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The use of plasmon amplification of nonlinear optical wave-mixing signals to generate optical images in which the position of the scattering point source can be determined with nanometer accuracy is described. Solid gold nanosphere dimers were used as a model system for the nonlinear medium, which converted the Ti:sapphire fundamental to its second harmonic frequency. Matching the fundamental wave energy to the localized surface plasmon resonance of the electromagnetically coupled nanospheres was critical for achieving the high localization accuracy. Our technique, named Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging, routinely yielded nonlinear optical images with 1-nm localization accuracy at rates ≥2 fps and can also be used as a photo-switching localization contrast method. This high level of accuracy in pinpointing the signal point source position exceeded that made possible using conventional diffraction-limited far-field methods by 160×. The NOLES technique, with its high temporal resolution and spatial accuracy that far surpass the performance typical of fluorescence-based imaging, will be relevant for imaging dynamic chemical, biological, and material environments. [ABSTRACT FROM AUTHOR]

TheNonlinear Optical Localization using Electromagnetic Surfacefields (NOLES) imaging technique was used to generate optical imagesin which the position of a chiral object could be determined withnanometer precision. Asymmetric gold bowtie nanostructures were usedas a model system with 2D chirality. The bowties functioned as a chiralnonlinear medium that converted the fundamental of a Ti:sapphire laserto its second harmonic frequency. The bowties consisted of two lithographicallyprepared equilateral triangles (base = 75 nm, height = 85 nm, thickness= 25 nm) separated by a 20 nm gap. Asymmetric bowties were formedby lateral displacement of one triangle by 10 nm, yielding C2point group symmetry. The chirality of thebowtie nanostructures was confirmed via nonzero second-harmonic generationcircular dichroism (SHG-CDR) ratios, which came from single-particleSHG measurements. The SHG-CDR ratios were validated using numericalfinite difference time domain simulations that quantified the relativemagnitudes of gap-localized electromagnetic fields at the harmonicfrequency resulting from excitation by left and right circularly (LCPand RCP) and linearly polarized fundamental waves. The relative electricdipolar and magnetic dipolar contributions to the SHG responses weredetermined using single-particle continuous polarization variation(CPV) SHG measurements. The spatial localization precision obtainablefor individual chiral nanostructures was determined by statisticalanalysis of the SHG image point spread function. Our results demonstratedthat both the chiral image contrast, which resulted from LCP and RCPexcitation, and the corresponding localization precision was dependentupon the relative magnetic dipole/electric dipole ratio (G/F). A localization precision of 1.13 ± 0.13nm and left-to-right image enhancements of 400% were obtained forbowties with the highest G/Fratiosusing 5 s frame exposure times. The polarization dependence and magneticdipole amplification confirmed here demonstrate that the NOLES imagingtechnique is a powerful method for studying chiral specimens withhigh spatial precision. [ABSTRACT FROM AUTHOR]

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (<100) of these C 8v heterodimers (ratio of radii R 1/R 2 = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures. [ABSTRACT FROM AUTHOR]