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Physical Review A

Recoil-induced lasing of cold atoms

Péter Horák, Klaus M. Gheri, Helmut Ritsch

Published Jul 1, 1995• 10 citations
Publisher

Abstract

We present a review and extension of a recently made proposal [K. M. Gheri et al., Phys. Rev. Lett. 74, 678 (1995)] to utilize cold atoms in a Raman configuration as a uv-laser gain medium in the limit of the photon recoil shift exceeding the natural atomic linewidth and the Doppler broadening. A discussion of model assumptions is given as well as an analysis of the dependence of the gain on detunings and decay rates. Special emphasis is put on a calculation of the nonlinear gain that allows a self-consistent determination of the uv-laser intensity as a function of the model parameters. For a deep uv transition, the gain turns out to be of approximately the same order of magnitude as for comparable optical transitions including a significant enhancement through Raman coherences especially at weak laser intensities. Our calculations suggest the possibility of a cw deep uv laser, based on laser cooled metastable (e.g., helium) atoms.

DOI
10.1103/physreva.52.554

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We present a review and extension of a recently made proposal [K. M. Gheri et al., Phys. Rev. Lett. 74, 678 (1995)] to utilize cold atoms in a Raman configuration as a uv-laser gain medium in the limit of the photon recoil shift exceeding the natural atomic linewidth and the Doppler broadening. A discussion of model assumptions is given as well as an analysis of the dependence of the gain on detunings and decay rates. Special emphasis is put on a calculation of the nonlinear gain that allows a self-consistent determination of the uv-laser intensity as a function of the model parameters. For a deep uv transition, the gain turns out to be of approximately the same order of magnitude as for comparable optical transitions including a significant enhancement through Raman coherences especially at weak laser intensities. Our calculations suggest the possibility of a cw deep uv laser, based on laser cooled metastable (e.g., helium) atoms.
We calculate the weak probe field absorption spectrum for ultracold three-level atoms in a Raman gain configuration in the limit of the recoil shift exceeding the natural linewidth and the Doppler broadening. Our results show that even in the limit of very large recoil gain persists with its maximum shifted toward the red of the atomic line by the recoil energy, as is the spontaneous emission line. Some preliminary calculations suggest the possibility of a cw deep uv laser, based on cooled metastable helium atoms.
Subrecoil Raman laser cooling of lt;suplt;suplt;supgt;6lt;/suplt;/suplt;/supgt;Li atoms
We realize an efficient one-dimensional Raman cooling of 6Li atoms using 2S→2P Raman transition pulses and 2S→3P ultraviolet (UV) optical repumping. In this cooling scheme, we directly cool an initial cold atomic sample of 350 μK released from a standard magneto-optical trap down to a velocity spread of 0.8 recoil velocity, corresponding to an effective temperature of 4.5 μK, an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling. This technique may provide an ideal initial sub-recoil atomic sample for the application of the large-momentum-transfer in the recoil-sensitive lithium atom interferometers.
The recent development of laser cooling and trapping techniques has made possible the controlled realization of dense and cold atomic samples, thus opening the way for spectroscopic investigations in the low and ultra-low temperature regimes not accessible with conventional techniques. In our lab, an apparatus for laser cooling and Bose-Einstein condensation of rubidium atoms is in operation, producing a sample large enough to perform reliable investigations by absorption or emission techniques. Moving from previous photoionization experiments of laser-cooled atoms in a magneto-optical trap irradiated by cw-laser radiation [1] and stimulated by the interest grown recently on ultra-cold plasma [2], we have started a series of experiments aimed at investigating the interaction of intense laser pulses with the ultra-cold neutral sample, boty aboeand below the condensation temperature. Radiation from an excimer-pumped dye laser was used at a wavelength close to 594 nm, corresponding to the ionization threshold of rubidium for two photon non-resonant absorption from the ground state, with intensities in the tens of MW/cm^2 range and a pulse duration of 16 ns. Preliminary results, attained through the dynamical analysis (atom number, temperature) of the cold atom sample after the arrival of the laser pulse, suggest the occurrence of different processes, initiated by multi-photon ionization, involving collisions between cold electrons and ultra-cold neutrals, recombination at low temperatures and collective processes in the weak and cold plasma. Interpretation of those results demonstrates that this class of experiments, based on a variety of topics typical of atomic, plasma and statistical physics, can open the way for novel and intriguing investigations of laser-matter interaction. Further experimental developments will include duplication of the laser pulse frequency and one photon ionization of ground state rubidium laser-cooled atoms. [1] O. Maragò et al., Phys. Rev. A 57 R4110 (1998); E., Arimondo et al., Appl. Surf. Sci. 154-155 527 (2000); F. Fuso et al., Opt. Commun. 173 223 (2000). [2] T.C. Killian et al., Phys. Rev. Lett. 83 4776 (1999); S. Kulin et al., Phys. Rev. Lett. 85 318 (2000).
We demonstrate a Raman laser using cold (87)Rb atoms as the gain medium in a high-finesse optical cavity. We observe robust continuous wave lasing in the atypical regime where single atoms can considerably affect the cavity field. Consequently, we discover unusual lasing threshold behavior in the system causing jumps in lasing power, and propose a model to explain the effect. We also measure the intermode laser linewidth, and observe values as low as 80 Hz. The tunable gain properties of this laser suggest multiple directions for future research.
The possibility of realizing an efficient gaseous laser-beam-generating medium that utilizes $\ensuremath{\Lambda}$-type coherently phased (i.e., ``dressed'') atoms for the active laser species, but that does not inherently require the use of external laser beams for pumping, is explored. Specifically, it is investigated if multiphoton stimulated hyper-Raman scattering (SHRS) processes driven by fluorescence radiation generated in a continuous electrical discharge present within the vapor-containing cell could produce continuous-wave (cw) optical gain at the $\ensuremath{\Lambda}$-atom resonance frequencies ${\ensuremath{\omega}}_{o}$ and ${\ensuremath{\omega}}_{o}^{\ensuremath{'}}$. It is deduced that such gain could result from nnn-photon $(n\ensuremath{\geqslant}4)$ SHRS processes only if absorption of fluorescence pump light occurs in the first three transitions of the nnn-photon sequence representing the process unit step. Estimates of the amount of optical gain that could be produced in such a system indicate that it should be sufficient to allow multiwatt cw laser operation to occur on one set of $\ensuremath{\Lambda}$ transitions connecting levels in a ``double-$\ensuremath{\Lambda}$'' structure, with the pump light being discharge-produced fluorescence centered about the transitions of the other $\ensuremath{\Lambda}$ pair. However, to initiate operation of such a device would require injection into the laser optical cavity of intense ``starter'' laser pulses at both lasing frequencies. What should be an optimal experimental configuration for determining feasibility of the proposed laser device is described. In the suggested configuration, Cs-atom $6{S}_{1∕2}\text{\ensuremath{-}}6{P}_{1∕2}$ transitions form the double-$\ensuremath{\Lambda}$ structure.
Based on the numerical solution of the quantum kinetic equation for the atomic density matrix, which makes it possible to accurately take into account the recoil effects in the interaction of atoms with field photons, we have studied the limits of laser cooling of atoms using closed optical transitions characterised by different recoil parameters (the ratio of the recoil energy to the natural linewidth). It is shown that for optical transitions with an insufficiently small recoil parameter, the polarisation effects, which lead to the possibility of sub-Doppler laser cooling, lose their efficiency and do not ensure an attainment of the temperature below the Doppler limit. The analysis performed allows one to outline the boundaries of the sub-Doppler theory of laser cooling of atoms.