Nat. Phys. (2018)

Localization calculations for determining the position of light emitters or scatterers that are not linearly polarized may need to revisited thanks to analysis performed by scientists in Austria and Australia. Gabriel Araneda and co-workers have shown that spin–orbit coupling of light can introduce wavelength-scale systematic discrepancies between the actual position of the emitter and the estimate of traditional centroid calculations relying on the paraxial approximation. For certain polarizations and small numerical apertures the error can become arbitrarily large. The team tested their analysis by imaging a single trapped atom and also 100-nm-diameter gold nanoparticles. In the latter case, the apparent positions of the nanoparticle imaged by right- and left-circularly polarized light are displaced relative to each other by 145 ± 6 nm for a numerical aperture of 0.41 that increases to a value as large as 430 ± 7 nm (four times the size of the nanoparticle) for elliptically polarized light. The findings may affect super-resolution imaging schemes that rely on emitter localization algorithms.