Light confinement in scanning near-field optical microscopy

Light propagation and light confinement in scanning near-field optical microscopy (SNOM) are studied by means of the multiple multipole method (MMP). Helmholtzʹ vector wave equation is solved in three dimensions for four different near-field optical probes: a bare glass tip, a metal-clad aperture probe, an “entirely coated” tip and a novel plasmon probe. The latter two are capable of producing a near-field light spot of less than 20 nm (FWHM) extension. The spots have a single intensity peak at the centre, in contrast to the field distribution behind an aperture, which is doubly peaked. The effect of various parameters on light throughput and confinement is investigated for these probes.