Fig. 1. Schematic diagram of the experimental setup.

Fig. 2. Experimental frequency spectra of a 1 ps laser pulse recorded after propagation through a supersonic gas jet (normalized intensity versus normalized angular frequency). A reference spectrum, recorded with the gas jet turned off, has been included in each plot. Graph (a) is the spectrum recorded with only the seed beam at an intensity of interacting with the gas jet at , without a counter-propagating pump beam. Graph (b) was recorded with the two counter-propagating beams interacting, the seed at an intensity of and the pump at , at . Graph (c) was recorded with the seed at an intensity of and the pump at , at . The generation of a downshifted peak can be observed through the interaction of the laser pulses and the gas jet, with its relative intensity compared to the fundamental peak strongly depending on the plasma density and the presence of a counter-propagating pump pulse.

Fig. 3. Simulated spectra corresponding to each of the experimental regimes presented in Figure 2 (normalized intensity versus normalized wavevector). The electron density varied from to . Graph (a) is the spectrum simulated with a single laser of intensity interacting in a neon-like argon plasma with an electron temperature of about 20 eV and density of . Graph (b) was calculated with the two counter-propagating beams interacting in a deuterium plasma of density , the seed at intensity and the pump at , with an electron temperature of 120 eV. Graph (c) was simulated with the seed at intensity and the pump at , in an argon plasma with an electron temperature of 5 eV and a density of .