Strong four-photon absorption corresponding to Im[x(7) (-⍵; ⍵, -⍵, ⍵, -⍵, ⍵, -⍵, ⍵)] 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.
© 1998 Optical Society of America
Hoon Shim, Mingguo Liu, Chang Hwangbo, and George I. Stegeman, "Four-photon absorption in the single-crystal polymer bis(paratoluene) sulfonate," Opt. Lett. 23, 430-432 (1998)