Near field focusing by edge diffraction.

Spherical microparticles have the ability for nonresonant focusing of light in the near field zone, forming nanojet (NJ) beams. Arbitrary-shaped microstructures, with wavelength-scale dimensions, may offer similar functionality with lower fabrication complexity. The focusing properties are ruled by the edge diffraction phenomenon. The diffraction of light on the edge of a dielectric microstructure forms a tilted focused beam whose deviation angle depends on the index ratio between the structure material and host medium. The beam geometry and field intensity enhancement can be tuned by varying the curvature of the edge line. Interference of edge diffracted waves from different segments of the edge line creates a condensed beam in the near field zone, the photonic nanojet.