Light confinement effect of nonspherical nanoscale solid immersion lenses

We report on the light confinement effect observed in nonideally shaped (i.e., nonspherical) nanoscale solid immersion lenses (SIL). To investigate this effect, nanostructures of various shapes are fabricated by electron-beam lithography. When completely melted in reflow, these noncircular pillars become spherical, while incomplete melting results in nonspherically shaped SILs. Optical characterization shows that nonideal SILs exhibit a spot size reduction comparable with that of spherical SILs. When the size of the SIL is of wavelength scale or smaller, aberrations are negligible due to the short optical path length. This insensitivity to minor variations in the shape implies a large tolerance in nano-SIL fabrication. 1 Introduction The immersion technique, which improves the imaging performance of a microscope, was first proposed by Hooke in his book Microscopium in 1678. 1 Sir Brewster later proposed the immersion of the objective lens in 1812. 2 Similarly, the concept of homogeneous immersion and the immersion objective preceded Ernst Abbe's pioneering work, in which he demonstrated the first oil-immersion lens in developing the imaging theory of microscopy. 3 Abbe also developed the standard measure of performance of an objective lens, the numerical aperture (NA), which is defined as