| The classical theory of plasmonics envisions spherical nanoparticles obeying classical electrodynamics. Modern colloidal synthesis of noble metal nanoparticles, in tandem with emerging methods of nanoparticle assembly, transcends the assumptions of this theory. First, strongly nonspherical particles give rise to optical spectra with complicated orientation dependence. An interpolation method is introduced to connect electrodynamic simulation results, generally carried out at fixed orientations, with experimental optical spectra, such as those of randomly oriented ensembles. Second, the ability to manipulate and arrange multiple spherical particles in solution with optical binding demands efficient calculation of the optical forces giving rise to their preferred geometries. A coupled-dipole model is developed to allow for rapid optical force calculations that predict many of the phenomena seen in the laboratory. Third, the prospect of attaching semiconductor quantum dots to metal nanoparticles in the electromagnetic near-field raises new questions about how the quantum behavior of localized surface plasmons affects the nonlinear optical response of the coupled system. Investigating such questions yields several new predictions about the optical response of plasmon-exciton systems. Under ultrafast pulsed illumination, a reversal of a Fano resonance is predicted, turning a dip into a spike in the pulsed optical spectrum. When two quantum dots are coupled to the same metal nanoparticle, it is found that their individual couplings to a quantized plasmon can give rise to coherence between the quantum dots, in particular a state enriched in an antisymmetric dark excitation that can be prepared with pulsed laser illumination. These theoretical tools and predictions, in addition to providing basic insight into plasmonic systems, will serve to guide further developments in colloidal synthesis, nanoparticle assembly, and optical applications. |
