10 Super-Resolution Imaging Based on Nonlinear Plasmonic Scattering
253
dissipation is proportional to the surface area of the particle, it is reasonable to expect
higher temperature for larger particles at equilibrium.
Although we focus on super-resolution imaging in this article, we would like to
mention that one potential application of the plasmonic all-optical switch is towards
realisation of high-density photonic circuits. Our novel plasmonic all-optical switch
has a mode volume less than 0.001 µm
3 . The modulation depth of this ultra-small
switch reaches 80%, and the switching behaviour is active within the broad band
of plasmonic resonance. Our work could open the possibility to realise high-density
integrated photonic nanocircuits in the future.
Now, based on our recent discovery of plasmonic nonlinear scattering, let us
continue the journey to explore how one can utilise the property of nonlinear emission
due to plasmonic nanoparticles to achieve super-resolution imaging.
10.4 Super-Resolution Based on Nonlinear Scattering
In the previous section, we have unravelled three novel nonlinearities in plasmonic
scattering, that is saturation, reverse saturation, and all-optical switching. Here we
are going to present three methods to enhance spatial resolution based on these
nonlinearities. The first one is to use the high-order nonlinearity of reverse saturation,
which causes significant reduction of PSF directly under a laser scanning microscope.
The second one is the combination of saturable scattering and SAX, where the latter
extracts nonlinear components of the former, and thus enhances spatial resolution.
The third method is to adopt all-optical switching of scattering into a STED-like
setup, to “turn off” scattering around the centre of a PSF, leading to resolution
enhancement.
10.4.1 Super Resolution Based on Reverse Saturation
of Scattering
Figure 10.6 presents a very interesting shape change of PSF from a single 80 nm
GNS, as the excitation intensity gradually increases. At low excitation intensity in
Fig. 10.6a, that is the linear response region, the PSF fits well to a Gaussian profile,
as we expected. As excitation intensity exceeds 10
5 W/cm
2 in Fig. 10.6b, flattening
at the centre of PSF is observed, manifesting that saturation of scattering starts from
the region with largest intensity. Figure 10.6c is the situation of deep saturation,
where scattering intensity reduces with increase of excitation intensity. This unusual
phenomenon creates an unexpected donut-like PSF, whose side lobes exhibit width
of only 40 nm (λ/13).
When the excitation intensity further increases to above 10
6 W/cm
2 , the scattering signal at the centre of PSF rises, showing an onset of reverse saturation, while
253
dissipation is proportional to the surface area of the particle, it is reasonable to expect
higher temperature for larger particles at equilibrium.
Although we focus on super-resolution imaging in this article, we would like to
mention that one potential application of the plasmonic all-optical switch is towards
realisation of high-density photonic circuits. Our novel plasmonic all-optical switch
has a mode volume less than 0.001 µm
3 . The modulation depth of this ultra-small
switch reaches 80%, and the switching behaviour is active within the broad band
of plasmonic resonance. Our work could open the possibility to realise high-density
integrated photonic nanocircuits in the future.
Now, based on our recent discovery of plasmonic nonlinear scattering, let us
continue the journey to explore how one can utilise the property of nonlinear emission
due to plasmonic nanoparticles to achieve super-resolution imaging.
10.4 Super-Resolution Based on Nonlinear Scattering
In the previous section, we have unravelled three novel nonlinearities in plasmonic
scattering, that is saturation, reverse saturation, and all-optical switching. Here we
are going to present three methods to enhance spatial resolution based on these
nonlinearities. The first one is to use the high-order nonlinearity of reverse saturation,
which causes significant reduction of PSF directly under a laser scanning microscope.
The second one is the combination of saturable scattering and SAX, where the latter
extracts nonlinear components of the former, and thus enhances spatial resolution.
The third method is to adopt all-optical switching of scattering into a STED-like
setup, to “turn off” scattering around the centre of a PSF, leading to resolution
enhancement.
10.4.1 Super Resolution Based on Reverse Saturation
of Scattering
Figure 10.6 presents a very interesting shape change of PSF from a single 80 nm
GNS, as the excitation intensity gradually increases. At low excitation intensity in
Fig. 10.6a, that is the linear response region, the PSF fits well to a Gaussian profile,
as we expected. As excitation intensity exceeds 10
5 W/cm
2 in Fig. 10.6b, flattening
at the centre of PSF is observed, manifesting that saturation of scattering starts from
the region with largest intensity. Figure 10.6c is the situation of deep saturation,
where scattering intensity reduces with increase of excitation intensity. This unusual
phenomenon creates an unexpected donut-like PSF, whose side lobes exhibit width
of only 40 nm (λ/13).
When the excitation intensity further increases to above 10
6 W/cm
2 , the scattering signal at the centre of PSF rises, showing an onset of reverse saturation, while