Direct determination of the two-photon absorption cross section by the intracavity laser spectroscopy method

The intracavity laser spectroscopy method was used to determine the two-photon absorption of potassium vapor. The two-photon transitions from the ground state 4²S_1/2 to the states 6²S_1/2 and 4²D_3/2,5/2 were detected from absorption in the output spectrum of a wide-band dye laser emitting in the range 760–770 nm; the absorption occurred at a frequency corresponding to the difference between the energies of the upper state and the energy of a ruby laser photon. The ratio ofthe absorption cross section to the square ofthe field was σ₂/E² = 1.5 × 10^–33,1.3 × 10^–35,and 2.8 × 10^–36 m⁴/V², in satisfactory agreement with calculations carried out on the basis of perturbation theory.