Enhancement of third harmonic yield in fused filaments due to Gouy shift suppression

Abstract

We have measured the absolute energy of the third harmonic generated by multiple filaments in the focusing geometry of a 744-nm femtosecond pulse in air. The subdivision of the initial 744-nm beam into several beamlets and their fusion in the vicinity of the geometrical focus breaks up the symmetry of the fluence distribution before and after the focus and suppresses the Gouy shift. As a result, the third harmonic yield as the function of 744-nm pump pulse energy undergoes branching. An order of magnitude higher branch corresponds to 248-nm pulse energy obtained after four, three, or two colliding beamlets. The lower branch corresponds to 248-nm pulse energy for a single beamlet of the same initial energy as the energy of several ones added together. The two branches of the third harmonic yield converge back as the peak power after a single opening reaches the critical power for self-focusing in air.