Photonic Nanojet Generated by a Coated Hemisphere with Tunable Core

Photonic nanojets are generally generated by a dielectric microcylinder or dielectric microsphere illuminated by a plane wave. When the plane wave incident on the microsphere, the parallel components of the wave vector cancel each other out, and the perpendicular component has a very long scale due to the superposition effect. Therefore, in the backward direction of the microsphere, we get a focusing spot with a transverse scale smaller than the wavelength and a longitudinal scale of several wavelengths, which is the photonic nanojet, and it can greatly enhance the field strength on the basis of the incident field [1]. Such high-intensity photonic jets are diffraction-free, high-intensity, and able to break through the diffraction limit, and this phenomenon is now widely used in optical imaging [2 - 5], optical tweezers [6 - 8], nanolithography [9 - 10], and optical switches [11]. Without changing the incident field parameters, the morphology of the photon jet is most affected by the scattering object. Therefore, this paper discusses the influence of the radius of the scattering hemisphere on the morphology of the photon jet based on the finite difference frequency domain (FDFD) method. It has some application prospects in the design of optical devices such as optical tweezers.