Third-harmonic generation at 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 the laser intensity with an exponent of ∼7 and saturates at an intensity Is∼4×1014 W/cm2. The conversion efficiency was found to increase with the pressure for laser intensities smaller than the saturation intensity Is 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.