Multiple increase in the efficiency of picosecond stimulated Raman scattering excited by Bessel laser beams in water

Stimulated Raman scattering of second harmonic picosecond pulses (60 ps, 532 nm) of an Nd$^{3+}$:YAG laser in water when focusing a Bessel beam behind a conical concentrator with the displacement of the beam caustic through the free surface is studied. The generation of two Stokes (650 and 836 nm) and two anti-Stokes (390 and 450 nm) components of stimulated Raman scattering with an axisymmetric ring beam structure in the cross section is achieved. A significant decrease in the spectral bandwidth of stretching OH vibrations of water molecules in the first circular Stokes component of forward stimulated Raman scattering has been detected (to $\sim$ 70 cm$^{-1}$ compared to $\sim$ 400 cm$^{-1}$ for spontaneous Raman scattering). For the first time, a fourfold increase in the efficiency of converting the pump pulse energy into the first Stokes component of forward stimulated Raman scattering has been achieved using Bessel beams for excitation instead of Gaussian beams.