Electromagnetic Nanophotonics: Superlens Imaging of Dipolar Emittersand Cloaking in Weak Scattering

Two novel topics of nanophotonics, near-field imaging by superlenses and invisibility cloaking with slab scatterers, are investigated within the context of classical electromagnetic theory. In superlens imaging the objects are radiating point dipoles or externally excited dipolar emitters. The imaging element is a metallic or a slightly lossy negative-index material (NIM) slab with thickness of a few tens of nanometers. The electromagnetic angular spectrum representation is used to derive the Green tensors for the slab’s transmission and reflection. With this formalism the point-spread function of the imaging system is numerically evaluated, which enables one to assess resolution and image brightness. The dependence of image quality on the system parameters, dipole orientations, and near-field interactions among the objects and the lens is investigated. It is shown that both metallic and lossy metamaterial superlenses allow for image definitions beyond the usual diffraction limit of half the wavelength λ. High image quality requires a low-absorption slab and a good impedance match of the lens and its surroundings. In the immediate vicinity of the slab the dipole-slab interaction prevents the dipole from radiating. With low-loss NIM the interaction is weak and of short range. For silver slabs the interactionis stronger and reaches over the near-field zone, adversely influencing the imaging capabilities. With two dipole-like objects the emission is also suppressed by dipole-dipole near-field interactions, in particular with molecular objects while the effect is weak for glass or metallic nanoparticles. Due to interference subwavelength definition can only be attained for dipolesaligned predominantly orthogonal to the slab. Such a situation is achieved with excitation by total internal reflection. In optimal circumstances, resolutions of about λ/5 for silver and λ/10 for metamaterial lens are reached in three-dimensional configurations. Invisibility cloaking is considered within weak optical scattering in slab geometry. The conditions for cloaking in forward and backward directions are established, enabling the determination of the cloak’s refractive-index distribution for stratified objects. For any absorbing object forward cloaking is achieved with a lossy NIM or gainy ordinary-material slab. The cloaking is perfect for incident fields of any spatial structure and bandwidth. Backward cloaking is found possible with self-imaging fields. In both cases the cloak’s dispersive properties resemble those of the object.