10 Super-Resolution Imaging Based on Nonlinear Plasmonic Scattering
251
a laser-scanning microscope, that is xy-scan. Therefore, if the nonlinear response can
be separated from linear response, the effective PSF would be reduced, leading to
enhancement of spatial resolution, as demonstrated in Sect. 10.4.
For SNS, its diameter is 80 nm, leading to two resonance peaks at 400 and 490 nm.
Figure 10.4c shows a clear nonlinear behaviour of a single 80 nm SNS using 473 nm
excitation of wavelength, which is located inside the broad 490 nm resonance. Similar
to the response of gold nanostructures, both saturation and reverse saturation are
observed. Saturation in scattering is observed within the excitation range of 5 ×
10
5 –7 × 10
5 W/cm
2 , and higher intensity induces reverse saturation. Once again,
the nonlinear response can be visualised in the laser scanning PSF, that is xy-scan,
as given in Fig. 10.4d. Here deep saturation is observed, resulting in an unusual PSF
profile.
Based on Mie theory calculation, in the 80 nm SNS, absorption dominates at
the 400 nm resonance peak, while scattering is the main component of the 490 nm
resonance. It thus provides an interesting opportunity to clarify the mechanism of
nonlinear scattering. Figure 10.4e shows the comparison of threshold laser intensities for nonlinear scattering using 405, 473, and 785 nm lasers. More than 100
nanoparticles are analysed, and the percentages of nanoparticles that start to show
nonlinear scattering at certain excitation intensity is presented in the vertical axis. A
dashed horizontal line marks the threshold intensity when 50% of the SNSs show
nonlinear scattering, and apparently 405 nm gives lower threshold intensity. That is,
nonlinear scattering is more probable to occur with the 405 nm laser, manifesting
that SPR absorption is dominant. On the other hand, for the 785 nm laser, which is
far from the SPR absorption of SNSs, no nonlinear scattering is observed at all. Since
the consequence of absorption is temperature rise, the major mechanism behind the
nonlinear scattering should be photo-thermal effect.
One particular interesting application of plasmonics is the hot spot created by
sharp structures or neighbouring nanoparticles. Figure 10.4f presents the scattering
intensity dependence of a gold bowtie antenna, which is composed of two triangular
gold nanostructures, whose edge length is 150 nm, and the gap between them is
10 nm. It is interesting to see that saturation of scattering is again observed in the
antenna, manifesting that nonlinear plasmonic scattering is universal with different
materials and structures.
10.3.3 All-Optical Switch on a Plasmonic (Au) Nanospheres
In the preceding section, we have shown that saturation of plasmonic scattering is
ubiquitous, and have found that deep saturation, that is scattering reduces as excitation
intensity increases, is obtained when excitation wavelength is close to the plasmonic
absorption peak. Here we show that the deep saturation affects not only the excitation
wavelength, but also other wavelengths inside the plasmonic band, thus enabling alloptical switch of plasmonic scattering for the first time [26].
251
a laser-scanning microscope, that is xy-scan. Therefore, if the nonlinear response can
be separated from linear response, the effective PSF would be reduced, leading to
enhancement of spatial resolution, as demonstrated in Sect. 10.4.
For SNS, its diameter is 80 nm, leading to two resonance peaks at 400 and 490 nm.
Figure 10.4c shows a clear nonlinear behaviour of a single 80 nm SNS using 473 nm
excitation of wavelength, which is located inside the broad 490 nm resonance. Similar
to the response of gold nanostructures, both saturation and reverse saturation are
observed. Saturation in scattering is observed within the excitation range of 5 ×
10
5 –7 × 10
5 W/cm
2 , and higher intensity induces reverse saturation. Once again,
the nonlinear response can be visualised in the laser scanning PSF, that is xy-scan,
as given in Fig. 10.4d. Here deep saturation is observed, resulting in an unusual PSF
profile.
Based on Mie theory calculation, in the 80 nm SNS, absorption dominates at
the 400 nm resonance peak, while scattering is the main component of the 490 nm
resonance. It thus provides an interesting opportunity to clarify the mechanism of
nonlinear scattering. Figure 10.4e shows the comparison of threshold laser intensities for nonlinear scattering using 405, 473, and 785 nm lasers. More than 100
nanoparticles are analysed, and the percentages of nanoparticles that start to show
nonlinear scattering at certain excitation intensity is presented in the vertical axis. A
dashed horizontal line marks the threshold intensity when 50% of the SNSs show
nonlinear scattering, and apparently 405 nm gives lower threshold intensity. That is,
nonlinear scattering is more probable to occur with the 405 nm laser, manifesting
that SPR absorption is dominant. On the other hand, for the 785 nm laser, which is
far from the SPR absorption of SNSs, no nonlinear scattering is observed at all. Since
the consequence of absorption is temperature rise, the major mechanism behind the
nonlinear scattering should be photo-thermal effect.
One particular interesting application of plasmonics is the hot spot created by
sharp structures or neighbouring nanoparticles. Figure 10.4f presents the scattering
intensity dependence of a gold bowtie antenna, which is composed of two triangular
gold nanostructures, whose edge length is 150 nm, and the gap between them is
10 nm. It is interesting to see that saturation of scattering is again observed in the
antenna, manifesting that nonlinear plasmonic scattering is universal with different
materials and structures.
10.3.3 All-Optical Switch on a Plasmonic (Au) Nanospheres
In the preceding section, we have shown that saturation of plasmonic scattering is
ubiquitous, and have found that deep saturation, that is scattering reduces as excitation
intensity increases, is obtained when excitation wavelength is close to the plasmonic
absorption peak. Here we show that the deep saturation affects not only the excitation
wavelength, but also other wavelengths inside the plasmonic band, thus enabling alloptical switch of plasmonic scattering for the first time [26].