Abstract
We present a theoretical study of the interaction between light and a cold gasof three-level, ladder configuration atoms close to two-photon resonance. In particular, weinvestigate the existence of collective atomic recoil lasing (CARL) instabilities in differentregimes of internal atomic excitation and compare to previous studies of the CARL instabilityinvolving two-level atoms. In the case of two-level atoms, the CARL instability is quenchedat high pump rates with significant atomic excitation by saturation of the (one-photon)coherence, which produces the optical forces responsible for the instability and rapid heatingdue to high spontaneous emission rates. We show that in the two-photon CARL schemestudied here involving three-level atoms, CARL instabilities can survive at high pump rateswhen the atoms have significant excitation, due to the contributions to the optical forces frommultiple coherences and the reduction of spontaneous emission due to transitions betweenthe populated states being dipole forbidden. This two-photon CARL scheme may form thebasis of methods to increase the effective nonlinear optical response of cold atomic gases.
Highlights
The rapid progress over recent years in the ability to cool atomic gases down to temperatures of a fraction of a degree Kelvin has stimulated intense interest in fundamental aspects and applications of the interaction of light with cold and ultracold matter.In contrast to matter-light interactions involving “hot” atomic gases or vapours, in cold atomic media, the centre-of-mass motion produced by optical forces can have a sufficiently long coherence time such that new regimes of collective behaviour can result
Increasing the level of atomic excitation in a two-level atomic system is usually undesirable for two reasons: the first is that saturating the atomic transition makes the optically-induced dipole force acting on the atoms disappear [2,37], and the second is that in two-level atoms, a significant population of the excited state leads to substantial spontaneous emission, the stochastic nature of which heats the atomic gas
We extend the analysis of collective atomic recoil lasing (CARL)-type instabilities to a cold gas of atoms whose internal energy structure consists of three levels in a ladder configuration, as shown schematically in Figure 1, where the frequency of the light is tuned to be close to two-photon resonance, i.e., 2ω ≈ ωeg, where ω is the frequency of the optical field and hωeg is the energy difference between the upper and ground state of the atom
Summary
The rapid progress over recent years in the ability to cool atomic gases down to temperatures of a fraction of a degree Kelvin has stimulated intense interest in fundamental aspects and applications of the interaction of light with cold and ultracold matter. Increasing the level of atomic excitation in a two-level atomic system is usually undesirable for two reasons: the first is that saturating the atomic transition makes the optically-induced dipole force acting on the atoms disappear [2,37], and the second is that in two-level atoms, a significant population of the excited state leads to substantial spontaneous emission, the stochastic nature of which heats the atomic gas For these reasons, we extend the analysis of CARL-type instabilities to a cold gas of atoms whose internal energy structure consists of three levels in a ladder configuration, as shown schematically, where the frequency of the light is tuned to be close to two-photon resonance, i.e., 2ω ≈ ωeg , where ω is the frequency of the optical field and hωeg is the energy difference between the upper (excited) and ground state of the atom. We present an outline derivation of our working equations
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