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 similar to 800 nm superposed by its second harmonic in air. The third-harmonic signal shows a periodic modulation with a period of similar to 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 (3 omega = omega + omega + omega) and a four-wave mixing (FWM) channel (3 omega = 2 omega + 2 omega - omega). 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.