A. Mandatori

Negative refraction and sub-wavelength focusing in the visible range using transparent metallo-dielectric stacks

We numerically demonstrate negative refraction of the Poynting vector and sub-wavelength focusing in the visible part of the spectrum using a transparent multilayer, metallo-dielectric photonic band gap structure. Our results reveal that in the wavelength regime of interest evanescent waves are not transmitted by the structure, and that the main underlying physical mechanisms for sub-wavelength focusing are resonance tunneling, field localization, and propagation effects. These structures offer several advantages: tunability and high transmittance (50% or better) across the visible and near IR ranges; large object-image distances, with image planes located beyond the range where the evanescent waves have decayed. From a practical point of view, our findings point to a simpler way to fabricate a material that exhibits negative refraction and maintains high transparency across a broad wavelength range. Transparent metallo-dielectric stacks also provide an opportunity to expand the exploration of wave propagation phenomena in metals, both in the linear and nonlinear regimes.

Birefringence in one-dimensional finite photonic bandgap structure

The spectral and dispersive behaviors of anisotropic layered structures forming a one-dimensional polarization-dependent photonic bandgap are discussed. The finite dimension of the structure is taken into account. Interesting field-localization properties are found when the optical axes of layers are not aligned each with each other, i.e., principal axes of layers are rotated with respect to each other. The field-localization behavior is also discussed through a suitable definition of density of modes for the anisotropic layered structure. A discussion about the behavior of the dispersion law for such a finite periodical structure is also presented.

Asymmetric transmission of some two-dimensional photonic crystals

Some particular geometries are discussed in which the transmission spectrum is different for light traveling from right to left or from left to right. The conditions that allow this transmission asymmetry are discussed.

Application of ray-path geometry to the design of an optical cloaking structure

Cloaking structures and invisibility-devoted media are receiving much attention. Although the main efforts are devoted to the realization of 3D structures, still many interesting features can arise from the development of monodirectional 2D cloaking structures. We discuss a structure designed by using a numerical method based on geometrical optics that is able to hide any object smaller than the cloaking envelope but much bigger than the wavelength of the electromagnetic field, using normal materials. Some numerical examples are presented.

Analytical demonstration of omnidirectional transmission enhancement in dispersive birefringent photonic-bandgap structures

We have analytically studied uniaxial, birefringent, one-dimensional multilayer structures and found that in such structures birefringence leads to an interesting phenomenon of omnidirectional transmission. In particular, we have demonstrated analytically that the bigger the birefringence in the layers is, the better the omnidirectional transmission is. We demonstrated this property for all kinds of multilayers with absorption and dispersion.

Analysis of negative refraction from anomalous phase in transmission spectrum

The dependence of the negative refraction of a simple Fabry–Perot system as a function of the sign of material permittivity and permeability is analyzed. It is shown that negative refraction is possible every time the transmission phase of the system shows an anomalous behavior. However if the permittivity or the permeability are complex the anomalous phase is no longer univocally related to the presence of a negative refractive index.

A way to super resolution by a sampling method using the spectral properties of one-dimensional multilayer systems

A way to perform sampling of the evanescent spectrum of an object is considered by using a photonic bandgap (PBG). The coupling between the scattered field from the object and the PBG is discussed, showing a connection of the guide modes with selected spectral components of the scattering object in free space. Some useful examples have been discussed, showing good agreement between numerical results and theoretical previsions.

Considerations on cloaking in geometrical optics

It is shown that by properly using the geometrical optics approximation it is possible to design particular optical structures able to shape an optical beam in some wanted way. We discuss the application to hide to an incident plane wave, objects contained in a finite space region.

Coherence effects in propagation through one-dimensional photonic bandgap structures with a rough glass interface

The effect of the coherence of a beam traveling through a photonic 1D structure coupled with a rough glass is studied. The analysis is made for the case of spatial coherence showing the possibility to determine the coherence characteristics of the beam by an examination of the output field. We have shown that starting from a simple plane wave with Gaussian partial spatial coherence, we still obtain a partial coherence process on the output of the system, which, however, is no more Gaussian. The output process is no more a stationary Gaussian process. The output field correlation now depends not only on the distance between two points, but also on their position.

The Effect of Coherence in the Propagation through Periodic Structures

The effects of the coherence properties of light beams propagating through periodic structures are discussed. In particular, spatial coherence may influence the transmitted field. Some 2D structures which give transmittances strongly influenced by the coherence of the incoming field are discussed