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 I_s ̃4 x 10¹⁴ W/cm². 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.