Multi-octave spanning conical emission has been numerically predicted to be generated from ultrafast LWIR pulse propagation in various bulk gaseous media. The gUPPEcore propagator was used to simulate the filamentation collapse in xenon. A flat dispersive landscape near the fundamental at 10 μm allows for efficient high-harmonic generation and slow walkoff of generated spectral components due to a high cutoff frequency and slowly varying GVD. Enough energy is converted to higher harmonics that many of the generated harmonics carry enough power to propagate nonlinearly themselves. As the pulse collapses into a filament, the evolution of the far-field, (angle-resolved) spectrum reveals a conical emission feature that is localized around many high harmonics and generates a tail that spans more than four octaves after the collapse. The x-wave dispersion relation was used to fit three distinct conical emission features generated from three different high harmonics (5th, 7th, and 9th) during collapse. The integrated spectrum exhibits a supercontinuum during collapse, but not the on-axis spectrum, indicating that most of the spectral contribution between harmonics comes from the off-axis conical emission. Pulses with various durations (34 − 500 fs) exhibit the broadband far-field spectral feature, but the signal is stronger with shorter pulses due to spectral broadening. We conclude that there exists a conical emission feature with a tail that spans multiple octaves that is formed from the interference of conical emission generated from individual harmonics using an ultrafast 10 μm pulse as a seed.
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.
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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