Asymmetric spectral broadening of sub-picosecond laser pulse in BaWO₄ crystal: interplay of self-phase modulation, stimulated Raman scattering, and orientational Kerr nonlinearity

Abstract

Spectral broadening of 0.3 ps 515 nm laser pulse in a highly Raman-active B a W O 4 crystal and fused silica demonstrates significantly different behavior with the incident pulse energy. While the broadening in fused silica is fairly symmetric with respect to the pump laser pulse wavelength, the Stokes wing broadening in the B a W O 4 crystal is 2 times wider than that of anti-Stokes wing, the former demonstrating a step-like increase with the pulse energy. To the best of our knowledge, the obtained data are the first clear evidences of the following facts: (i) stimulated Raman scattering with sufficiently high efficiency of conversion to Stokes components slows down spectral broadening induced by self-phase modulation, and (ii) the mechanism of Kerr nonlinearity, which is responsible for self-phase modulation in B a W O 4 , is of orientational nature. The nonlinear refraction coefficient and its decay time following from our experiments with B a W O 4 were estimated as n 2 ≈ 6.4. 10 − 15 c m 2 / W and τ N L ≈ 0.35 p s .

Highly efficient transient stimulated Raman scattering on secondary vibrational mode of BaWO4 crystal due to its constructive interference with self-phase modulation

“…The spectra demonstrate a trend of broadening with the E p increase, which is asymmetric with respect to Stokes and anti-Stokes wings, as in Ref. [12]. On such a background, the first pronounced peak appears at λ ≈ 524 nm (corresponding to the ν 2 secondary Raman mode) at E p between 0.1 μJ and 0.12 μJ, i.e., well before the peak appearing at λ ≈ 540 nm corresponding to SRS on the about 5 times stronger ν 1 mode (see Fig.…”

“…Based on the presented above results and gained knowledge in Refs. [12,13], we propose the following scenario for the information presented above and observed earlier in Refs. [12,13] experimental data.…”

“…All these features look very similar to those observed in Refs. [12,13] for SRS on ν 1 mode. Except for decay of the peaks with increase of E p , these features were accounted for as a result of seeding SRS by the pump spectrally broadened via self phase modulation (SPM).…”

“…Another unusual observation in Ref. [12] was (i) a low pump pulse energy for the ν 2 Raman mode spectral peak emerging, which was much lower than that for the strongest ν 1 Raman mode, and (ii) decay of that peak at higher pump pulse energy. In this Letter, we study these effects in detail, quantify them, and present a model explaining them.…”

“…An appearance of notable SRS on the secondary Raman mode ν 2 alone in a conventional single-pass scheme of interaction may be found in Ref. [12]; however, emphasis there was given to the nature of asymmetric spectral broadening of pump radiation in BaWO 4 (BWO) crystal. Another unusual observation in Ref.…”