Efficient forward stimulated Raman scattering (SRS) was observed along 400-nm femtosecond (fs) laser filaments in water. SRS conversion dominated over self-phase modulation induced continuum generation as the input pulse energy was above 4 lJ (30 Pcr), implying that plasma in the aqueous filamentation channel played an important role in compensating for the group velocity walk-off
between the pump and Stokes pulses. By overlapping two synchronous fs 400-nm filaments to form plasma grating in water, significant enhancement of SRS conversion was observed. Such a SRS enhancement originated from the ultrahigh plasma density in the intersection region of the preformed plasma grating.
FIG. 1. Schematic of plasma grating formation in water. The inset shows the interference fringes of plasma grating when two beams were synchronized (zero time delay) in dilute aqueous colloidal gold with the total input pulse energy of 25 lJ.
FIG. 2. Forward SRS emission during 400-nm fs filamentation in water. (a) Spectra of the probe beam before and after it underwent filamentation in water. The inset spectrum shows the clean Raman peak after passing the 450 nm long-pass filter. (b) and (c) are the light spots of two asynchronous beams after propagation in water and projected onto a white paper after collimation without (b) and with (c) the 450nm long-pass filter, respectively. The pulse energy of the probe beam and the pump beam was 10 lJ and 7 lJ, respectively.
FIG. 3. (a) Raman enhancement of the probe beam by plasma grating when the time delay was 1 ps. (b) Raman intensities of the two beams as a function of the time delay. Positive values of the time delay denoted the pump beam travelling ahead of the probe beam. Note that the Raman spectral intensities of two beams cannot be compared crosswise, because the spectra collecting efficiency is different in two cases. The pulse energies of the probe beam and the pump beam were 50 and 35 lJ, respectively.
