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A. V. Maslov and V. N. Astratov
the similar approach can be used for characterizing the resolution in the case of superresolution imaging with the difference that the PSF width in the latter case would
become narrower than the diffraction-limited resolution. Initially, this approach was
realized using simplified convolution with 1D PSF [33]. Soon after it was generalized
for arbitrarily shaped objects using convolution with 2D PSF [34, 37].
Experimentally, the resolution depends on many factors: (a) type of the object—
near fields can be resonantly enhanced in nanoplasmonic structures, (b) refractive
indices of the microlens and surrounding medium—optimal index contrast for virtual
imaging is within 1.3−1.7 range, but higher background index can facilitate higher
resolution, and (c) type of illumination—confocal can provide a better signal-to-noise
ratio compared to widefield, the excitation of Mie resonances in the microsphere and
special (oblique or coherent) illumination can favor higher resolution. Since in different experimental studies these conditions and parameters varied greatly, presenting a
coherent picture of the developments in this area is rather complicated. We selected
only papers which included some resolution quantifications and combined the results
in Table 16.1. Besides the resolution values, the table contains the information about
the imaged objects, the size and material of the microspheres, the refractive indices
of the microspheres and the surrounding media, and the type of illumination used.
The main conclusion of Table 16.1 is that microspherical nanoscopy provides an
extremely simple and versatile method of imaging which works similar to SIL, but
with somewhat higher resolution. The resolution advantage is particularly well studied for nanoplasmonic objects where the resolution ∼λ/6 − λ/7 was reproduced in
several studies using rigorous resolution criteria. Microspheres provide good nearfield coupling conditions with the nanoscale objects (sphere-to-object distance below
λ/2) due to small size of the contact area. The drawback, however, is that the field
of view is limited by about a quarter of the sphere diameter [32]. It has been demonstrated that the field of view can be increased by translating the microspheres by a
variety of techniques [48–52] followed by stitching the corresponding images.
The simplicity of microspherical imaging implies that it can be combined with
practically any other method of resolution enhancement known in standard widefield
microscopy. Combination with the confocal microscopy [33, 45] allows increasing the signal-to-noise ratio. Improving the resolution is also possible in confocal
microscopy, but it requires a significant reduction of the confocal pinhole with the
associated reduction of the signal. Another example is the combination with interferometric measurements which allows increasing the axial resolution [54–56]. One
more example is the combination with structured illumination microscopy (SIM)
[7–11], especially in the case of plasmonic structures with localized surface plasmon
resonances (LSPRs). This idea was implemented for imaging through microspheres,
where the resolutions ∼λ/10 was reported using localized plasmonic structured
illumination microscopy (LPSIM) with post-imaging processing [57]. A different
approach was developed for imaging nanoscale objects with emission resonantly
coupled to LSPRs in the underlying plasmonic metasurfaces with ultrashort periods
[44]. It can be shown that for periods shorter than 150 nm, the coupling to the socalled surface lattice resonances becomes inefficient in the given detection range,
and the fluorescence enhancement can be achieved due to coupling to the closest
A. V. Maslov and V. N. Astratov
the similar approach can be used for characterizing the resolution in the case of superresolution imaging with the difference that the PSF width in the latter case would
become narrower than the diffraction-limited resolution. Initially, this approach was
realized using simplified convolution with 1D PSF [33]. Soon after it was generalized
for arbitrarily shaped objects using convolution with 2D PSF [34, 37].
Experimentally, the resolution depends on many factors: (a) type of the object—
near fields can be resonantly enhanced in nanoplasmonic structures, (b) refractive
indices of the microlens and surrounding medium—optimal index contrast for virtual
imaging is within 1.3−1.7 range, but higher background index can facilitate higher
resolution, and (c) type of illumination—confocal can provide a better signal-to-noise
ratio compared to widefield, the excitation of Mie resonances in the microsphere and
special (oblique or coherent) illumination can favor higher resolution. Since in different experimental studies these conditions and parameters varied greatly, presenting a
coherent picture of the developments in this area is rather complicated. We selected
only papers which included some resolution quantifications and combined the results
in Table 16.1. Besides the resolution values, the table contains the information about
the imaged objects, the size and material of the microspheres, the refractive indices
of the microspheres and the surrounding media, and the type of illumination used.
The main conclusion of Table 16.1 is that microspherical nanoscopy provides an
extremely simple and versatile method of imaging which works similar to SIL, but
with somewhat higher resolution. The resolution advantage is particularly well studied for nanoplasmonic objects where the resolution ∼λ/6 − λ/7 was reproduced in
several studies using rigorous resolution criteria. Microspheres provide good nearfield coupling conditions with the nanoscale objects (sphere-to-object distance below
λ/2) due to small size of the contact area. The drawback, however, is that the field
of view is limited by about a quarter of the sphere diameter [32]. It has been demonstrated that the field of view can be increased by translating the microspheres by a
variety of techniques [48–52] followed by stitching the corresponding images.
The simplicity of microspherical imaging implies that it can be combined with
practically any other method of resolution enhancement known in standard widefield
microscopy. Combination with the confocal microscopy [33, 45] allows increasing the signal-to-noise ratio. Improving the resolution is also possible in confocal
microscopy, but it requires a significant reduction of the confocal pinhole with the
associated reduction of the signal. Another example is the combination with interferometric measurements which allows increasing the axial resolution [54–56]. One
more example is the combination with structured illumination microscopy (SIM)
[7–11], especially in the case of plasmonic structures with localized surface plasmon
resonances (LSPRs). This idea was implemented for imaging through microspheres,
where the resolutions ∼λ/10 was reported using localized plasmonic structured
illumination microscopy (LPSIM) with post-imaging processing [57]. A different
approach was developed for imaging nanoscale objects with emission resonantly
coupled to LSPRs in the underlying plasmonic metasurfaces with ultrashort periods
[44]. It can be shown that for periods shorter than 150 nm, the coupling to the socalled surface lattice resonances becomes inefficient in the given detection range,
and the fluorescence enhancement can be achieved due to coupling to the closest