Most of the known methods to measure the nonlinear optical properties of materials deal with the bulk properties, but there are many demanding applications that require those measurements to be done on a single particle or a single molecule. We report a novel application of nonlinear optics to measure the third-order nonlinear optical susceptibility of nanoparticles in solutions. By measuring the power of the third harmonic generated in a diluted solution of nanoparticles, both the size and chi(3) can be extracted from a simple set of measurements.
We present an analytical model for describing optical third-harmonic generation from a sphere that is small compared with the wavelength of light. Analysis of the problem shows that the power of the third harmonic from a sphere that is small compared with the waist size and the confocal parameter of the beam is proportional to the fourth power of a sphere's size. Experiments with different spheres both in index-matching and non-index-matching liquids are performed and confirm theoretical predictions.
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