ChiralNanostructures Studied Using Polarization-DependentNOLES Imaging.

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]