Nonlinear Characterization of Very Thin Polymer Films by Surface Plasmon Resonance

Abstract

The investigation of new polymers with large third order nonlinearities is of great interest in view of applications to all optical processing. It was early recognized that, depending on the characteristics of the film under investigation different techniques are appropriate for measuring its third order susceptibility χ(3). In practice the synthesis of a small amount of a novel material is costly and time consuming, as well as its subsequent processing to realize films; as a consequence, at least at an early stage, the nonlinear tests have usually to deal with small quantities of material and with the poor quality of the obtained samples. For these reasons the Surface Plasmon Resonance (SPR) technique is very attractive for the linear and nonlinear characterization of very thin films. Limiting ourselves to the nonlinear characterization, to our knowledge only few tests were reported so far, although the intrinsic drawbacks of SPR (TM polarized light and a metal film are required) are largely counterbalanced by a number of advantages, in particular the capability of monitoring sign, real and imaginary part of the nonlinearity of extremely thin layers with a high degree of insensitivity to their defects. Here we report on the use of SPR to evaluate the χ(3) of thin polymer films (10–150nm). The potential of this method was investigated by numerical simulations. Experimental tests, which were performed at λ=1065nm, by using thin film samples of a novel polydiacetylene, PDCHD-HS, confirmed such potential provided that also the behaviour of the metal film (typically Ag), at low and high intensity, is taken into account. The tests on PDCHD-HS showed a very large value of the real part of the χ(3) which turned out to be ~ -1.2 x 10 – 17 m2/V2. If this encouraging result is confirmed by further tests at 1300–1500nm, this material could be an interesting candidate for future applications.