Characterization of excited states of centrosymmetric and noncentrosymmetric squaraines by third-harmonic spectral dispersion

We measured the third-harmonic spectrum for each of four squaraine dyes in chloroform. By fitting the experimental dispersion of the third-order susceptibility, gamma, to a four-level sum-over-states model, we determine the strength and the location of the lowest-lying two-photon-like transition. In each case, we find that the lowest two-photon-like state appears just above the dominant linear absorption peak in energy and that the transition moment to that state makes a significant contribution to the nonlinearity, as do transition moments to one or more higher-lying two-photon-like states in the ultraviolet. The spectra of two of these dyes contain additional features, evidencing a nonzero dipole moment difference between the ground and the first excited-state dipole moments that we attribute to asymmetry in their structures. From our measurements, we also predict the dispersion of the real and the imaginary optical Kerr susceptibilities in the near infrared for two of these dyes. (C) 1995 Optical Society of America