Bunching-induced optical nonlinearity and instability in cold atoms [Invited]

Abstract

Abstract:We report a new nonlinear optical process that occurs in a cloud of cold atoms at low-light-levels when the incident optical fields simultaneously polarize, cool, and spatially-organize the atoms. We observe an extremely large effective fifth-order nonlinear susceptibility of χ (5) = 7.6 × 10 −15 (m/V) 4 , which results in efficient Bragg scattering via six-wave mixing, slow group velocities (∼ c/10 5 ), and enhanced atomic coherence times (> 100 µs). In addition, this process is particularly sensitive to the atomic temperatures, and provides a new tool for in-situ monitoring of the atomic momentum distribution in an optical lattice. For sufficiently large light-matter couplings, we observe an optical instability for intensities as low as ∼ 1 mW/cm 2 in which new, intense beams of light are generated and result in the formation of controllable transverse optical patterns.

Steady-state, cavityless, multimode superradiance in a cold vapor

“…We observe that decreasing |∆| and increasing OD result in smaller values of I thresh , since these changes enhance the lightmatter coupling strength [19,21]. The measured values of I thresh agree well with the model developed in Refs.…”

“…During this time, we leave the MOT repump beams and magnetic fields on to ensure that atoms are not pumped into a dark state and to enable continuous cooling and trapping, respectively. We have verified that the MOT magnetic fields do not affect the superradiance when the MOT beams are off [19]. While the MOT beam intensities are reduced, we illuminate the atomic cloud with a pair of pump beams that are intensity-balanced (with single beam intensity I p ), frequency degenerate, and counterpropagate at an angle θ = 10 • relative to the trap's long axis (see Fig.…”

“…This configuration allows us to exploit a gain mechanism in which atomic cooling enhances the loading of atoms into the optical lattice formed by the interference of the pump and self-generated optical fields [19,21]. The resulting atomic spatial organization constitutes a density grating, which is phase-matched for coupling the pump and superradiant fields.…”

“…The measured values of I thresh agree well with the model developed in Refs. [19] and [21] to describe the light-matter interaction in our system, where we take I thresh as the value of I p for which the gain becomes infinite. The lowest threshold we measure is I thresh = 1.1 ± 0.25 mW/cm 2 , which occurs for ∆/Γ = −3 and OD = 20 ± 1 and is comparable to the threshold intensity observed for a Bose-Einstein condensate [8].…”

“…Unfortunately, the relative frequency jitter between the AOM drivers (typically ∼ 20 kHz FWHM) limits our measurement resolution so that we cannot determine the linewidth of the superradiant fields. Nevertheless, the relative spectral width is nearly a factor of two smaller than the Dopplerbroadened gain spectrum observed below the superradiant transition [19], which indicates that the coldest atoms participate preferentially in the collective scattering [31].…”