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Schematic diagram of the experimental setup. The fundamental laser pulse is focused by a lens through a BBO crystal, generating a second harmonic pulse. Through a dc-biased two-color filament in air, the THz pulse transmits through a Si filter and is detected coherently via EOS.
THz peak-to-peak amplitude [(c) and (d)], waveform [(e) and (f)] and frequency spectra [(g) and (h)] from a two-color laser-induced filament in air with horizontal (a) and vertical (b) external dc field. The THz waveforms and frequency spectra are for the components polarized along to the direction of external dc field.
Polarization trajectories of THz emission from a two-color laser-induced filament with a dc field along the Y axis of (a) 0, (b) 1, (c) 3, and (d) 5 kV/cm. THz polarizations in (e)–(g) were obtained by subtracting the waveform without dc field (a) from (b)–(d), respectively. Each solid square corresponds to the measured THz amplitudes in two orthogonal directions at one specified time delay (Ref. 14); two successive solid squares are separated by 33 fs.
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