Terahertz ring beam independent on ω –2ω phase offset in the course of two-color femtosecond filamentation

We measured and simulated the spectrally resolved angular distributions of 0.3–1-THz emission from the two-color filament with its plasma length (∼40 mm) exceeding the dephasing length (∼25 mm) between the fundamental (740 nm) and the second harmonic (370 nm) pulses in air. We show that only the forwardly propagating on-axis terahertz (THz) radiation is sensitive to the variation of the phase offset φ between fundamental and second harmonics, while the ring-like THz beam carrying ≳80% of the overall THz yield is independent of φ. Utilization of the THz ring allows one to omit the tedious adjustment of the frequency-doubling crystal position in the experiment.