Internal electromagnetic waves, energy trapping, and energy release in simple time-domain simulations of single particle scattering.

Abstract

During the decay phase of the interaction of a femto-second Gaussian pulse with a single spherical particle, the presence of quasi-periodic short oscillatory bursts of electromagnetic energy at points in the near field outside the particle have been observed in three-dimensional simulations. Analogous behavior can be very easy produced and understood in simple one-dimensional scattering calculations. In two dimensions the situation immediately becomes more complicated and interesting. Here we discuss results from two-dimensional pseudo-spectral time-domain simulations of scattering from circular, elliptical, and hexagonal particles with the real index of refraction m = 1.3. Our focus is on how energy initially trapped within a particle after interaction with an incident Gaussian pulse is released over time, and we show two kinds of events that can result in “bursts” of energy release from the particles: (i) the coalescence of counter-propagating wave-packet-like electromagnetic field structures that have maximum amplitude near the surface of the particle, and (ii) encounters of individual packets with surface regions of high curvature. The coalescence events in the circular case show the dynamical origin of a two-dimensional form of “photonic nanojet.” The two-dimensional simulations make clear the reason for quasi-periodic intermittent bursts at fixed near-field points outside the particle. Examination of field evolution shows that distinct near-surface internal field maxima, ostensibly the “source” of the emission bursts, are in fact inter-connected by caustic-like internal field structures that extend throughout the particle and have complex time evolution. The revealed intricacy of these connections suggests that understanding the origins of pulsed emissions in three dimensions, even for simple particle geometries, may be quite challenging.