Four-photon absorption in the single-crystal polymer bis(paratoluene) sulfonate

Strong four-photon absorption corresponding to $\mathrm{Im}\left[{\chi }^{\left(7\right)}\left(-\omega ; \omega , -\omega , \omega , -\omega , \omega , -\omega , \omega \right)\right]$ has been measured for the first time to the authors’ knowledge with 100-fs pulses at 1600 nm in the single-crystal polymer poly[bis($p$-toluene sulfonate)] of 2,4-hexadiyne-1,6-diol. The transition involved is from the even-symmetry ground state into the vibronic subband of the dominant, even-symmetry, excited two-photon state at an energy of 2.7 eV.