Nonlinear plasmonic scattering and resolution enhancement with gold nanoparticles.
(A) Left panel is PSF of a single particle scattering in x-z plane, schematically representing saturation at the central region while the periphery is unsaturated. The right panel demonstrates that, with a temporally modulated excitation, higher harmonic components in frequency domain are generated at the central region of the scattering profile, while the scattering signals from the periphery follow the modulation frequency linearly. (B) Resolution enhancement by extracting different modulation harmonic components of the scattering signal. Reprinted with permission from ref . (B1) is taken with fundamental modulation frequency 1fm, corresponding to the conventional confocal image. A white arrow indicates two indistinguishable particles. (B2) depicts the image reconstructed from 2fm component of the same scattering signal, where the resolution is improved, and the two particles can be resolved. (B3) depicts the image reconstructed from the 3fm component, and the two nearby nanoparticles are clearly resolved. (B4) The detailed line profiles of a single particle in the scattering images show the resolution enhancement. (C) PSF of scattering from isolated gold nanoparticles with increasing excitation intensity. Reprinted with permission from ref . In (C1–C3), the left panel of each figure shows back-scattering images and the right panel shows the signal proﬁle of a single selected nanoparticle along a white dashed line. (C1) When the excitation intensity is low, the PSF profile ﬁts Gaussian distribution well. (C2) As the intensity increases to 5×105 W/cm2, the scattering signals become saturated, manifested by the valley in the center of the profile. (C3) When the intensity is higher than 106 W/cm2, the scattering signal at the center of PSF increases again, showing the reverse saturated scattering. The FWHM of this PSF is much smaller than the unsaturated one, showing great potential for high-resolution microscopy. Scale bar: 500 nm. (C4) is scattering signal (in red circles) variation against excitation intensity. The green line is a theoretical fitting based on a polynomial equation, and the blue dashed line shows the linear trend.