Optical Third Harmonic Generation in Gases by a Focused Laser Beam

Optical third-harmonic generation at 2314 Å using a focused ruby laser beam (6943 Å) has been observed in a number of gases. A theory of third-harmonic generation by focused beams is presented which is used to derive atomic (or molecular) third-harmonic coefficients from the experimental data. An ambiguity in choice of a sign leads to two alternative sets of coefficients. One set is preferred on the basis of general agreement with data given in or derived from the literature: Theoretical calculations of the third-harmonic coefficients, measurements of the Kerr effect and theoretical dc hyperpolariabilities. For helium the measured absolute value is ${\chi }_{\mathrm{He}}=(7\mathrm{}\times 3^{\pm })\times {10}^{-39\mathrm{}}$ esu/atom. For other gases the experimental values of $\chi$ in units of ${10}^{-39\mathrm{}}$ esu/atom and scaled to ${\chi }_{\mathrm{He}}=4.0\mathrm{}$ (a theoretical value due to Sitz and Yaris which is thought to be reliable to about 1%) are: Ne 8.9 ± 1.5; Ar 126 ± 20; Kr 386 ± 75; Xe 979 ± 150; ${\mathrm{H}}_2$ 80 ± 12; C${\mathrm{O}}_2$ 156 ± 23; ${\mathrm{N}}_2$ 107 ± 11. Some data for ${\left(\mathrm{C}{\mathrm{H}}_2\right)}_2$, ${\mathrm{Cl}}_2$, ${\mathrm{O}}_2$, air, and glass are also presented.