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T. C. Jagadale and S.-W. Chu
enhances evanescent waves across the lens, the goal of a hyper-lens is to project
the evanescent waves into the far-field by converting them into propagating
waves. A plasmonic hyper-lens [31] consists of alternating layers of a metal and
dielectric to form an anisotropic plasmonic material, which has the unique property of propagating high spatial frequencies. These lenses offer the possibility
of imaging sub-diffraction-limited objects in real time. However, no exclusive
reports on multiple objectives were found for super-resolution imaging.
(b) Non-uniform illumination/collection: The tailoring of excitation light is
achieved by the strong coupling between light and plasmonic materials. The
concentration of light to small volume localised at the plasmonic surface is
possible. This small region of strongly enhanced EM fields at the plasmonic
surface is called as hot spot. The hot spots effectively serve as nanometre-sized
excitation sources for any molecule that enters within this small excitation volume. As these hot spots are spatially localised to regions 1–10 nm in size, so,
the molecules separated by distances smaller than the diffraction limit can be
selectively excited, enabling super-resolution imaging of molecules that would
otherwise overlap in a traditional far-field technique [32].
Similar to localised SPR hot spots, surface plasmon polaritons (SPP) standing
wave interference patterns were also proved as near-field excitation sources.
Using waves with different phases to shift the interference patterns and illuminate different regions of the sample allows reconstruction of a super-resolved
image. A particularly interesting application of SPPs to super-resolution imaging is that radiation travelling as an SPP has a wavelength smaller than the
original excitation light. This sets up the possibility to use SPPs as both small
volume excitation sources as well as coupling agents for projecting high spatial
frequencies into the far-field via interference [33].
SPP interferences can improve the resolution in conventional SIM by creating structured illumination patterns with higher spatial frequencies than typical
excitation grid patterns. However, in 2010, Lui and co-workers proposed that
surface plasmons could form interference patterns for illumination with much
higher spatial frequencies, based on the fact that SPPs have larger wave-vectors
than free space light. A silver film with periodic slits spaced by 7.6 µm is used to
launch SPPs in opposing directions that form a standing wave, which interfere
to double the spatial frequency of the surface plasmon. Liu and co-workers in
2014 [34] has experimentally demonstrated PSIM, which offered an advantage
that the resolution is not strictly limited by the numerical aperture of an objective
lens but rather by the emission wavelength, SPP wavelength, and interference
pattern. But the major limitation of above two methods is the post-processing
of images to get final super-resolved image.
Another strategy is to adopt non-uniform collection, such as dark-field
microscopy, to resolve individual plasmonic NP within a diffraction-limited
spot. Huang et al. devised a technique called photo-stable optical nanoscopy
(PHOTON) [35], which relied on deconvolution of overlapping LSPR spectra
T. C. Jagadale and S.-W. Chu
enhances evanescent waves across the lens, the goal of a hyper-lens is to project
the evanescent waves into the far-field by converting them into propagating
waves. A plasmonic hyper-lens [31] consists of alternating layers of a metal and
dielectric to form an anisotropic plasmonic material, which has the unique property of propagating high spatial frequencies. These lenses offer the possibility
of imaging sub-diffraction-limited objects in real time. However, no exclusive
reports on multiple objectives were found for super-resolution imaging.
(b) Non-uniform illumination/collection: The tailoring of excitation light is
achieved by the strong coupling between light and plasmonic materials. The
concentration of light to small volume localised at the plasmonic surface is
possible. This small region of strongly enhanced EM fields at the plasmonic
surface is called as hot spot. The hot spots effectively serve as nanometre-sized
excitation sources for any molecule that enters within this small excitation volume. As these hot spots are spatially localised to regions 1–10 nm in size, so,
the molecules separated by distances smaller than the diffraction limit can be
selectively excited, enabling super-resolution imaging of molecules that would
otherwise overlap in a traditional far-field technique [32].
Similar to localised SPR hot spots, surface plasmon polaritons (SPP) standing
wave interference patterns were also proved as near-field excitation sources.
Using waves with different phases to shift the interference patterns and illuminate different regions of the sample allows reconstruction of a super-resolved
image. A particularly interesting application of SPPs to super-resolution imaging is that radiation travelling as an SPP has a wavelength smaller than the
original excitation light. This sets up the possibility to use SPPs as both small
volume excitation sources as well as coupling agents for projecting high spatial
frequencies into the far-field via interference [33].
SPP interferences can improve the resolution in conventional SIM by creating structured illumination patterns with higher spatial frequencies than typical
excitation grid patterns. However, in 2010, Lui and co-workers proposed that
surface plasmons could form interference patterns for illumination with much
higher spatial frequencies, based on the fact that SPPs have larger wave-vectors
than free space light. A silver film with periodic slits spaced by 7.6 µm is used to
launch SPPs in opposing directions that form a standing wave, which interfere
to double the spatial frequency of the surface plasmon. Liu and co-workers in
2014 [34] has experimentally demonstrated PSIM, which offered an advantage
that the resolution is not strictly limited by the numerical aperture of an objective
lens but rather by the emission wavelength, SPP wavelength, and interference
pattern. But the major limitation of above two methods is the post-processing
of images to get final super-resolved image.
Another strategy is to adopt non-uniform collection, such as dark-field
microscopy, to resolve individual plasmonic NP within a diffraction-limited
spot. Huang et al. devised a technique called photo-stable optical nanoscopy
(PHOTON) [35], which relied on deconvolution of overlapping LSPR spectra