We present a comparative experimental study of supercontinuum generation in undoped KGW and YVO4 crystals pumped with near-IR femtosecond laser pulses. We demonstrate that KGW and YVO4 crystals, compared to commonly used sapphire and YAG, have significantly lower supercontinuum generation thresholds, produce remarkably larger red-shifted spectral broadenings and exhibit durable damage-free long-term operation at 2 MHz and 76 MHz repetition rates. Our results show that KGW and YVO4 crystals are excellent nonlinear materials for high repetition rate infrared supercontinuum generation which could be used for the design of high average power optical parametric amplifiers as well as for the development of ultrafast ultrafast spectroscopic and high-speed imaging systems.
We show that high repetition rate supercontinuum generation in solids is accompanied by the emission of conical third harmonic, whose occurrence serves as straightforward indication of the onset of in-bulk optical damage at its early stage. It is shown that conical third harmonic generation obeys noncollinear phase matching condition, which involves reciprocal lattice vector of a nanograting inscribed by femtosecond filament in the volume of transparent material. The universality of phenomenon is justified by the experiments in various transparent crystals and glasses and under various settings of focusing condition, pulse energy, repetition rate, exposure time and laser wavelength.
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