The impact of thermal effects on filamentation and supercontinuum generation in bulk materials at MHz pulse repetition rates.

We experimentally study the impact of beam focusing geometry-related heat accumulation on supercontinuum spectra in YAG crystal pumped with 210 fs, 1030 nm pulses by varying pulse repetition rate in the 200 kHz - 2 MHz range. It is demonstrated that in the case of converging pump beam, the red-shifted (long-wavelength) portion of SC spectrum shrinks considerably with the increase of pulse repetition rate, almost completely vanishing at 2 MHz repetition rate, and such spectral shrinking takes place on a very fast, microsecond time-scale. In contrast, in the case of diverging pump beam, stable and broad supercontinuum spectrum is generated regardless of pulse repetition rate. The contrasted supercontinuum generation performances are attributed to large differences of deposited energy due to nonlinear losses experienced by converging and diverging pump beams, which set different thermal regimes of beam filamentation. The experimental observations were backed-up with numerical simulations unveiling the role of heat accumulation and thermal lensing on the nonlinear propagation of the leading pulse, which is responsible for red-shifted spectral broadening. Our findings are of importance for better understanding of underlying phenomena and optimization of supercontinuum generation setups driven by the state-of-the-art high repetition rate ultrafast Yb lasers.