Nonmonotonic Dependence of the Stimulated Raman Scattering Threshold of 240-fs Pulses Near the Air-PMMA Surface

A nonmonotonic dependence of the stimulated Raman scattering (SRS) threshold of 240-fs second-harmonic pulses of an ytterbium fiber laser (515 nm, 3 Hz, up to 11 μJ/pulse) in organic glass (polymethylmethacrylate, PMMA) on C-H stretching vibrations has been discovered when moving the waist of the focused beam (lens F = 16 mm) from air into the sample volume. The first minimum of the threshold (~2 μ J) was found when the waist combined with the surface. Here, the maximum intensity and the maximum value of the nonlinear refractive index (nonlinear lens) supported channeling of the pump in the medium at the maximum length of the SRS exponential gain, taking into account that half of the beam caustic was in air. The second minimum was achieved by moving the entire beam caustic, stretched along the beam axis because of spherical aberration, in a layer of ~1.8 mm near the sample surface. Subsequent movement of the waist was accompanied by a five-fold increase in the SRS threshold due to conversion of a part of the pump pulse energy (515 nm) into the Stokes component (605 nm), the pulse of which was shifted forward because of dispersion of the refractive index and group velocities. This delay of the pump pulse destroyed the synergetic factor of action, which reduced the probability of a breakdown with a five-fold increase in pump energy. The delay reached a value of ~480 fs at a PMMA sample thickness of 10 mm. The breakdown was observed with pump fluctuations in the vicinity of the minimum threshold (2 μJ) at the input surface of the PMMA sample.