cooling without significantly populating the intermediate excited state. Unlike SWAP cooling on a narrow transition, Raman SWAP cooling allows for a final 1D temperature well below the Doppler cooling limit (here, 25 times lower); and the effective excited state decay rate can be modified in time, presenting another degree of freedom during the cooling process. We also develop a generic model for Raman Landau-Zener transitions in the presence of small residual free-space scattering for future applications of SWAP cooling in other atoms or molecules.

Sawtooth-wave adiabatic-passage slowing of dysprosium

“…Recently, a novel cooling technique called sawtooth wave adiabatic passage (SWAP) was demonstrated for strontium [15,16] on a 2π × 7.5 kHz wide transition and rubidium on a Raman transition [17]. In this work, we demonstrate its application for dysprosium (Dy).…”

Laser cooling using adiabatic rapid passage

“…Recently, a sawtooth-wave adiabatic passage (SWAP) scheme was demonstrated to cool atoms by rapidly sweeping the laser frequency of counter-propagating beams with Doppler shifts providing time-ordering to ensure photon recoils oppose the particle's motion [7][8][9]. This technique, when adopted to more complex multi-level systems, is shown to be resultative for cooling molecules [10].…”

“…The approach in this work differs from the one in Ref. [8] in using the frequency ramp covering the entire hyperfine manifold and addressing all sublevels in each ramping process. The work suggests the solution for a weak Doppler cooling force problem relevant for multi-level type-II transitions in which the ground state total angular momentum quantum number is greater or equal to that of the excited state.…”

Phase-space-density limitation in laser cooling without spontaneous emission

“…This framework provides satisfying predictions for Doppler cooling where the change of semiclassical PMD maximum is essentially attributed to spontaneous emission. However even without spontaneous emission, several semi-classical studies suggest that the maximum of a PMD can be modified (π-pulse, rapid adiabatic passage (RAP), Stimulated RAP, bichromatic fields [3,5,6,8,10]). Their common idea is that a coherent force, resulting from absorption and stimulated emissions, depends on the particle velocity via the Doppler effect.…”

“…Still, a sample initially prepared in a thermal state and thereby without quantum correlation can exhibit a gain of the PMD maximum or PSD up to the number M of internal states (or ultimately M 2 if initial correlations exist in the initial state, see SM [19]). The direct and fundamental consequence of this analysis, holding for any kind of free particles or particles in time-dependent trapping potential is that cooling mechanisms based on coherent field momentum transfer without spontaneous emission (such as adiabatic passages, bichromatic, π-pulses [5,6,8,10,11,28]) have a limited efficiency and could only lead to a position-momentum PSD gain of M . This is still of interest for studies that need more particles in a same phase space area regardless of the internal distribution (for laser manipulation, detection, collisional studies, ...).…”