New Nonlinear Optical Effect: 20-Fold Local Increase in Intensity in the Field of a Backward SRS Picosecond Pulse in Heavy Water

We report for the first time, to our knowledge, the discovery of a new nonlinear optical effect in a stimulated Raman scattering (SRS) laser in heavy water as a Raman active medium: a 20-fold local increase in intensity of the backward SRS field up to the optical breakdown threshold (~40 TW/cm²) due to Kerr beam compression as the beam waist moves from bulk water to the surface at a constant pump pulse energy (57 ps, ~2 mJ, and 532 nm). The optical breakdown of water accompanied by ejection of droplets normally to the surface is achieved in a thin layer when the pump beam waist is located at a depth of 1‒3 mm. Outside this layer, the radiation intensity needed to achieve SRS does not exceed 2 TW/cm². The detected energy concentration in a small volume indicates a high degree of spatiotemporal localization of summed Kerr nonlinear optical contributions to the refractive index and two-photon absorption coefficient, which leads to a local increase in field intensity and to an optical breakdown at a pulse energy 20 times lower than that in the absence of such summation of nonlinear optical contributions.