Elliptically polarized light could cause large errors in superresolution microscopy

Scientists at the Austrian Academy of Sciences (Vienna, Austria), TU Wien (also in Vienna), and the University of Innsbruck (Innsbruck, Austria) have demonstrated that elliptically polarized light can produce measurement errors in the optical position estimation of objects (for example, when using various forms of microscopy) of up to several hundred nanometers. The error-inducing wave effect is attributed to spin-orbit coupling of light in the emission of elliptically polarized emitters. The work could have consequences for optical microscopy and optical astronomy.

With modern superresolution optical imaging techniques, the position of objects can be measured with a precision that reaches a few nanometers. These techniques are used in the laboratory, for example, to determine the position of atoms in quantum experiments. “The elliptical polarization causes the wavefronts of the light to have a spiral shape and to hit the imaging optics at a slight angle,” says Yves Colombe, one of the Innsbruck researchers. “This leads to the impression that the source of the light is somewhat off its actual position.” The effect could be relevant, for example, in biomedical research, where luminous proteins or nanoparticles are used as markers to determine biological structures. This now-proven effect would possibly lead to a distorted image of the actual structures.

At Innsbruck, physicists determined, through single-photon emission, the position of a single barium atom trapped in an ion trap. Physicists at Vienna determined the position of a small gold sphere, about 100 nm in size, by analyzing its scattered light. In both cases, there was a difference between the observed and the actual position of the particle. “The deviation is on the order of the wavelength of the light and it can add up to a considerable measurement error in many applications,” says Stefan Walser, one of the TU Wien researchers. “Superresolution light microscopy, for example, has already penetrated far into the nanometer range, whereas this effect can lead to errors of several 100 nm.” The scientists believe it is very likely that this fundamental systematic error will play a role in superresolution microscopy applications, but this has yet to be proven in separate studies. Reference: G. Araneda et al., Nat. Phys. (2018); doi:10.1038/s41567-018-0301-y.