Monochromatic Conical IR Emission from Decaying KrF Laser Filaments in Xenon as Coherent Stimulated Four-Wave Mixing Process

Igor V Smetanin,Vladimir D Zvorykin,Nikolay N Ustinovskii,Alexey V Shutov,Alexander M Popov,Anna V Bogatskaya

doi:10.3390/photonics8020047

Abstract

We develop theoretical background for the new nonlinear optical phenomenon of narrowly directed monochromatic IR conical emission which has been recently observed when 248-nm UV filaments propagate in xenon (V. D. Zvorykin, et al., Laser Phys. Lett. 13, 125404 (2016)). We treat it as coherent stimulated four-wave mixing process in which two pump KrF laser photons are converted into the coupled pair of resonance IR(828 nm) and VUV (147 nm) photons through 5p5(2P3/2)6p[1/2]0→5p5(2P3/2)6s[3/2]1o and 5p5(2P3/2)6s[3/2]1o→1S0 transitions. We explore the coherent interaction regime which proceeds at a time scale shorter than transverse relaxation time T2. The momentum and energy conservation laws determine the characteristic angle of conical emission. We find that the threshold of this coherent process is determined by the KrF laser pump pulse area.

Highlights

Conical emission (CE) of frequency-shifted laser radiation is one of the very fascinating effects in nonlinear optics
We developed a theory of coherent resonance-enhanced four-wave mixing (FWM) parametric difference frequency conversion of the ultrashort UV laser pulse in xenon emerging when the pulse width is less than the transverse relaxation times
It is considered the parametric frequency conversion process which proceeds through the two-photon resonance excitation of the 5p5(2P3/2)6p[1/2]0 state followed by stimulated coupled emission of IR (828 nm) and VUV (147 nm) quanta in 5p5(2P3/2)6p[1/2]0 → 5p5(2P3/2)6s[3/2]o1 and 5p5(2P3/2)6s[3/2]o1 →1 S0 transitions

Summary

Introduction

Conical emission (CE) of frequency-shifted laser radiation is one of the very fascinating effects in nonlinear optics. In contrast to the mentioned above experiments [30,31], narrowly directed monochromatic CE at the 828 nm wavelength arising from the filaments of KrF laser radiation at intensities ∼1011 W/cm2 [32] We treat this monochromatic CE as a resonance-enhanced coherent stimulated four-wave mixing process, in which two pump UV photons are converted into a coupled pair of phase-correlated IR and VUV photons corresponding to the 5p5(2P3/2)6p[1/2]0 → 5p5(2P3/2)6s[3/2]o1 and 5p5(2P3/2)6s[3/2]o1 →1S0 transitions. Xenon has negative nonlinear Kerr coefficient at the KrF laser wavelength which is caused by the 2-photon resonance of 248-nm radiation with the certain 6p atomic state [36] For this reason UV beam exhibits self-defocusing of filaments and intensity homogenization.

Theory of Coherent Resonance Four-Wave Mixing

Spectrum Narrowing of the Amplified IR Pulse

Findings

Conclusions