Formation of Photonic Nanojets by Two-Dimensional Microprisms

The focusing of laser radiation with triangular dielectric prisms has been numerically studied using the finite element method implemented in the COMSOL Multiphysics software package. It has been shown that two-dimensional triangular prisms make it possible to focus light in free space into spots smaller than the scalar diffraction limit. For example, a quartz glass prism with a base width of 60 μm and a height of 28.5 μm illuminated by light with a wavelength of 4 μm produces a photonic nanojet with the maximum intensity that exceeds the incident radiation intensity by a factor of 6 and has a full width at half maximum of 0.38 of the focused radiation wavelength. The focal spot size can be decreased by selecting the height of the prism so as to the maximum intensity was located inside it. In particular, a barium titanate prism with a height of 21 μm and a base width of 60 μm forms a focal spot directly behind its apex with a width at half maximum equal to 0.25 of the focused radiation wavelength. It has been shown that the use of a prism weakens the wavelength dependence of the focal spot width as compared with the case of a microcylinder. In particular, for a quartz glass microcylinder 60 μm in diameter, a change in the wavelength from 3 to 5 μm leads to the spot width variation by 0.09 of the focused radiation wavelength, while for a prism, by 0.05 of the focused radiation wavelength on average.