Tung-Yu Su

Effect of Lorentz local field for optical second order nonlinear susceptibility in ZnO nanorod

Nonlinear optical properties of ZnOnanorods (NRs) are strongly influenced by its dimension and aspect ratio. Size-dependent second harmonic generation(SHG) in ZnO NRs has been investigated with polarized excitations recently. However, detailed description to the SHG dependency with NR dimensions has not yet been given. In this paper, the relationship between rod diameter/length and corresponding χ(2) values based on Lorentz local field is established, both theoretically and experimentally, for the first time. Theoretically,Lorentz local field induced spectral red shift and the consequent dielectric constant modification is used to elucidate the size effect for χ(2) under the condition that both excitation and SHG wavelengths are far from the band gap. Experimentally, χ(2) of ZnOnanorods with various sizes is measured via Maker fringe technique, and the results fit well to our theory.

Plasmon saturation induced super-resolution imaging

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.

Size and wavelength dependency of saturable scattering by a single gold nanosphere embedded in dielectric material

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.

Reverse saturable scattering of a single gold nanoparticle

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.

Super-resolution imaging of gold nanoparticles based on saturation of plasmonic light scattering

We observed the saturation of plasmonic light scattering on gold nanoparticles. Extracting the nonlinear signals due to the saturation, we demonstrated super-resolution imaging of two nearby gold nanoparticles.

Saturable scattering and its application to superresolution microscopy

Recently, several superresolution methods have been demonstrated to revolutionize the way we access the nano-world. However, these techniques are based on switching or saturation of fluorescence, and consequently are limited by switching reversibility and photobleaching. Here we show first superresolution far-field imaging based on scattering, which is a universal phenomenon without bleaching. Our principal finding is that scattering from plasmonics particles is saturable. By extracting the saturated part, sub-80-nm resolution is achieved. This work not only expands the horizon of superresolution imaging from fluorescence to scattering, but also points out the possibility of resolution enhancement for all imaging modalities that utilize plasmon properties.

Relationship of second order susceptibility to the dimensions of ZnO nanorods based on Lorentz local field

With the aid of Maker fringe technique, we have observed two nonlinear optical (NLO) phenomena separately on diameter and length of ZnO nanorod (NR). One is second harmonic generation (SHG) saturation in rod diameter, and the other is SHG enhancement in rod length. Besides that, the model based on Lorentz local field is proposed for the first time to elucidate the above phenomena. The deduced second order susceptibility χ(2) with various sizes of ZnO NR matches well to our theory, demonstrating that the size effect on χ(2) is governed by Lorentz local field. Our theory provides a theoretical basis to explain the mechanism of light-material interaction in nano-dimensions and is readily to be extended to other kind of semiconductor nanostructures when addressing NLO properties in them.

Saturable scattering of localized surface plasmon resonance in a single gold nanoparticle

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.

Dependence of third-harmonic generation on melanin concentration in solution

In this study, we performed theoretical analysis and experimental measurement of third harmonic generation (THG) in melanin solution with different concentrations. As predicted by theory, only THG at glass/solution interface was observed due to Guoy phase shift effect. We have shown that this interfacial THG intensity is strongly affected by index matching condition between the two media, leading to minimal THG at a certain melanin concentration. By fitting the dependence of THG intensity versus melanin concentration, linear and nonlinear electric susceptibilities of melanin are obtained, providing a valuable tool to characterize optical properties of biological molecules.

On the possible origin of bulk third harmonic generation in skin cells

We studied third harmonic generation (THG) of melanin solution with concentrations similar to melanocytes in human skin. In contrast to conventional observation of THG at interface, bulk THG was detected inside the solution due to the formation of melanin hydrocolloids. A linear relationship between melanin concentration and THG intensity was found, suggesting THG originated from high-order hyper-Rayleigh scattering. By fitting this linear relationship, third-order hyperpolarizability of melanin hydrocolloids was determined to be three orders larger than that of water. Our result will be useful for interpretation of THG signals in skin and other tissues containing colloidal particles.