Phase and amplitude apodization induced by focusing through an evanescent gap in a solid immersion lens microscope

We characterize phase and amplitude apodization induced when a converging beam passes through an evanescent gap. The apodization is caused by vector transmission and reflection properties. General characteristics are described for solid immersion lens indices from n_SIL = 1.5 to 3.1, gap heights from h = 0 to 300 nm, and marginal ray angles α_m = 0.7 or 0.84, where α_m is the direction cosine of the marginal ray angle inside the solid immersion lens. A small amount of defocus is found to be a good compensator of the phase apodization for low n_SIL and h. After proper defocus is applied, asymmetry of focus in the spot may remain, primarily due to the uncompensated amplitude apodization. Simplification of the phase and amplitude characteristics is accomplished by applying a Jones-matrix expansion to the transmission coefficient through the gap. Simulation and experiment quantify the effect with a simple solid immersion lens geometry.