Parametric Self-Oscillation via Resonantly Enhanced Multiwave Mixing

We demonstrate an efficient nonlinear process in which Stokes and anti-Stokes components are generated spontaneously in a Raman-like, near resonant media driven by low power counter-propagating fields. Oscillation of this kind does not require optical cavity and can be viewed as a spontaneous formation of atomic coherence grating. PACS numbers: 42.50.-p;42.65.-k;42.50.Gy Typeset using REVTEX present address: National Institute of Standards and Technology, Boulder, Colorado, 80303 1 Theoretical and experimental work of past few years on atomic coherence and interference has demonstrated a potential to improve significantly the existing nonlinear optical techniques [1]. In the present Letter we demonstrate efficient parametric generation accompanying the spontaneous formation of the coherent superposition states. Specifically, the present work reports the observation of the spontaneous parametric self-oscillation in resonant Raman media. Such generation does not involve optical cavity and appears under remarkably simple circumstances, when two low-power counter-propagating fields interact with the medium. Oscillation manifests itself as Stokes and anti-Stokes components generated with a frequency shift corresponding to that of the Raman transition. As the oscillator goes over threshold, dramatic increase and narrowing of the beat note between the input field and generated components takes place. The principal possibility of mirrorless parametric oscillation with counter-propagating signal and idler fields has been suggested in 1960’s by Harris [2]. The original proposal based on non-degenerate frequency mixing has not been realized up to now due to small values of nonlinearities in available materials and difficulties in achieving phase matching [3]. It is easier to achieve mirrorless oscillation in degenerate four-wave mixing. The possibility of self-oscillation in such interactions has been predicted in [4], and a number of the related effects, such as conical emission or transverse pattern formation have been observed in a vapor driven by very strong, off-resonant counter-propagating laser beams [5]. Workers in the field have also noted the importance of Raman nonlinearities in the early experiments on polarizations instabilities [6]. As compared to the above work the presently reported results utilize atomic coherence gratings [7,8] in a resonant double-Λ atomic system (Fig.1a). Coupling of counterpropagating Stokes and anti-Stokes fields via such a grating appears to be the main physical mechanism resulting in Raman self-oscillation [8]. Similar to several related studies [1,7,9,10] the present work operates in a so-called strong coupling regime in which nonlinearities can not be derived from a usual perturbation expansion. In this regime quantum coherence and interference have a profound influence on nonlinear parametric amplification. For ex-