Intense terahertz pulses by four-wave rectification in air

We describe a new four-wave rectification method for the generation of inte nse, ultrafast terahertz (THz) pulses from gases. The fundamental and secon d-harmonic output of an amplified Ti:sapphire laser is focused to a peak in tensity of similar to 5 x 10(14) W/cm(2). Under these conditions, peak THz fields estimated at 2 kV/cm have been observed; the measured power spectrum peaks near 2 THz. Phase-dependent measurements show that this is a coheren t process and is sensitive to the relative phases of the fundamental and se cond-harmonic pulses. Comparable THz signals have been observed from nitrog en and argon as well as from air. (C) 2000 Optical Society of America. OCIS codes: 320.7110, 320.7160, 190.4380, 260.3090.