Second-Harmonic Generation of Light and Evidence of Phase Matching in Thin Films of P(VDF-TrFE) Copolymers

We report in this paper detailed measurements of the linear and nonlinear optical properties of thin films of P(VDF-TrFE) copolymers (with composition 70/30 mol%). The polymer films are cast from solutions using the spin coating technique and poled at saturation using very low-frequency a.c. fields. The refractive index ( n = 1.42) is determined from measurements of the Brewster incidence, and the thickness from measurements of the optical path close to the normal incidence. Recording of the Maker fringes and calibration of the set-up with a quartz plate allow us to determine the second-harmonic coefficients: d 33 = 4.3, d 31 = 0.22 · 10 −12 m/V and the optical dispersion n (2ω) − n (ω) = 0.0077 (ω is the frequency of the incident laser beam at 1060 nm). The spin-coated copolymer films are optically unaxial due to a preferential orientation of the chain axes parallel to the plane of the film, and measurement of their birefringence gives n e − n o = −0.0083. Therefore, this feature allows us to find a direction of phase matching along which the first- and second-harmonic waves have an infinite coherence length. It is observed using an index-matching liquid.