Non-monotonic dependence of the 240-fs pulses stimulated Raman scattering threshold near the AIR–PMMA surface

For the first time to our knowledge, a non-monotonic dependence of the stimulated Raman scattering 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 the stretching C-H 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 μJ) is found when the waist combines with the surface. Here, the maximum intensity and the maximum value of the nonlinear refractive index (nonlinear lens) supports channeling of the pump in the medium at the maximum length of the Raman exponential gain, taking into account that half of the beam caustic is in air. The second minimum is achieved by moving the entire beam caustic, stretched along the beam axis due to spherical aberration in a layer of ~1.8 mm near the sample surface. The subsequent movement of the waist is accompanied by a 5-fold increase in the Raman threshold due to conversion of a part of the pump pulse energy (515 nm) into the Stokes component (605 nm), the pulse of which is shifted forward due to the dispersion of the refractive index and group velocities. This delay of the pump pulse destroys the synergetic factor of action, which reduces the probability of a breakdown with a 5-fold increase in pump energy. At the PMMA sample thickness (10 mm), the delay reaches a value of ~480 fs. Note that the breakdown is observed with pump fluctuations in the vicinity of the minimum threshold (2 μJ) at the input surface of the PMMA sample.

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