Stimulated Raman Back-Scattering and Self-Guiding of Femtosecond Laser Pulses

Abstract

Plasma has been proposed as the amplification medium for the next generation of ultra-high intensity lasers as it can sustain several orders of magnitude higher intensities than the thermal damage threshold of the solid-state optical elements, which is below $10^{12} \SI{}{\W\per\square\centi\m}$. Plasma-based Stimulated Raman Back-Scattering, SRBS, also known as Raman Amplification, seems to be a very efficient approach, although an energy transfer efficiency to the amplified seed did not reach $10\%$ yet.

Experiments and simulations on increasing efficiency and exploring better control of SRBS seed amplification were conducted at Princeton University. For example, particle-in-cell simulations help reveal the splitting of the amplified seed as a result of resonance slipping induced by the large pump chirp. In addition, a new scheme for SRBS was proposed, Stimulated Raman Near-Back-Scattering (SRNBS), while using a three-wave model, in which, by varying the pulse-front tilt angle of the pump, the length of the pump beam passing a plasma can be controlled. As a result, this new scheme may efficiently enhance the amplification, and at the same time, it could reduce the spontaneous Raman radiation that may pre-deplete the pump pulse. Simulations using the three-wave model was also applied to better understand the novel double-pass SRBS experiments. Landau damping and the frequency shift of Langmuir waves were identified as possible reasons for the low efficiency of single-pass amplifiers. In the double-pass scheme, those problems can be alleviated, due to plasma cooling between the two passes.

Ionization assisted self-guiding of very tightly focused beams for more than 30 Rayleigh lengths was demonstrated with the transmission up to $80\%$. A cylindrical shock wave is necessary for the self-guiding and it is generated following the expansion of the plasma filament created by a laser line focus. As an intense femtosecond laser pulse propagates inside the shock wave, a small portion of the pulse's leading edge ionizes the neutrals near the inner wall of the cylindrical shock wave. Such created free electrons form a guiding structure for the trailing part of the pulse, which could enhance some laser-plasma interactions, including the recombination X-ray lasers developing in our lab.