Mid-infrared spectroscopy of laser filamentation in optics

This work seeks to explore the interplay between material differences in the nonlinear index of refraction n2 and zero group velocity dispersion wavelength λGVD in the formation of spectral broadening and supercontinuum generation. We present mid-wave infrared (MWIR) spectroscopic data of the transmitted 200 fs (FWHM) laser pulse through polycrystalline zinc selenide and polycrystalline zinc sulfide (Cleartran™) optical materials. Using MWIR laser wavelengths between 3.3 and 3.8 μm, we vary the laser energy between roughly 1 and 126 μJ and focus the laser pulses into the materials using lenses of four different focal lengths. This allows us to analyze not only how the nonlinear frequency broadening of the fundamental wavelength varies with increasing pulse energy, but also how it varies with increasing peak intensities when the pulse energy is kept fixed. These results are discussed along with possible implications on the importance of both laser energy and peak intensity in producing supercontinuum for a material with a given n2 and λGVD. Understanding of these practical considerations is essential for assessing material effects to MWIR ultrashort pulsed lasers.