Optical Properties of Nanoparticle Arrays and their Application to Control Light

Metal nanoparticles offer a potential means of fabricating subwavlength optical elements with tunable birefringence. Combining genetic algorithms with a finite-difference time-domain solution of the Maxwell equations, Seideman and coworkers demonstrated an approach to numerical design of plasmonic devices with predetermined optical properties and hence desired functionalities.

Panel A in the figure below exemplifies an extensive study of silver L-shaped nanoparticles and their optical properties, carried out in collaboration with the groups of R. Van Duyne (IRG 3) and K. Spears (Chemistry, Northwestern University). It illustrates the intensity distribution in the plane of the particle (the xy-plane) for four different plasmon resonances, each of which has a different physical origin: (I) a volume plasmon resonance, (II) a dipole point resonance, (III) and (IV) high- and low-energy surface resonance bands. Panel B shows simulations of the intensity enhancement by a metallic tip, carried out in the framework of collaborative studies with an experimental group at the University of Pittsburg. A gold tip with an apex of 20 nm diameter is placed 40 nm away from a gold surface and is excited by a plane wave whose k-vector points perpendicular to the figure plane. Shown is the steady-state electromagnetic intensity enhancement distribution in logarithmic scale. Panel C illustrates a 25 parameter genetic algorithm optimization of a metallic slab to design a nanolens that focuses an electromagnetic plane wave to a pre-specified point (shown as a star on the output side). Notably, the goal was to focus the light to an arbitrarily chosen point, remote from the surface of the particle. The optimal lens is strongly asymmetric with respect to the x=0 plane and focuses the light by coupling surface plasmon waves on the input and output sides of the construct. Panel D illustrates a 3D genetic algorithm optimization of an array of asymmetric nanoparticles whose aim is to convert a linearly polarized plane wave into an elliptically polarized source with maximal y-component. The main frame shows the converged construct. The inset illustrates the convergence pattern of the algorithm to the construct shown in the main frame, in which the intensity ratio W_y/W_x increases from 0 to 53%.

(A) Collaborative research with IRG 3 experimental groups explores the birefringence of L-shaped particles and reports the first observation of volume plasmons.
(B) Numerical design of a tip for SERS.
(C) Design of a metallic lens with predetermined properties via multi-parameter genetic algorithm optimization.
(D) Design of a nanoscale source with predetermined polarization properties via 3D genetic algorithm optimization.