Abstract

Since the demonstration a decade earlier by Cook and Hochstrasser [1] of intense THz emission from air ionized with a two-color (ω + 2ω) laser field, this phenomenon remains in the focus of attention both as a source of intense single-cycle THz pulses and because the underlying mechanism(s) of THz emission are hotly disputed. Experimental evidence points toward both the four-wave mixing mechanism based on stationary or nonstationary third-order susceptibilities [2] and toward the model of micro electric currents in which electrons experience directional drift in an “AC bias” of the broken-symmetry electric field [3]. To date, only commensurate ω + 2ω schemes were in use, which corresponds to a degenerate case and thus complicated the delineation of different emission mechanisms. In this contribution we show that by fixing the carrier envelope phases and detuning the frequencies of two optical fields the timing of sub-cycle optical field ionization in gas can be controlled yielding continuous frequency tuning of the emitted THz radiation. Our results suggest that this particular mechanism of THz emission is closely related to two prominent phenomena, ATI and HHG generation that are also based on quasi-periodic sub-cycle ionization followed by electron acceleration in the E-field.

© 2010 Optical Society of America