Theoretical analysis of numerical aperture increasing lens microscopy

We present a detailed theoretical analysis and experimental results on a subsurface microscopy technique that significantly improves the light-gathering, resolving, and magnifying power of a conventional optical microscope. The numerical aperture increasing lens (NAIL) is a plano-convex lens placed on the planar surface of an object to enhance the amount of light coupled from subsurface structures within the object. In particular, a NAIL allows for the collection of otherwise inaccessible light at angles beyond the critical angle of the planar surface of the object. Therefore, the limit on numerical aperture increases from unity for conventional subsurface microscopy to the refractive index of the object for NAIL microscopy. Spherical aberration associated with conventional subsurface microscopy is also eliminated by the NAIL. Consequently, both the amount of light collected and diffraction-limited spatial resolution are improved beyond the limits of conventional subsurface microscopy. A theoretical optical model for imaging structures below the planar surface of an object, both with and without a NAIL, is presented. Experimental results demonstrating the predicted improvements in resolution of subsurface imaging are also presented.