The dependence of external focusing geometries and polarization in generation of supercontinuum by femtosecond laser pulse in air

Supercontinuum (SC) generation in ambient air using different focusing geometries and laser energy under linear and circular polarization was investigated experimentally. Under the same input laser energy, the SC spectra in the visible range became wider when smaller f-number convex lens are used and the linearly polarized laser is used as the pump source, the main nonlinear mechanism for asymmetric spectral broadening of femtosecond laser pulses towards the higher frequencies in isotropic media is that the cascade generation with GHz spectral shift for gases and the four-photon parametric wave mixing processes. Besides, the temporal differential of electron density Ne(t) play a more important role in extending the SC spectra to the blue side than the interaction length l. Under the same magnitude of the optical field and the critical power for self-focusing of linearly and circularly polarized beams, the SC spectra range is wider when circularly polarized beams as the pump source.