Although there are optical methods such as stimulated emission depletion microscopy that circumvent the diffraction limit, common sense tells that position determination at ångstrom precision is not possible with visible light. Nevertheless, with the help of precisely tailored laser light and a spherical silicon nanoparticle, Bag et al. have now been able to measure the displacement of this nanoantenna with sub-ångstrom precision.
The researchers base their work on an effect known as Kerker scattering. An incoming wave that simultaneously excites electric and magnetic dipoles of a dielectric particle is scattered asymmetrically, with reduced or enhanced transverse scattering. Earlier, the same group proposed the use of radially polarized light focused on the nanoparticle. The interference of the scattering transverse to the light’s propagation direction then produces an intensity distribution in the far field that contains information about the position of the scatterer. Now, they have designed an analytical model taking into account not only the particle but also the interface to its support. Their model can simulate the effects of various parameters, such as particle size and shape or the wavelength of the light, for the most sensitive position determination. With an optimized wavelength and a well-focused laser, the researchers were then able to follow the displacement of a single nanoantenna by a couple of ångstrom with sub-ångstrom precision.