We use evaporation within a microfluidic device to extract the solvent of a (possibly very dilute) dispersion of nanoparticles and concentrate the dispersion until a solid made of densely packed nanoparticles grows and totally invades the microfluidic geometry. The growth process can be rationalized as an interplay between evaporation-induced flow and kinetic and thermodynamic coefficients which are system-dependent; this yields limitations to the growth process illustrated here on two main cases: evaporation- and transport-limited growth. Importantly, we also quantify how colloidal stability may hinder the growth and show that care must be taken as to the composition of the initial dispersion, especially regarding traces of ionic species that can destabilize the suspension upon concentration. We define a stability chart, which, when fulfilled, permits us to grow and shape-up solids, including superlattices and extended and thick arrays of nanoparticles made of unary and binary dispersions, composites, and heterojunctions between distinct types of nanoparticles. In all cases, the geometry of the final solid is imparted by that of the microfluidic device.

6 pagesInternational audienceWe present high refractive index optical metamaterials assembled via a microfluidic evaporation technique. This technique enables fabrication of truly three-dimensional bulk samples from a suspension of nanoparticles with a number of layers well in excess of 600, surpassing rival techniques by at least an order of magnitude. In addition to their large dimensions, the assembled metamaterials show a high degree of homogeneity and warrant an easy and rapid optical characterization using spectroscopic ellipsometry. We believe that the suggested inexpensive method considerably reduces the complexity in assembling optical metamaterials and opens new avenues in engineering bulk optical devices by choice of nanoparticle composition and geometry

We measure linear absorption, circular dichroïsm, second harmonic and sum frequency generation in the [-Fe(phen) 3 ](Δ-As 2 (tartarate) 2), [-Fe(phen) 3 ](-As 2 (tartarate) 2) enantiomers of an Fe(II) complex. In the solid state, the chirality of this compound results from the introduction of the (As 2 (tartarate) 2) chiral anions. Linear absorption and X-ray diffraction indicate that Fe(II) is in the low-spin state. Circular dichroïsm reveals that in the solid state, these compounds are chiral, whereas the complexes racemize in solution. A large second harmonic generation signal is recorded using thin films from these two enantiomers. The second-order susceptibility of these compounds is evaluated across the visible spectral range. It displays a resonance at 520 nm, which is associated with the metal-to-ligand charge transfer occurring within the complex. At its maximum, = 6.4 pm V-1 is more than 1.4 times larger than the well-known beta-BaB 2 O 4 nonlinear crystal (~4.4 pm V-1). Finally, we demonstrate a useful application for a thin film from this compound, which characterizes the cross-correlation of two femtosecond laser pulses.