We have measured the third-harmonic response, γ, of a centrosymmetric squaraine dye (ISQ) in chloroform at a range of frequencies for which the third harmonic is above the strong, narrow peak in the dye’s linear absorption spectrum but below the UV absorption band. By fitting the experimental dispersion of γ using a four-level model, we determine the strength, location, and width of the lowest-lying two-photon transition. We find that the 21Ag state appears just above the 11Bu state in energy and that the 11Bu–21Ag transition moment is somewhat smaller than the transition moment between the ground state and the 1 1Bu state but much larger than previously predicted for comparable squaraine dyes.
© 1994 Optical Society of America
Original Manuscript: January 31, 1994
Published: July 1, 1994
James H. Andrews, John D. V. Khaydarov, and Kenneth D. Singer, "Contribution of the 21Ag state to the third-order optical nonlinearity in a squaraine dye," Opt. Lett. 19, 984-986 (1994)