Power- and polarization-dependent supercontinuum generation inα−BaB2O4crystals by intense, near-infrared, femtosecond laser pulses

Abstract

We report generation of a broadband supercontinuum (SC) in a birefringent crystal upon irradiation by intense, near-infrared, femtosecond laser pulses and investigate its dependence on incident laser power and the initial laser polarization. We choose as our sample $\ensuremath{\alpha}\ensuremath{-}{\mathrm{BaB}}_{2}{\mathrm{O}}_{4}$ ($\ensuremath{\alpha}$-BBO), a material in which the third order is the lowest order of nonvanishing nonlinear optical susceptibility. Three different SC generation mechanisms are identified under our experimental conditions; these are found to depend upon the pump power ${P}_{pu}$. Close to the critical power for self-focusing (${P}_{pu}\ensuremath{\sim}5$ MW), the spectrum is dominated by Kerr nonlinearity or self-phase modulation, whereas at intermediate pumping ${P}_{pu}\ensuremath{\sim}125$ MW, Raman amplification on the Stokes side is a dominating mechanism. At higher pump power $\ensuremath{\sim}300$ MW, significant asymmetric broadening extending up to $\ensuremath{\sim}570$ nm is observed due to the coherent anti-Stokes Raman scattering. Our study provides an intuitive explanation for the observed power- and polarization-dependent SC generation in $\ensuremath{\alpha}$-BBO crystals.