Measurement of nanoparticle sizes by conventional optical microscopy with standing evanescent field illumination

The size of a particle smaller than the diffraction limit is measured using a conventional optical microscope by adopting a standing evanescent field illumination. The scattering intensity from a nanoparticle is periodically modulated by shifting the intensity fringes of the standing evanescent field. By measuring contrast of scattering intensity variation during one cycle of modulation, particle sizes can be estimated easily. Furthermore, material dependence can be canceled using contrast as an evaluation factor. From the experimental results, particle sizes ranging from 20 to 250 nm are successfully determined. This technique offers a low-cost size measurement for nanoparticles.