We experimentally and theoretically investigate the nonlinear frequency conversion of transparent chalcogenide optical materials using ultrashort midwave infrared laser pulses at 3.6 microns. Evidence of the structure of second through sixth harmonic generation demonstrates different levels of filamentation related to laser intensity, sample thickness, and sample position. Simulations using a (3+1)D model with experimentally measured n2 values and random quasi phase matching provide good qualitative agreement with experimental data. Together, the data suggests that focusing geometry and material structure play a significant role in harmonic generation in these materials.
We investigate the nonlinear optical properties of transparent optical materials using ultrashort midwave infrared laser pulses between 3 and 4 microns. Random quasi-phase matching in polycrystalline materials generates multiple frequency harmonics of both odd and even orders throughout the transmission window of the target. We also investigate single crystal and amorphous materials and demonstrate a range of frequency conversion and pulse broadening. Simulations using a nonlinear polarization model enhanced with ionization and experimentally measured n2 values provide good qualitative agreement with experimental data.
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.
We investigate the nonlinear optical properties of ZnSe and ZnS using ultrashort (pulse duration approximately 200 fs) midwave infrared laser pulses between 3 and 4 μm. Multiple harmonic generation in both materials was observed, as well as significant spectral modification of the fundamental pulse. Simulations using a nonlinear polarization model enhanced with ionization compared favorably with experimental data. Random quasi phase matching in the materials is the likely generator of the observed harmonics.
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