Terahertz ring beam independent on -2 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 (similar to 40 mm) exceeding the dephasing length (similar to 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 phi between fundamental and second harmonics, while the ring-like THz beam carrying greater than or similar to 80% of the overall THz yield is independent of phi. Utilization of the THz ring allows one to omit the tedious adjustment of the frequency-doubling crystal position in the experiment.