Third-harmonic generation at a atmospheric pressure in methane by use of intense femtosecond pulses in the tight-focusing limit

An experimental study of third-harmonic generation in methane with a 100-fs, 820-nm, Ti:sapphire laser in a tight focusing geometry is presented. The harmonic intensity and bandwidth were measured in a range of intensities extending from below to far above the first ionization threshold and at pressures as high as 10 atm (1 atm = 760 Torr). The harmonic signal follows a power-law dependence on laser intensity with an exponent of similar to 7 and saturates at an intensity I-s similar to 4 x 10(14) W/cm(2). The conversion efficiency was found to increase with the pressure for laser intensities smaller than the saturation intensity I-s and to decrease with the pressure at larger intensities. At laser intensities larger than the saturation intensity a substantial modification in the third-harmonic bandwidth and structure was observed. (C) 1999 Optical Society of America [S0740-3224(99)00905-4].