Concepts to build nonlinear optical biomaterials in a bottom-up approach

We have performed Hyper-Rayleigh Scattering (HRS) experiments to measure the quadratic hyperpolarizability of several natural amino acids, in particular tryptophan and tyrosine. Values of (29.6+/-0.4)x10^-30 esu for tryptophan and (25.7+/-0.03)x10^-30 esu for tyrosine have been found. We have then investigated the dependence of the quadratic hyperpolarizability of tryptophan-rich short peptides as a function of the number of tryptophans in the sequence. The experimental findings indicate that the resulting quadratic hyperpolarizability in these peptides cannot be assumed as the mere coherent superposition of the hyperpolarizabilities of the tryptophans contained in the peptide. Our results unambiguously demonstrate that there must be strong interactions between the tryptophans contained in these short peptides. We have also investigated the case of the collagen triple helix. A second order hyperpolarizability of (1.25+/- 0.05)x10^-27 esu for rat-tail type I collagen has been measured. In this case, we have been able to model this effective quadratic hyperpolarizability by summing coherently the nonlinear response of elementary moieties forming the triple helix, as opposed to the previous case of the tryptophan-rich peptides.