New developments in molecular photonics: from photoswitchable nonlinearities to nonlinear nanoplasmonics

We propose new strategies to increase and control molecular second-order nonlinearities, byusing photoswitching processes or plasmonic resonance. First, we report on reversible switching of nonlinear optical (NLO) properties, based on the use of dipolar or octupolar metal complexes containing photochromic ligands like dithienylethene (DTE) derivatives. DTE undergo a reversible interconversion between a non-conjugated open form and a π-conjugated closed form. A strong relative change of the quadratic molecular nonlinearity upon visible light exposure (more than one order of magnitude) is evidenced by second harmonic generation experiments. This process is fully reversible. On another hand, we explore the quadratic NLO properties of highly NLO dyes (stilbazolium derivatives) grafted onto the surface of gold nanoparticles (NP). In spite of the fact that the plasmon resonance wavelength of gold NPs lies far from that of the second harmonic signal at 820 nm, a significant increase of the nonlinear response of this dye (when linked to a gold nanoparticle) due to surface Plasmon resonance with gold NPs is shown by Harmonic Light Scattering at 1.64 μm. The nature of the linker has a significant influence of hyperpolarizability values, and a similar, but weaker enhancement effect, is also reported in the case of dye/NP mixtures.