Properties of the spectra of relativistically strong laser pulses in an underdense plasma

An experimental study and a particle-in-cell (PIC) simulation of the plasma-wave breaking and stimulated backward Raman scattering (SBRS) in the interaction of sub-picosecond laser pulses with an underdense plasma are carried out. The results of the experiment show that the spectrum of backscattered radiation is strongly modulated; the structure of modulations depends on the plasma density and the laser pulse intensity and exhibits pronounced symmetry. It is found that the SBRS saturates at the same pulse intensity at which the modulations of the SBRS spectrum appear. A one-dimensional PIC simulation also demonstrates the spectral modulation of scattered radiation as well as the appearance of modulations in the transmitted radiation spectrum during the pulse propagation.