Subwavelength focusing by a microsphere array

Utilizing the diffractive effect of the output light fields from photonic nanojets, a new scheme for a microsphere array is designed to focus an incident wave to achieve subwavelength resolution in the far field. The three-dimensional finite-difference time-domain (FDTD) method has been introduced to investigate the beam focusing phenomenon along the propagation direction. According to the numerical calculation results, we find that a focal spot with a full width at half maximum (FWHM) of 0.63 lambda can be achieved at a distance of 20.4 lambda away from the exit plane of the microsphere array with a refractive index n = 1.45. We show that the properties of the focal spot will be diverse with the changes of the refractive index, the incident wavelength, and the arrangement of the array. This potential relationship is explicitly analyzed.