Phase-stable sub-cycle mid-infrared conical emission from filamentation in gases.

Sub-single-cycle pulses in the mid-infrared (MIR) region were generated through a conical emission from a laser-induced filament. Fundamental and second-harmonic pulses of 25-fs Ti:sapphire amplifier output were focused into argon to produce phase-stable broadband MIR pulses in a well-focusable ring-shaped beam. The beam profile and spectrum of the MIR field are accurately reproduced with a simple calculation based on a four-wave mixing process. The ring-shaped pattern of the MIR beam originates from a dramatic confocal-parameter mismatch between the MIR field and the laser beams.