The mechanisms of super-resolution imaging by contact microspherical or microcylindrical nanoscopy remain an enigmatic question since these lenses neither have an ability to amplify the near-fields like in the case of far-field superlens, nor they have a hyperbolic dispersion similar to hyperlenses. In this work, we present results along two lines. First, we performed numerical modeling of super-resolution properties of two-dimensional (2-D) circular lens in the limit of wavelength-scale diameters, λ ≤ D ≤ 2λ, and relatively high indices of refraction, n=2. Our preliminary results on imaging point dipoles indicate that the resolution is generally close to λ/4; however on resonance with whispering gallery modes it may be slightly higher. Second, experimentally, we used actin protein filaments for the resolution quantification in microspherical nanoscopy. The critical feature of our approach is based on using arrayed cladding layer with strong localized surface plasmon resonances. This layer is used for enhancing plasmonic near-field illumination of our objects. In combination with the magnification of virtual image, this technique resulted in the lateral resolution of actin protein filaments on the order of λ/7.
Recently, it was experimentally demonstrated (K.W. Allen et al., APL 108, 241108 (2016)) that microspheres can be used as contact microlenses to enhance the efficiency of collection of light by individual pixels in mid wave infrared (MWIR) focal plane arrays (FPAs). In this work, using finite difference time domain (FDTD) modeling, we optimized the designs of such FPAs integrated with microspheres for achieving maximal angle of view (AOV) as a function of the index of refraction and diameter of the spheres. We also designed structures where the spheres are partly immersed in a layer of photoresist. Our designs are developed for both front-side and back-side illuminated structures. Compared to standard microlens arrays, our designs provide much larger angle of view reaching ~15 degrees for front-illuminated and ~4 degrees for back-illuminated structures. Our designs allow decreasing the sizes of photosensitive mesas down to wavelength-scale dimensions determined by the minimal waists of the focused beams produced by the dielectric microspheres, so-called photonic jets. This opens a principle possibility to reduce the dark current and increase the operating temperature of MWIR FPAs. We also discuss the techniques of fabrication of such FPAs integrated with a large number of microspheres and show that suction assembly of microspheres is a promising method of obtaining massive-scale integration of microspheres onto the individual pixels with very small concentration of defects.
In recent years, optical super-resolution by microspheres and microfibers emerged as a new paradigm in nanoscale label-free and fluorescence imaging. However, the mechanisms of such imaging are still not completely understood and the resolution values are debated. In this work, the fundamental limits of super-resolution imaging by high-index barium-titanate microspheres and silica microfibers are studied using nanoplasmonic arrays made from Au and Al. A rigorous resolution analysis is developed based on the object’s convolution with the point-spread function that has width well below the conventional (~λ/2) diffraction limit, where λ is the illumination wavelength. A resolution of ~λ/6-λ/7 is demonstrated for imaging nanoplasmonic arrays by microspheres. Similar resolution was demonstrated for microfibers in the direction perpendicular to the fiber axis with hundreds of times larger field-of-view in comparison to microspheres. Using numerical solution of Maxwell’s equations, it is shown that extraordinary close point objects can be resolved in the far field, if they oscillate out of phase. Possible super-resolution using resonant excitation of whispering gallery modes is also studied.