Backward SRS suppression of picosecond pulses in water upon moving the pump beam waist from the water volume through the surface

We report, for the first time to our knowledge, suppression of backward stimulated Raman scattering (BSRS) of picosecond pulses (57 ps, 532 nm) due to the development of optical breakdown in the surface (0 – 3 mm) water layer with a shift of the beam waist (lens focal length of 83 mm) to the water – air interface without changing the pump pulse energy (~1.3 – 1.5 mJ). In this case, SRS generation in forward direction is observed even in the presence of breakdown. When the focal plane coincides with the surface, the BSRS generation is restored without optical breakdown, despite an increase in the pump radiation intensity due to a decrease in the beam diameter. It is significant that the optical breakdown threshold in the water volume was unattainable even with an increase in the pump pulse energy by more than an order of magnitude – up to 16 mJ. The mechanism of self-consistent summation of nonlinear optical processes, such as electrostriction, beam self-focusing, beam phase conjugation, and BSRS pulse compression is discussed.