Power- and polarization-dependent supercontinuum generation in $\alpha -{\mathrm{BaB}}_2{\mathrm{O}}_4$ crystals by intense, near-infrared, femtosecond laser pulses

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 $\alpha -{\mathrm{BaB}}_2{\mathrm{O}}_4$ ($\alpha$-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 ($P_{pu}\sim 5$ MW), the spectrum is dominated by Kerr nonlinearity or self-phase modulation, whereas at intermediate pumping $P_{pu}\sim 125$ MW, Raman amplification on the Stokes side is a dominating mechanism. At higher pump power $\sim 300$ MW, significant asymmetric broadening extending up to $\sim 570$ 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 $\alpha$-BBO crystals.

Figure 1

(a) Normalized SC spectra generated in a 2-mm-thick $\alpha$-BBO crystal by femtosecond pulses centered at ${\lambda }_{pu}=801$ nm and at selected pump powers ($P_{pu}\sim 5$ MW, magenta; $P_{pu}\sim 50$ MW, blue; $P_{pu}\sim 175$ MW, green; and $P_{pu}\sim 300$ MW, red). The laser polarization is parallel to the optic axis. The spectra are shifted vertically for clarity. Pump pulse spectrum (black dotted line) and SC spectrum generated in 1-cm-long path length in water (cyan dashed line) are also shown for comparison. (b) Spectral extents ($▵{\lambda }_b$, blue) and ($▵{\lambda }_r$, red) of the SC spectra as a function of $P_{pu}$. Spectral extents for the pump pulses ($▵{\lambda }_{b,r}^{pu}$, gray dash-dotted line) and for water are also noted. At $P_{pu}\sim P_{cr}$, the SC generation is mainly due to self-phase modulation as in water. At intermediate values of $P_{pu}$, Raman amplification of Stokes lines is a dominating effect, whereas $▵{\lambda }_r$ becomes significant at $P_{pu}\sim 300$ MW due to the coherent anti-Stokes Raman scattering.

Figure 2

(a) Raman scattering spectra of a 2-mm-thick $\alpha$-BBO crystal for two crystal orientations recorded in transmission. The spectra are shifted vertically for clarity. Scattering cross sections are much higher when the crystal is oriented with its optic axis parallel to the laser polarization (bottom, red) compared to the perpendicular (top, blue) orientation. (b) Comparison between the observed (blue dotted line) and simulated (red solid line) SC spectra. The observed spectrum is recorded at $P_{pu}\sim 125$ MW with polarization parallel to the optic axis. The simulated spectrum is obtained by considering only the amplification of strong Raman lines on the Stokes side.

Figure 3

Polarization-dependent, normalized $\alpha$-BBO SC spectra recorded at (a) $P_{pu}\sim 125$ MW and (b) $P_{pu}\sim 300$ MW. At lower power, the spectral extents are comparable for laser polarization parallel (red solid line) and perpendicular (blue dotted line) to the optic axis. At higher pumping, there is a significantly enhanced emission in the visible range with broader $▵{\lambda }_b$ for parallel polarization.