Design and Fabrication of Microscale, Thin-Film Silicon Solid Immersion Lenses for Mid-Infrared Application.

Lens-based optical microscopes cannot resolve the sub-wavelength objects overpass diffraction limit. Recently, research on super-resolution imaging has been conducted to overcome this limitation in visible wavelength using solid immersion lenses. However, IR imaging, which is useful for chemical imaging, bio-imaging, and thermal imaging, has not been studied much in optical super-resolution by solid immersion lens owing to material limitations. Herein, we present the design and fabrication schemes of microscale silicon solid immersion lenses (µ-SIL) based on thin-film geometry for mid-infrared (MIR) applications. Compared with geometrical optics, a rigorous finite-difference time-domain (FDTD) calculation of proposed silicon microlenses at MIR wavelengths shows that the outstanding short focal lengths result in enhanced magnification, which allows resolving objects beyond the diffraction limit. In addition, the theoretical analyses evaluate the influences of various structural parameters, such as radius of curvature (RoC), refractive index, and substrate thickness, in µ-SIL. In particular, the high refractive index of µ-SIL is beneficial to implement the outstanding near-field focusing, which corresponds to a high numerical aperture. On the basis of this theoretical background, novel methods are developed for the fabrication of a printable, thin-film silicon microlens array and its integration with a specimen substrate. From the result, we provide a physical understanding of near-field focusing phenomena and offer a promising tool for super-resolution far-field imaging in the MIR range.