10 Super-Resolution Imaging Based on Nonlinear Plasmonic Scattering
245
from silver nanoparticles of varying sizes. An LSPR spectrum is taken for a
single diffraction-limited spot, which has multiple scatterers and the spectral
deconvolution of the spot extracts the peak wavelengths of each NP within it.
The corresponding diffraction-limited images are acquired at each peak wavelength, which are then fit into two-dimensional Gaussian to localise the emitter
associated with each peak wavelength. This process is repeated multiple times
to yield a super-resolution image. These results show very good agreement with
TEM images.
(c) Nonlinear absorption and emission (Saturation of emission and all-optical
switching): It is well known that fluorescence exhibits saturation at high excitation intensity. In 2007, Fujita et al. [36] developed a super-resolution technique,
saturation excitation microscopy (SAX), which is based on extraction of saturated emission. Since saturation starts from the centre of focus, such extraction
provides resolution enhancement. In 2013–2014, we found that scattering signals from individual plasmonic nanostructures show strong nonlinearity, and
applied SAX to significantly enhance spatial resolution. More details will be
given in the following sections.
STED nanoscopy is a well-known strategy for super-resolution imaging based
on optically switching on/off fluorophores. In traditional STED, higher STED
laser powers lead to higher spatial resolution, but at the expense of dye photostability. Sivan et al. [37] proposed that using plasmonic NPs could address
both of these issues. First, a plasmonic NP’s light-concentrating ability from
the far-field to small near-field volumes could lower the power needed from
the STED beam to achieve a particular resolution. Second, if the triplet-state
emission wavelength of the fluorophore overlapped with the LSPR spectrum
of the NP, the dye stability would be increased due to an increased triplet-state
decay rate. Thus, plasmonic-enhanced STED can work both at the lower STED
beam power threshold with increased fluorophore photo-stability, but it is still
based on fluorescence switching.
In 2016, all-optical switching of pure plasmonic scattering was found in our lab
[26]. A simple two-colour method for modulating the scattering intensity from
80 nm gold nanoparticles with a plasmon resonance near 590 nm is used. In
that work, a scanning confocal geometry was used to measure backscattering
of a 543 nm laser from a gold nanoparticle. Our team developed a modified
plasmonic-based STED approach called SUSI, that is suppression of scattering
imaging, that can provide λ/9 resolution.
In the following, we focus the discussion on super-resolution based on nonlinear absorption and emission. We will start by introducing fluorescence-based SAX
and STED microscopy in Sect. 10.2, and then address the discovery of nonlinear
emission from plasmonic nanoparticles in Sect. 10.3. The combination of nonlinear
plasmonics and super-resolution techniques will be given in Sect. 10.4, along with a
brief summary in Sect. 10.5.
245
from silver nanoparticles of varying sizes. An LSPR spectrum is taken for a
single diffraction-limited spot, which has multiple scatterers and the spectral
deconvolution of the spot extracts the peak wavelengths of each NP within it.
The corresponding diffraction-limited images are acquired at each peak wavelength, which are then fit into two-dimensional Gaussian to localise the emitter
associated with each peak wavelength. This process is repeated multiple times
to yield a super-resolution image. These results show very good agreement with
TEM images.
(c) Nonlinear absorption and emission (Saturation of emission and all-optical
switching): It is well known that fluorescence exhibits saturation at high excitation intensity. In 2007, Fujita et al. [36] developed a super-resolution technique,
saturation excitation microscopy (SAX), which is based on extraction of saturated emission. Since saturation starts from the centre of focus, such extraction
provides resolution enhancement. In 2013–2014, we found that scattering signals from individual plasmonic nanostructures show strong nonlinearity, and
applied SAX to significantly enhance spatial resolution. More details will be
given in the following sections.
STED nanoscopy is a well-known strategy for super-resolution imaging based
on optically switching on/off fluorophores. In traditional STED, higher STED
laser powers lead to higher spatial resolution, but at the expense of dye photostability. Sivan et al. [37] proposed that using plasmonic NPs could address
both of these issues. First, a plasmonic NP’s light-concentrating ability from
the far-field to small near-field volumes could lower the power needed from
the STED beam to achieve a particular resolution. Second, if the triplet-state
emission wavelength of the fluorophore overlapped with the LSPR spectrum
of the NP, the dye stability would be increased due to an increased triplet-state
decay rate. Thus, plasmonic-enhanced STED can work both at the lower STED
beam power threshold with increased fluorophore photo-stability, but it is still
based on fluorescence switching.
In 2016, all-optical switching of pure plasmonic scattering was found in our lab
[26]. A simple two-colour method for modulating the scattering intensity from
80 nm gold nanoparticles with a plasmon resonance near 590 nm is used. In
that work, a scanning confocal geometry was used to measure backscattering
of a 543 nm laser from a gold nanoparticle. Our team developed a modified
plasmonic-based STED approach called SUSI, that is suppression of scattering
imaging, that can provide λ/9 resolution.
In the following, we focus the discussion on super-resolution based on nonlinear absorption and emission. We will start by introducing fluorescence-based SAX
and STED microscopy in Sect. 10.2, and then address the discovery of nonlinear
emission from plasmonic nanoparticles in Sect. 10.3. The combination of nonlinear
plasmonics and super-resolution techniques will be given in Sect. 10.4, along with a
brief summary in Sect. 10.5.