LiNa5Mo9O30: A New Active Crystal for Ultrashort-Pulse SRS Lasers Featuring a Rare Frequency Shift of 947 cm–1

Cubically nonlinear properties of a LiNa5Mo9O30 crystal were studied under ultrafast laser excitation at a wavelength of 1030 nm with a peak power (>4 MW) exceeding the critical power of self-focusing Pcr. Transient stimulated Raman scattering (SRS) in LiNa5Mo9O30 was observed in competition with nonlinear effects associated with a nonlinear refractive index n2. This competition was controlled by adjusting the external chirp of the pump pulses. A new fast method for measuring the nonlinear refractive index of Raman media using a change in the chirping of the pump pulse is proposed and compared with a known fast method using a change in the energy of the pump pulse. The proposed technique enabled measurement of n2 = (6.5 ± 1.7) × 10–16 cm2/W and Pcr = 1.4 ± 0.4 MW at 1030 nm for LiNa5Mo9O30 with twice the accuracy of the previous method. Raman conversion into the Stokes wavelength occurred with an optical efficiency of up to 9%, accompanied by a blue (red) shift of the Stokes wavelength relative to the theoretical value of 1141 nm corresponding to the crystal’s Raman mode of 947 cm–1 at the positive (negative) external chirp. This shift depended on the sign of the external chirp and arose due to the transient SRS regime, where the Stokes pulse was generated with a delay relative to the chirped pump pulse. This shift increased significantly for pump pulse durations shorter than 3 ps, owing to the internal chirp induced by nonlinear laser–matter interactions, including self-phase modulation of the pump and cross-phase modulation of the Stokes wave. Controlling the external chirp of the pump pulse enabled fine tuning of the Stokes peak wavelength in the range of 1129–1135 nm (for positive chirp) and 1143–1148 nm (for negative chirp).