Analysis of Photonic Nanojets Formed by a Focused Incident Beam Using Vector Spherical Harmonics Expansion

The analysis of photonic nanojets formed by dielectric spheres generally assumes that the incident field is a plane wave. In this work, using vector spherical harmonics representations, we analyze the case of a more realistic incident field consisting of a focused beam formed by microscope objectives with low as well as high numerical aperture. The nanojet solution is exact with practical number of spherical harmonic terms. In contrast with a plane wave analysis, we are able to include in the analysis cases in which the sphere is at the focal plane of the focus beam and away from the focal plane. We found that the nanojet beam waist dimension is less sensitive with respect to the azimuthal angle when compare with the plane wave case. Also, by shifting the particle away from the focal plane, the nanojet beam waist can be positioned outside the particle which otherwise would be inside or at the particle surface. Different relative positions of the particle and the focused beam along the propagating axis are discussed as well as different particle size. The bwam waist can achieve 130 nm when the diameter of the particle is 2 um. The results are compared with the plane wave case