Third-harmonic generation in relative-phase-controlled two-color laser field

We experimentally demonstrate third-harmonic generation (THG) by intense femtosecond laser pulse at a central wavelength of ∼800 nm superposed by its second harmonic in air. The third-harmonic signal shows a periodic modulation with a period of ∼0.67 fs when the delay between fundamental and second-harmonic wave is continuously changed. The periodic modulation of THG can be attributed to the interference of third-harmonic signals generated from a direct THG channel (3ω=ω+ω+ω) and a four-wave mixing (FWM) channel (3ω=2ω+2ω-ω). With high pump intensity, the fitting of the measured TH spectrum as a function of delay implies that the pump pulse undergoes a strong pulse splitting and self-phase modulation at the focus.