Abstract

We have investigated the nonlinear optical properties of nanosized grains of Fe(lO₃)₃, recently synthesized with a simple and inexpensive process [1]. The measurements have been carried out using a Ti:Sapphire femtosecond oscillator coupled to an inverse microscope modified to detect the polarization dependence of the signal. We show that Fe(lO₃)₃ crystals (20-70 nm) can be efficiently employed as local probes for second-harmonic generation (SHG) microscopy, owing to their high non-linear coefficients. Contrary to fluorescent dye molecules, which need resonant excitation, Fe(lO₃)₃ crystals double the frequency of incident light in a large spectral range, they do not undergo photo-bleaching, and they possess a permanent dipole moment that bears information about the orientation of each nanocrystal. In this respect, we are currently investigating their possible use as sensors of local electric field in biological samples. The dependence of SHG for different polarizations of the excitation light and of the detection has been numerically simulated and fitted to the data, using a physical model which takes into account the crystal symmetry, the spatial orientation of the crystals, and the polarization response of the optical set-up [2]. This way, we have been able to derive quantitative information about the second-order susceptibility tensor of this novel material, and retrieve the orientation of individual crystals. Moreover, the recently assessed possibility to detect an NMR signal opens the way to employ Fe(l0₃)₃ crystals as dual SHG-NMR probes.

© 2007 IEEE