Recently, we realized a simple technique, which spontaneously bypasses the diffraction limit in a conventional confocal microscope by exploiting super-linear effects in nanoparticle bio-markers: super-linear excitation-emission (SEE) microscopy. Here, we present a theoretical framework and its practical implementation for optimizing and expanding this technique. We accurately predict the expected 3D super-resolution by accounting for all crucial parameters affecting the resolution: the empirically measured/modelled excitation-emission curve, the filling factor of the microscope objective back pupil, the polarization and the pinhole setting. The presented theoretical framework is a practical tool, which enables end-users to augment their own confocal microscopes with super-resolution capabilities.
We achieve spontaneous 3D super-resolution on a standard confocal microscope by exploiting bio-friendly fluorescent markers with super-linear excitation-emission dependence (upconversion nanoparticles of NaYF4: Yb, Tm). We refer to this approach as upconversion super-linear excitation-emission (uSEE) microscopy. To demonstrate the applicability of the method for biological applications, we image sugar-coated upconversion nanoparticles in neuronal cells and we achieve resolution twice better than the diffraction limit both in lateral and axial directions. We envision that due to the application simplicity of the developed methodological toolbox, uSEE microscopy can be widely incorporated as an everyday super-resolution method in biological laboratories.
The development of innovative photonic devices and metamaterials with tailor-made functionalities depends critically on our capability to characterize them and understand the underlying light-matter interactions. Thus, imaging all components of the electromagnetic light field with nanoscale resolution is of paramount importance in this area. Nowadays, the electric and the vertical magnetic field components of light can be measured with sub-wavelength resolution. This is achieved by scanning the sample surface with specific probes in a method known as scanning near-field optical microscopy (SNOM). However, within this toolbox, an unambiguous way of visualizing the horizontal magnetic field component has been missing.
We have answered this challenge by demonstrating experimentally that a hollow-pyramid circular aperture probe SNOM can directly image the horizontal magnetic field of light in simple plasmonic antennas – rod, disk and ring. These results are also confirmed by numerical simulations, showing that the probe can be approximated, in the first order, by a magnetic point-dipole source. This approximation substantially reduces the simulation time and complexity and facilitates the otherwise controversial interpretation of near-field images. Further, we use the validated technique to study complex plasmonic antennas and to explore new opportunities for their engineering and characterization. The applicability of this methodology is currently being extended beyond plasmonics structures.
Thus, the presented hollow-pyramid circular aperture based SNOM approach complements the existing techniques for imaging the different electromagnetic field components, by providing an opportunity to explore the tangential magnetic field of light with sub-wavelength resolution.
We report mapping of the lateral magnetic near-field distribution of plasmonic resonant modes in different nanostructure geometries by hollow-pyramid probe aperture-SNOM. Using full-field simulations we investigate how the near-field probe acts as a confined light source and how it efficiently excites surface plasmons. This excitation occurs at lateral magnetic field maxima, enabling the visualization of the lateral magnetic near-field distribution with subwavelength spatial resolution. Our approach complements the available methods for imaging the different field components of light.
[1] D. Denkova, N. Verellen et al., ACS nano 7(4), 3168-3176 (2013).
[2] D. Denkova, N. Verellen et al., Small, accepted (2013).
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