Supercontinuum Generation from Elliptically Polarized Laser Filaments Formed at Low Pressure

When an ultrashort pulse propagates through a nonlinear medium a low density plasma can be formed. The formation of this plasma is the result of a complex balance between nonlinear self-focusing and plasma defocusing, known as filamentation. During filamentation, self-phase modulation, four-wave mixing, and ionization of the medium lead to supercontinuum generation (SCG). Through the examination of SCG we seek to gain insight into the nonlinear optical response and ionization characteristics of air. This is achieved by controlling the polarization state of the laser and density of air. While previous work has separately examined the effects of polarization of the laser field on SCG and low pressure conditions on filamentation, we have observed that combining the ellipticity and pressure results in a complex dependence on pulse intensity. Specifically, changing the input laser polarization alters the bound electron response of the air molecules which simultaneously changes the nonlinear optical processes and the ionization rate of the gas, which determines the density and size of the resulting plasma channel. By introducing ellipticity to the input beam, we observe spectral broadening in the supercontinuum. Varying the gas pressure where the laser creates the plasma channel modifies the generated spectrum due to differences in the ionization process and the nonlinearity of air. By combining the insights gained from this parameter space, our results will provide a more comprehensive understanding of the filamentation dynamics of air.