Lorentz Nanoplasmonics for Nonlinear Generation.

While past works have suggested that the Lorentz magnetic contribution to second harmonic generation from metal nanostructures is negligible as compared to other terms, here we demonstrate a dominant Lorentz contribution from T-shaped apertures in a gold film. The apertures are designed to have overlapping magnetic and electric near-field intensities at the plasmonic resonance. This gives 65% greater nonlinear generation from the Lorentz term than the sum of the other two terms. We demonstrate this effect experimentally by milling of nanoapertures of different size and orientation in a metal film and measuring their second harmonic generation. Good agreement is seen between the experiments and comprehensive calculations. In the development of highly efficient nonlinear metasurfaces, careful optimization of the Lorentz contribution should be considered in addition to all other contributions. Following the approach of this work, the Lorentz contribution may also be optimized for THz generation.

Characterization of Individual Magnetic Nanoparticles in Solution by Double Nanohole Optical Tweezers.

We study individual superparamagnetic Fe3O4 (magnetite) nanoparticles in solution using a double nanohole optical tweezer with magnetic force setup. By analysis of the trapping optical transmission signal (step size, autocorrelation, the root-mean-square signal, and the distribution with applied magnetic field), we are able to measure the refractive index, magnetic susceptibility, remanence and size of each trapped nanoparticle. The size distribution is found to agree well with scanning electron microscopy measurements, and the permeability, magnetic susceptibility and remanence values are all in agreement with published results. Our approach demonstrates the versatility of the optical tweezer with magnetic field setup to characterize nanoparticles in fluidic mixtures with potential for isolation of desired particles and pick-and-place functionality.