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/10⁵), 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/cm² in which new, intense beams of light are generated and result in the formation of controllable transverse optical patterns.
Highlights
One area of sustained activity in the field of nonlinear optics is the development of schemes that enable single photons to induce nonlinear optical (NLO) interactions
This effective χ(5) = 7.6 × 10−15 (m/V)4 is the largest ever reported, exceeding that obtained via electromagnetically-induced transparency (EIT) in a Bose-Einstein condensate by ∼ 105 [6], three-photon absorption in a zinc blend semiconductor by ∼ 1022 [7], and local-field-induced microscopic cascading in C60 by ∼ 1025 [8]
Expansion about Ip = 0 is not valid in this region. This effective χ(5) response can be understood as six-wave mixing (SWM) involving four optical and two matter wave fields, wherein the pump fields Bragg scatter off the density grating formed via optically-induced atomic cooling and localization
Summary
One area of sustained activity in the field of nonlinear optics is the development of schemes that enable single photons to induce nonlinear optical (NLO) interactions. We report a new dissipation-induced NLO process that leads to the generation of new optical fields when weak, frequency-degenerate optical fields are incident on a cloud of cold atoms These fields polarize the atoms as well as act on their center-of-mass motion to establish long-range spatial order. For intensities below Id, a gas transforms into a non-equilibrium system with two distinct temperature components: a localized cold fraction ( fc, mneumonic c) and a hot fraction ( fh, mneumonic h) undergoing anomalous diffusion [4] In this regime, the simultaneous spatial localization, polarization and cooling of the atoms leads to a very efficient NLO response that is dominated by an effective fifth-order (χ(5)) susceptibility.
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