Circular dichroism (CD) spectra of isotropic plasmonic nanoparticles with no structural dissymmetry have been measured in solution. We analyzed CD spectra of dispersed nanoparticles, having specific sizes ranging from 5 to 150 nm in diameter, high-symmetry shapes including spheres, rods, prisms, and octahedra, and different types of metal including gold, silver, and copper. While helical assembles of nanoparticles typically produce strong CD responses, we found that individual nanoparticles exhibit weak ellipticity. The underlying mechanism of optical activity of single nanoparticles is attributed to spatial dispersion (non-locality). In crystals, optical activity is the first-order spatial dispersion of electromagnetic waves, which has long been forgotten since CD responses of plasmonic metal nanoparticles gained attention a decade ago. To study the non-local optical property of crystals, in general, one needs to consider a wavevector-dependent polarization along with transverse dielectric components, and longitudinal electric fields play a crucial role. The theory of optical activity of molecules is the quantum mechanical theory based on single electron dispersions, which the direct relationship between ellipticity sign and structural dissymmetry has been well-established. In contrast, nanoparticles are many-body systems, and thus optical activity of single nanoparticles may not be correlated with nanoparticles' morphologies under spatial dispersion. We discuss the non-locality-induced CD mechanism in details, including coupling of longitudinal-transverse fields, right/left-handedness of dipolar/quadrupolar localized surface plasmons (LSPs), sum rules for the rotational strength, and CD-enhancement by Fano resonances. Also, the effects of multiple scattering and electron spin polarization are discussed. Noticeably, the field enhancement effect via chiral ligands adsorbed on nanoparticle surfaces was not observed, likely due to a screened Coulomb interaction between conduction electrons of the metals and molecular electrons in the presence of the positive jellium background. This study suggests that a field gradient exists in a nanoparticle in the direction of light propagation.

Non-locality driven circular dichroism of isotropic metal nanoparticles

254th National Meeting (2017)

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