Photonic nanojet generation under converging and diverging beams

In practical applications of the photonic nanojet (PNJ), a microscope objective can be used to illuminate a microsphere. Under such conditions, it is difficult to determine the absolute position of the sphere with respect to the focal point of the incident beam. A small change in the axial position of the sphere can change the form of the incident wavefront from diverging to converging and will significantly influence the evolution of PNJ. In our paper, we report a systematic study of the PNJ properties, including the effective focal length and the full-width at half maximum (FWHM) by changing the source curvature. We treat the cases of diverging and converging wavefronts with different NAs (from 0.1 to 0.8) and sphere diameters (from 1.6 λ to 33 λ ). We demonstrate that for a diverging source curvature, the effective focal length and FWHM of the PNJ increase as a function source NA for all the sphere diameters. By further increase of the source NA, for a NA of 0.8, the PNJ can be found only for sphere diameters less than 16 λ (refractive index n = 1.5 ). At larger diameters, the microsphere behaves like a diffractive–refractive lens, collimating the light and showing aberrations. For the converging source, the effective focal length and FWHM tend to decrease as a function of source NA, the PNJ localizing inside the sphere for N A > 0.2 . Also, the PNJ shows similar behavior with respect to the NA of a converging source and sphere refractive index under plane wave illumination. Finally, we compare our theoretical 2D simulation results with 3D sphere geometries for diameters of 1.6 λ and 8.3 λ ; we find similar behavior for converging and diverging source wavefront curvatures.