title: Observation of three-photon enhanced four-photon absorption.
abstract: We report the measurement of instantaneous resonant four-photon absorption in the polydiacetylene-PTS [polymer poly(bis para-toluene sulfonate) of 2,4-hexadyine-1,6 diol]. Although normally too weak to be observable, an exact degeneracy between three- and four-photon absorption into the dominant exciton and two-photon states, respectively, resonantly enhances this effect so that it can be measured (four-photon coefficient alpha(4)=0.01 cm(5)/GW(3)) at its peak wavelength of 1900 nm. Femtosecond measurements from 1200 to 2200 nm of two- and three-photon absorption, and the nonlinear refractive index coefficient are also reported.
authors.affiliation: School of Optics and Center for Research and Education in Optics and Lasers, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816-2700, USA.
authors.forename: Fumiyo
authors.initials: F
authors.lastname: Yoshino
authors.forename: Sergey
authors.initials: S
authors.lastname: Polyakov
authors.forename: Mingguo
authors.initials: M
authors.lastname: Liu
authors.forename: George
authors.initials: G
authors.lastname: Stegeman