Measuring the complex field scattered by single submicron particles

Abstract

We describe a method for simultaneous measurements of the real and imaginary parts of the field scattered by single nanoparticles illuminated by a laser beam, exploiting a self-reference interferometric scheme relying on the fundamentals of the Optical Theorem. Results obtained with calibrated spheres of different materials are compared to the expected values obtained through a simplified analytical model without any free parameters, and the method is applied to a highly polydisperse water suspension of Poly(D,L-lactide-co-glycolide) nanoparticles. Advantages with respect to existing methods and possible applications are discussed.

Detecting the shape of anisotropic gold nanoparticles in dispersion with single particle extinction and scattering

“…As an output, the SPES produces two-dimensional (2D) plots (histograms) providing the raw data distribution of single particle fields in the complex plane Re S(0) -Im S(0). 27 Typically, the scattered field depends on i) size, ii) refractive index and iii) shape and orientation of each particle. Figure 2 shows how spherical (GNP2) and branched gold nanoparticles (GNP3, GNP4) in the comparable size range occupy different regions of the two-dimensional plot.…”

“…This overcomes the limitations typical for DLS, making SPES an efficient tool for the in line characterisation of nanoparticle polydispersity in fluids, providing information on particle size distribution, agglomeration state and refractive index of the dispersed species. [26][27][28][29][30][31] In the present configuration, SPES can work with dielectric particles down to roughly 200 nm in diameter. In the case of metallic NPs, as for example gold, both scattering and extinction efficiencies are much larger than in the case of dielectric particles, increasing the sensitivity of the method for small sizes.…”

“…These two independent parameters are directly measured from the interference between the transmitted beam and the forward scattered light, thanks to the self-reference interference between the faint scattered and the intensely transmitted fields. 27,28 The raw data generated by SPES are described with real and imaginary parts of the electric field, Re S(0) and Im S(0) respectively. [27][28] The former is related to the ability of the particle to scatter and absorb the light and the latter to the capacity of the particle of being polarized under an external field, i.e.…”

“…27,28 The raw data generated by SPES are described with real and imaginary parts of the electric field, Re S(0) and Im S(0) respectively. [27][28] The former is related to the ability of the particle to scatter and absorb the light and the latter to the capacity of the particle of being polarized under an external field, i.e. polarizability.…”

“…[27][28] A 5 mW solid state laser (wavelength 635 nm) has been focused to a diameter of 5 µm into a flow cell and the emerging light collected by a segmented photodiode. A small particle passing through the focal region generates a scattered wave that superimposes to the intensely transmitted beam.…”