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Journal of the Optical Society of America B

Broadband laser cooling on narrow transitions

Hartmut Wallis, Wolfgang A. Ertmer

Published Nov 1, 1989• 52 citations
Publisher

Abstract

Laser cooling on transitions with a linewidth Γ much narrower than the recoil shift ℏk2/m is studied numerically according to a full quantum-mechanical treatment of the photon recoil. We propose and investigate a broadband spectral configuration that has the purposes of (1) shortening the long cooling time characteristic for monochromatic laser cooling on a slow transition and (2) reducing the kinetic energy of the stationary momentum distributions considerably below the one-photon recoil energy (ℏk)2/2m. The energies of the computed stationary distributions are limited only by the energy uncertainty of the narrow transition ℏΓ ≪ (ℏk)2/2m. Applications to laser cooling of alkaline earths on the narrow intercombination lines are discussed.

DOI
10.1364/josab.6.002211

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Laser cooling on transitions with a linewidth Γ much narrower than the recoil shift ℏk2/m is studied numerically according to a full quantum-mechanical treatment of the photon recoil. We propose and investigate a broadband spectral configuration that has the purposes of (1) shortening the long cooling time characteristic for monochromatic laser cooling on a slow transition and (2) reducing the kinetic energy of the stationary momentum distributions considerably below the one-photon recoil energy (ℏk)2/2m. The energies of the computed stationary distributions are limited only by the energy uncertainty of the narrow transition ℏΓ ≪ (ℏk)2/2m. Applications to laser cooling of alkaline earths on the narrow intercombination lines are discussed.
We present a quantum theory of one-dimensional laser cooling of free atoms using a transition with a J = 0 ground state and a J = 1 excited state. This treatment is valid both for broad lines (recoil energy small compared with the energy width ℏ Γ of the excited level) and for narrow lines. For broad lines we recover the well-known cooling limit for a two-level transition (∼ℏΓ/2), whereas for a narrow line the cooling limit is found to be of the order of the recoil energy. The stationary momentum distribution is obtained for both cases and is found to be close to the one obtained by Monte Carlo simulations.
The usual case in laser cooling is that the atomic linewidth ħγ is much greater than the recoil energy ER = ħ2k2/2 m. Considering the recent demonstration of laser cooling to kBT <<ħγ,1 It seems reasonable to reconsider laser cooling where ERħγ. In this case the Doppler shift ħk2/m of a recoil velocity is large compared to the linewidth and an analytic treatment is difficult. We have developed a Monte Carlo simulation of 3-D laser cooling (optical molasses) which applies to any value of ħγ/ER.
We present a cooling method that should be generally applicable to atoms with narrow optical transitions. This technique uses velocity-selective pulses to drive atoms towards a zero-velocity dark state and then quenches the excited state to increase the cooling rate. We demonstrate this technique of quenched narrow-line cooling by reducing the 1D temperature of a sample of neutral 40Ca{}^{40}\mathrm{Ca}40Ca atoms. We cool selected velocities with the 1S0(4s2)3P1(4s4p){}^{1}{S}_{0}{(4s}^{2}{)}^{3}{P}_{1}(4s4p)1S0​(4s2)3P1​(4s4p) 657-nm intercombination line and quench with the 3P1(4s4p)1S0(4s5s){}^{3}{P}_{1}{(4s4p)}^{1}{S}_{0}(4s5s)3P1​(4s4p)1S0​(4s5s) intercombination line at 553 nm, which increases the cooling rate eightfold. Limited only by available quenching laser power, we have transferred 18% of the atoms from our initial 2-mK velocity distribution and achieved temperatures as low as 4 \ensuremath{\mu}K, corresponding to a vrms{v}_{\mathrm{rms}}vrms​ of 2.8 cm/s or 2 recoils at 657 nm. This cooling technique, which is closely related to Raman cooling, can be extended to three dimensions.
Abstract The efficiency of the polarisation mechanisms of laser cooling of atoms is analysed using narrow optical transitions for which the recoil energy is greater than or comparable to the natural line width. By the example of atoms with the J g = 1 → J e = 2 optical transition (J g and J e are the total angular momenta of the ground and excited states) in the resonant light fields of σ + – σ – and lin⊥lin configurations, the analysis of the minimum achievable energies of laser-cooled atoms is performed. It is shown that the polarisation mechanisms of laser cooling under conditions of a significant influence of recoil effects become less efficient and do not lead to average kinetic energies of an ensemble of atoms below the Doppler limit.
We study laser cooling of atoms with narrow-line optical transition, i.e. in regimes when quantum nature of laser-light interactions results significant effect on atom kinetics. We demonstrate the minimum of laser cooing temperature is reached for the light field detuning close to -3 recoil frequency, i.e. is much differ from standard Doppler cooling theory is used for semiclassical description of laser cooling. We define a set of dimensionless parameters that describe the steady state solution of cold atom distribution in laser light and cooling time. The results can be used for analysis an optimal conditions of laser cooling of atoms with narrow lines Ca, Sr and Mg are of interest for optical time standards.
We propose and demonstrate a novel laser cooling mechanism applicable to particles with narrow-linewidth optical transitions. By sweeping the frequency of counter-propagating laser beams in a sawtooth manner, we cause adiabatic transfer back and forth between the ground state and a long-lived optically excited state. The time-ordering of these adiabatic transfers is determined by Doppler shifts, which ensures that the associated photon recoils are in the opposite direction to the particle's motion. This ultimately leads to a robust cooling mechanism capable of exerting large forces via a weak transition and with reduced reliance on spontaneous emission. We present a simple intuitive model for the resulting frictional force, and directly demonstrate its efficacy for increasing the total phase-space density of an atomic ensemble. We rely on both simulation and experimental studies using the 7.5 kHz linewidth 1 S 0 to 3 P 1 transition in 88 Sr. The reduced reliance on spontaneous emission may allow this adiabatic sweep method to be a useful tool for cooling particles that lack closed cycling transitions, such as molecules.