Photonic nanojets and microsphere lenses

A narrow, high-intensity beam that forms just behind a transparent microsphere or microcylinder lit by a plane wave. It can be narrower than half a wavelength and extends for a few wavelengths: a 3 µm silica sphere on glass at 532 nm concentrates the intensity about 67-fold (|E|²/|E₀|²).

The sphere refracts light like a ball lens, but because it is only a few wavelengths across, the focus forms at or just outside its surface and diffraction and interference shape it into the nanojet. Placed on a sample, the sphere collects evanescent (near-field) detail and forms a magnified virtual image, the basis of microsphere-assisted super-resolution microscopy.

Typically spheres from about 2 to 40 wavelengths across with a refractive index below about twice that of the surroundings (silica, polystyrene, BaTiO₃ glass in air or water). Higher index pulls the focus inside the sphere; larger spheres give longer, wider jets.

Super-resolution imaging (microsphere microscopy), nanopatterning and laser surface processing below the diffraction limit, detecting and sizing nanoparticles that cross the jet, enhancing Raman and fluorescence signals, and optical data storage concepts.

No. The nanojet is a propagating beam concentrated by interference behind the sphere; its narrow waist comes from the near-field and interference structure close to the surface, but it travels outward for several wavelengths.