title: Conical third-harmonic generation of optical vortex through ultrashort laser filamentation in air.
abstract: We experimentally generate third-harmonic (TH) vortex beams in air by the filamentation of femtosecond pulses produced in a lab-built Ti:sapphire chirped pulse amplifier. The generated TH beam profile is shown to evolve with increasing pump energy. At a sufficiently high pump energy, we observe a conical TH emission of the fundamental vortex and confirm that the conical radiation follows the conservation law for orbital angular momentum. We further investigate the far-field angularly resolved spectra of the TH wave to analyze the conical emission angle. We theoretically verify that the formation of the conical TH vortex results from the phase-matching between the fundamental and TH waves during the filamentation process.
authors.forename: Yu-Chieh
authors.initials: YC
authors.lastname: Lin
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Nabekawa
authors.forename: Katsumi
authors.initials: K
authors.lastname: Midorikawa