Recently, we discovered, for the first time, reverse saturable scattering in a single gold nanoparticle. When incident intensity increases, the scattering intensity dependence of 80-nm gold nanoparticles evolves from linear, to saturation, and to reverse saturation sequentially. The intensity dependence in reverse saturable scattering region is significantly steeper than that in the linear region. With the aid of a confocal microscope, the full width half maximum of the single-particle point spread function can be reduced down to 80 nm, which is beyond the diffraction limit. Our finding shows great potential for superresolution imaging application without bleaching.
The wavelength and size dependencies of nonlinear scattering by a single gold nanosphere immersed in oil are presented. We show that the wavelength dependency fits well with the scattering spectrum by Mie solution, reflecting that the nonlinear scattering is dominated by the field enhancement from plasmonic effects. The tendency for different sizes is consistent with the results of degenerate four-wave mixing in the literature, showing that the saturation behavior is governed by the Kerr nonlinearity resonantly enhanced via intraband transition. Thus we conclude that the saturable scattering in our case is attributed to intraband χ(3), with nonlinear behavior enhanced by LSPR.
Many biological systems are composed of chiral molecules and their functions depend strongly on their chirality. For example, most amino acids are of left-handed chirality while most polysaccharides are of right-handed chirality. Both of them are vital for human life, so it is important to perform chiral detection inside bio-tissues. Here we demonstrated second harmonic generation circular dichroism (SHG-CD) as a novel chiral imaging contrast in thick biotissue. Compared with conventional chiral detection, SHG-CD provides at least three orders higher contrast. In addition, due to the nonlinear nature of SHG, this technique provides optical sectioning capability, so the axial contrast is much better. The advantages of nonlinear optical microscopy are optical sectioning and deep penetration capabilities. The SHG-CD achieved 100% signal contrast with sub-micrometer spatial resolution. This method is expected to offer a novel contrast mechanism of imaging chirality inside complex bio-tissues.
Conventionally, super-resolution imaging is achieved by manipulating the on/off switching of fluorophores, or by saturation of fluorescence emission. To prevent the photobleaching of fluorophores, we demonstrate novel superresolution imaging based on saturation of scattering from plasmonic particles, for the first time. With spectral studies, we have confirmed the saturation is directly linked to surface plasmon resonance effect. With the aid of saturation excitation microscopy, plus field concentration due to nonlinear plasmon resonance, we have achieved optical resolution below 80-nm based on scattering. Our study will open up a completely new paradigm for super-resolution microscopy.
In this work, we investigated, both theoretically and experimentally, the saturable scattering in a single gold
nanoparticle for the first time. In theoretical part, we used different models of the nonlinear properties to explain the
nonlinear responses in gold material. In experimental part, multi-color confocal microscopy was used to observe the
scattering of a single gold nanoparticle. As a result, by a resonant excitation, saturable scattering was observed with
moderate excitation intensity (~107 W/cm2); with even higher excitation intensity (>109 W/cm2), reverse saturable scattering was observed, indicating the existence of higher order nonlinear properties. To completely comprehend the
mechanism of this saturable scattering, we applied three kinds of excitation wavelengths (405nm, 532nm and 671nm) and
four kinds of gold nanoparticle with different diameters (40nm, 50nm, 80nm and 100nm) to demonstrate the wavelength
dependence and size dependence. Since the scattering of gold nanoparticles is significantly enhanced by localized surface
plasmon resonance, we compared these dependencies with the spectral properties induced by LSPR and found that they
match the spectra, revealing that the saturation is dominated by plasmon resonance. Besides, by fitting the dependencies,
linear and nonlinear hyperpolarizability of a single gold nanoparticle were also deduced.