Accurate position sensing is of fundamental importance in various fields of science, such as microscopy and optical trapping. An established nonlinear method for localization is photoactivated localization microscopy (PALM) [1], which is utilized for high resolution imaging. Other localization methods use a linear response, for instance, the detection of directional signals created by interference of a scalar excitation field with the light scattered off a sub-wavelength particle [2]. We now present a new concept of high accuracy position sensing at nanoscale based on the intrinsic directional emission of an individual nanoparticle. This can be achieved by interfering the emission patterns of two multipoles supported by the particle [3]. Even in case of two dipoles, the scattering anisotropy can be enhanced if the interfering dipoles are of different type (electric and magnetic). Fig. 1 shows the corresponding scenario for electric and magnetic dipoles of the same strength close to a dielectric interface. The emission of each dipole individually results in a symmetric far-field pattern in the chosen plane, but their interference yields strongly directional emission (compare [4]). The strength of the scattering anisotropy can be changed by tuning the relative strength of the dipoles.
Fig. 1. Far-field emission pattern of a point-like (a) electric dipole normal to the air-glass interface and (b) transverse magnetic dipole. In the chosen plane, both dipoles emit symmetrically into the dielectric substrate; their mutual interference results in strong directional light emission (c).
For experimental realization, we use a sub-wavelength silicon sphere, supporting both electric and magnetic resonances [5]. If the excitation field is chosen to be spatially inhomogeneous consisting of both transverse and longitudinal components (achieved by tight focusing), the particle will emit differently at different positions relative to the beam (it senses only the local field). The particle position can be retrieved from the directionality of the emission measured in the far-field, achieving deep sub-wavelength localization accuracies on the order of several Angstroms to a few nanometers.
[1] E. Betzig et al., Science 313, 1642 (2006)
[2] F. Gittes et al., Opt. Lett. 23(1), 7 (1998)
[3] M. Neugebauer et al., Nano Lett. 14, 2546 (2014)
[4] M. Kerker et al., J. Opt. Soc. Am. 73(6), 765 (1983)
[5] A. I. Kuznetsov et al., Sci. Rep. 2, 492 (2012)
pawel.wozniak@mpl.mpg.de