Phase-mismatched degenerate four-wave-mixing: measurement of the complex third-order nonlinear optical susceptibility in thin films

Abstract

In degenerate four-wave mixing (DFWM), three beams of the same frequency ω overlap in a medium to generate a fourth , or more,1 new beams, also of frequency ω. At low beam intensities, the power in each newly generated beam is proportional to the product of the powers of the three incident intensities, and the constant of proportionality has as a factor the absolute square of an element of a third-order susceptibility tensor , χ(3)(−ω, ω, ω, −ω). As the beam frequency ω is increased to where it approaches or exceeds either (1) the lowest "one-photon" resonance frequency of the medium, or (2) half of the lowest "two-photon" resonance frequency of the medium, χ(3) passes from being a positive real number to a complex number that varies in a complicated way with frequency ω. Knowledge of the phase of χ(3), as well as of its magnitude, is relevant for the understanding of the electronic structure of materials as well as for the estimation of their device potential. We obtain this knowledge for a C60 film by interference of its generated beam with that from the CaF2 substrate.