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A classification of label-free super-resolution imaging mechanisms is given based on the nonlinear reduction of the point-spread function (PSF), near-field scanning, image magnification and gain, structured and sparse illumination, and information approaches. We argue that the super-resolution capability of contact microspheres stems from an image magnification effect taking place in close proximity to the object with contributions of its optical near-fields. We discuss several conditions for quantifying the super-resolution in a label-free microscopy: i) use of standalone objects or long-period arrays as opposed to subwavelength periodic structures, ii) use a convolution with two-dimensional PSF for calculating images, and iii) avoidance of coherent imaging which can lead to dramatic artifacts. We demonstrate a resolution of ∼λ/7 for imaging nanoplasmonic structures and propose a combination of microspherical nanoscopy with nanoplasmonic illumination for imaging biomedical samples. We applied these techniques for imaging actin protein filaments and yeast cells and observed a resolution advantage over standard microscopy.

Microspherical Nanoscopy uses a microsphere as a contact lens which allows for resolving nanometer sized objects using visible light. Nanoplasmonic structures have been used in combination with microspherical nanoscopy to achieve resolutions beyond the classical diffraction limit. The mechanisms of such super-resolution imaging can include the optical near field coupling and the virtual image magnification effects. In this work, we quantify the resolution of microspherical nanoscopy using a basic fluorescence microscope equipped with a standard 40× (NA = 0.6) objective which can be found in most of the labs performing histology or microscopy analyses in a clinical environment. We perform the resolution quantification in three distinct structures: (1) F-Actin proteins on a microscope slide, (2) F-Actin proteins imaged through a high index microsphere on a microscope slide, and (3) F-Actin proteins imaged through a high index microsphere on a nanoplasmonic array. The high index microspheres (n∼2) are embedded in an elastomer slab. Using a microscope we achieve a resolution of (1) ∼λ/0.7, (2) − λ/2.5, and (3) ∼λ/2.8 for each situation.

International audience; The resolution of microsphere-based nanoscopy is studied using fluorescently labeled nanospheres and F-actin protein filaments with the emission coupled to the localized surface plasmon resonances in the underlying Au nanodisk arrays. Virtual imaging is performed through high-index microspheres embedded in plastic coverslips placed in contact with the nanoscale objects. For 150 and 200 nm periods of nanoplasmonic arrays, the imaging has a solid immersion lens-limited resolution, whereas for shorter periods of 80 and 100 nm, the resolution was found to increase up to ∼λ/7, where λ is the emission wavelength. The results cannot be interpreted within a framework of a regular localized plasmonic structured illumination microscopy since the array period was significantly shorter than the wavelength and postimaging processing was not used. It is hypothesized that the observed super-resolution is based on coupling of the emission of nanoscale objects to strongly localized near-field maxima in the adjacent plasmonic metasurfaces followed by evanescent coupling to high-index microspheres.

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.

The use of contact microlenses first started being used as a simple method which allows resolving nanometer sized objects using visible light. The resolution beyond the classical diffraction limit has been reported for nanoplasmonic structures. However the mechanism is not fully understood whether it is from photonic nanojets, plasmon-polaritons, localized surface plasmon resonances, coherent modal excitation in metallic objects, or optical resonances in the microspherical lens itself is still debated in the literature. In this work, we applied contact high-index microspheres for fluorescence imaging of actin protein filaments. The microspheres were embedded in elastopolymer slabs. The fact that the filaments have nanometer-scale width, much smaller than the diffraction limit, significantly simplified the resolution analysis. Using microscope objective with a limited numerical aperture (NA=0.6), we achieve a resolution of ∼λ/1.5 for proteins through the microsphere and a resolution of ∼λ/0.5 without a microsphere.