Second-harmonic chiroptical scattering spectroscopy from plasmonic nanohelices

Chiral harmonic scattering has emerged as a powerful nonlinear optical tool for characterizing chiral nanostructures in solution, yet its applicability has been limited by narrow spectral coverage and a lack of comparative insight with linear chiroptical effects. Here, we demonstrate broadband second-harmonic (SH) chiroptical scattering using gold and silver plasmonic nanohelices, spanning a 150 nm fundamental wavelength range (710-860 nm). By resolving nonlinear ellipticity spectra at ten discrete wavelengths, we uncover spectral trends that differ between materials. Notably, we observe correspondences between the circular dichroism (CD) spectra in the linear optical regime and SH ellipticity spectra. This correspondence is consistent with an intuitive, minimal model of hyper-Rayleigh scattering. Furthermore, we compare right-angled and forward scattering geometries, revealing a geometry-dependent contrast again consistent with the hyper-Rayleigh scattering regime in both materials. This work establishes the feasibility of spectrally-resolved nonlinear chiroptical analysis and provides insights into the physical processes linking linear and nonlinear optical activity in plasmonic nanostructures.