Experimental Evidence of Backward Raman Scattering Driven Cooperatively by Two Picosecond Laser Pulses Propagating Side by Side

Abstract

This Letter investigates experimentally the backward stimulated Raman scattering (SRS) of two copropagating, 1-μm wavelength, 1.5-ps duration laser pulses focused side by side, but not simultaneously, in a preformed underdense plasma. When the two lasers do not interact, one of the pulses (so-called strong) yields a large SRS reflectivity, while the other weak pulse is essentially ineffective as regards SRS. By contrast, the weak pulse shows significant SRS activity if it is launched in the plasma slightly after the strong one, and for time delays as large as about 15 ps. For crossed polarizations and a lateral distance of 80-90 μm, the time delay has to be larger than 3-4 ps for the weak pulse to be active, while it has just to be positive when the polarizations are parallel. The experimental results are discussed with the help of large-scale particle-in-cell simulations.

Envelope equation for the linear and nonlinear propagation of an electron plasma wave, including the effects of Landau damping, trapping, plasma inhomogeneity, and the change in the state of wave

“…In particular, we have to exclude the situation when the EPW results from the stimulated Raman scattering of a spatially smoother laser. Indeed, as discussed in recent papers [39,40], such a situation would lead to complex couplings which are completely outside the scope of this paper. Then, let us introduce at any point M,…”

“…It is also noteworthy that, when V lim ≫ |γ|/k, at lowest order in γ/kV lim , this term matches the result derived from the expansion of ν 1 for large values of kV lim /γ. This shows that the envelope equation (40), derived for a growing wave, bears some relevance whatever the way the wave amplitude varies. However, the most accurate way to account for the nonlinear wave evolution in the general situation is not to make use of an effective damping rate, like ν 1 , but to solve an integro-differential equation, similar to Eq.…”

Self-consistent theory for the linear and nonlinear propagation of a sinusoidal electron plasma wave. Application to stimulated Raman scattering in a non-uniform and non-stationary plasma

“…Indeed, as shown experimentally in Refs. [52,53], the reflectivity of two co-propagating laser pulses is larger than the sum of each reflectivity calculated as though the pulses were propagating alone, because SRS is a collective process. In the experiment of Refs.…”

“…In the experiment of Refs. [52,53], two picosecond pulses propagating in the same direction, but at a transverse distance of about 80 µm from each other, are focused in a nearly homogeneous plasma at two different times. The intensity of one of the pulses, the so-called strong pulse, is large enough to induce a large reflectivity even when this pulse propagates in the plasma by itself.…”

Plasma Wave Seed for Raman Amplifiers

“…Although the technological production of the plasma wave seed is beyond the scope of this work, one straightforward methodology would be to employ low-power high-quality counterpropagating laser pulses to produce the plasma wave seed; this seed would then linger in the plasma (it has near zero group velocity) until the highpower pump laser (which needs not be of high quality) excites the parametric interaction. In a plasma setting, localized plasma waves have been generated by stimulated Raman scattering using a tightly focused intense laser pulse in preformed plasma [36][37][38][39]. The ability of plasma wave seeds to linger in plasma and then to scatter laser energy has already been exploited in a variety of settings, including plasma holography [40], plasma gratings [41] and plasma photonic crystals [42,43].…”