Sub-microscopic nonlinear optical probing of organized molecular assemblies

Abstract

Molecular engineering for nonlinear optics has been recently enriched by the possibility to design and explore molecular materials at the nanometer scale, by the use of supra-molecular arrangements, nanocrystals, and the development and nano-structuring techniques. New possibilities arise from the use of quadratic nonlinear effects at the nanometer-scale, such as probing molecular orientation in biological media[1] or the presence of local field effects in complex environments[2]. The new perspectives offered by downscaling nonlinear optics effects however require adapted optical microscopy techniques. We present in this work high resolution two-photon excitation microscopy studies using both two-photon fluorescence and second harmonic generation (SHG), in order to probe molecular organization and nonlinear optical responses at sub-microscopic scales in various environments. This technique, complemented by spectral and polarization resolution, is shown to be able to provide local tensorial information contained in complex molecular angular distribution symmetries in micro- and nano-scale arrangements. In particular, SHG is a sensitive probe for the detection of non-centrosymmetric local contributions in a molecular angular distribution. The experiment consists of a 120 fs-pulsed source excitation at a fundamental wavelength of ~987 nm, implemented in an inverted microscope with a high numerical aperture objective, yielding a spatial resolution of 400 nm in the detected visible spectral range.