Anomalous Decrease in the Threshold of Stimulated Raman Scattering near the Surface of Liquid Nitrogen

An anomalously strong (up to 13 times) reduction of the threshold of the stimulated Raman scattering of 15-ps laser pulses in liquid nitrogen has been found when moving the caustic of a pump beam from the bulk through the free surface. A nonmonotonic N-shaped dependence of the stimulated Raman scattering threshold on the distance of the beam waist to the surface has been recorded for the first time with a minimum reached when the plane front of the beam caustic coincides with the surface. The physical mechanism of the detected phenomenon has been discussed.