The existence of multi-octave spanning conical emission from ultrafast LWIR pulse filamentation

Abstract

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. © 2023 SPIE.