The Influence of Surface Roughness on Photonic-Nanojet Parameters of Dielectric Microspheres

All naturally found and man-made solid microparticles have a rough surface. Upon optical radiation scattering from such particles, in addition to geometric shape, the surface texture becomes an important morphological factor of the scatterer that determines its optical properties. We present the results of numerical FDTD simulation of an optical-wave focusing by a dielectric microsphere with a randomly distributed surface roughness. The cases of azimuthally symmetric and asymmetric distortions of the particle surface are analyzed. It is demonstrated that the key parameters of the near-field focal region (intensity, longitudinal and transverse dimensions, focal distance) referred to as the photonic nanojet (PNJ) turn out to be sensitive to changes in the sphere-surface texture. In the process, two parameters, the peak intensity of the PNJ and its length, experience the largest changes. The influence of the optical contrast (the relative refractive index) of the microsphere that scatters radiation on PNJ characteristics is analyzed, and the possibility of reducing the influence of surface roughness on the quality of the near-field focusing by means of microsphere watering (water-uptake) is demonstrated.