Highly efficient third-order optical nonlinearities and their frequency dependence in donor-substituted cyanoethynylethene molecules.

We report on a new class of organic molecules with record efficiency for application in third-order nonlinear optics (NLO). The third-order polarizability, $\gamma$, of several donor-substituted cyanoethynylethene molecules was determined at the off-resonant wavelength of 1.5 microns using four-wave mixing. The nonlinearities were found to be extraordinarily large relative to the small molecular masses and were found to be within 50 times Kuzyk's fundamental limit,\footnote{M. G. Kuzyk, Opt. Lett. 25, 1218 (2000)} with $53\times10^{-48}$ m$^{5}$V$^{-2}$ as the highest $\gamma$ value. Select molecules were further investigated at wavelengths on and surrounding their two-photon (TP) absorption peaks, revealing large TP cross sections and the resonant influence on the real and imaginary parts of $\gamma$. Several members of this molecular family can be vapor-deposited and are likely candidates for third-order NLO devices. When considering their small mass, the \emph{specific} $\gamma$ ($\gamma$ per molecular mass) for this family (off resonance, at 1.5$\mu$m) is up to $6.5\times10^{-23}$ m$^{5}$V$^{-2}$Kg$^{-1}$, approximately one order of magnitude larger than previously known large $\gamma$ molecules.\footnote{J. C. May et al, Opt. Lett. 30, 3057 (2005)}