Dielectric microspheres superenhance backscattering of light from nanoparticles

Enabling potential applications in visible-light ultramicroscopy and optical biosensing systems, in which nanoparticles consisting of as few as several hundred atoms could be tested and characterized, scientists at Northwestern University (Evanston, IL) have determined how and why the backscattering of light by nanoparticles from 1 to 100 nm in size positioned in close proximity to a dielectric microsphere is enhanced by 7 to 11 orders of magnitude.

A plane-wave-illuminated (wavelength of 300 nm) microsphere with a 3-µm radius and refractive index of 1.73 produces a photonic nanojet of strongly localized optical fields at its shadow-side surface. If a nanoparticle is introduced into the nanojet (surface-to-surface distance between microsphere and nanoparticle is 25 nm), analytical and perturbation analysis show that the nanoparticle is first excited by the nanojet, boosting its scattering intensity by two orders of magnitude; the fields reradiated by the nanojet-excited nanoparticle then interact with the electromagnetic modes of the microsphere, elevating the backscattered intensity of the nanoparticle by four to nine additional orders of magnitude. Contact Zhigang Chen at z‑[email protected] or [email protected].