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Complex micro- and nano-structured materials for photonic applications are designed and fabricated using top technologies. A completely different approach to engineering systems at the sub-micron-scale consists in recognizing the nanostructures and morphologies that nature has optimized during life’s history on earth. In fact, biological organisms could exhibit ordered geometries and complex photonic structures which often overcome the products of the best available fabrication technologies [1]. An example is given by diatoms, microalgae with a peculiar cell wall consisting of amorphous hydrated silica valves reciprocally interconnected in a structure called the frustule. Valve surfaces exhibit specie-specific patterns of regular arrays of chambers, called areolae, developed into the frustule depth. Areolae range in diameter from few hundreds of nanometers up to few microns, and can be circular, polygonal or elongate [2]. The formation of these patterns can be modeled by self-organised, genetically controlled processes. Despite of the high level of knowledge on the genesis and morphology of diatom frustules, their functions are not completely understood [2]. In this work, we show that the silica valves of marine diatoms, characterized by a photonic crystal-like structure, have surprising optical properties, being capable of filtering and focalizing light, as well as exhibiting optical sensing capabilities.

Digital holographic microscopy (DHM) has been successfully applied for the first time to characterize the radiative out-of-plane emission properties of a superdirective device. Complementarily to near-field microscopy, DHM allows us to reconstruct the beam in the far-field region. The angular dispersion of the light beam radiated from a grating composed of air and anti-air metamaterial has been determined, and the proposed technique has highlighted a collimation degree higher than 0.04 degrees, as already evaluated in a previous work. Further considerations on the retrieved phase map of the beam in the acquisition plane are presented.

We investigate from a theoretical point of view the photonic properties of a two dimensional photonic aperiodic crystal. These structures are obtained by removing the lattice points from a square arrangement, following the inflation rules emerging from the Thue-Morse sequence. The photonic bandgap analysis is performed by means of the density of states calculation. The mechanism of bandgap formation is investigated adopting the single scattering model, and the Mie scattering. The electromagnetic field distribution can be represented as quasi-localized states. Finally, a generalized method to obtain aperiodic photonic structures has been proposed.

Light passing through a photonic crystal can undergo a negative or a positive refraction. The two refraction states can be functions of the contrast index, the incident angle and the slab thickness. By suitably using these properties it is possible to realize very simple and very efficient optical components to route the light. As example we present two devices: a passive device acting as a polarizing beam splitter and a tunable switch. In the first device TM polarization is refracted in positive direction whereas TE component is negatively refracted, in the second device the light is positively refracted at room temperature and negatively refracted varying the local temperature of the device.

We show that the refracted wave at the exit surface of a Photonic Crystal (PhC) slab is periodically modulated, in positive or in negative direction, changing the slab thickness. In spite of an always increasing literature, the effect of the thickness in negative refraction on PhC's does not seem to be appropriately considered. However such an effect is not surprising if interpreted with the help of Dynamical Diffraction Theory (DDT), which is generally applied in the x-ray diffraction. The thickness dependence is a direct result of the so- called Pendellsung phenomenon. That explains the periodic exchange, inside the crystal, of the energy among direct beam ( or positively refracted) and diffracted beam ( or negatively refracted). The Pendellsung phenomenon is an outstanding example of the application of the DDT as a powerful and simple tool for the analysis of s electromagnetic interaction in PhC's.

Two different original theoretical approach for the analysis of vapour sensors based on a porous silicon optical microcavity are presented. The devices under analysis are based on a cavity with a high porosity layer of optical thickness lambdaB/2, where lambdaB is the Bragg resonant wavelength. This is enclosed between two distributed Bragg reflectors with seven periods made of alternate low and high porosity layers. When such a porous silicon microcavity is exposed to chemical vapours, a marked red-shift of its resonant peak, ascribed to capillary condensation of vapour in the pores, is observed. According to the first approach, the features of porous silicon microcavities are analyzed looking at the correspondent band structure. In particular, the microcavity structure is viewed as a 1-D photonic crystal with a defect of optical thickness lambdaB/2 giving rise to a narrow resonant transmittance peak at lambdaB in a wide transmittivity stop-band. We then compare the derivation of the band structure with an original approach based on the dynamical diffraction theory, the same widely used in x-ray diffraction. Using this approach we get an analytical expression of the reflectivity, giving the position but also the shape of the resonant peak.

X-ray diffraction spectra at different x-ray energies from a Si crystal subjected to a deformation produced by surface acoustic wave propagation have been modeled using the general framework of dynamical diffraction theory. The simulations have been successfully compared with the corresponding experimental results confirming the accuracy of the elastic model describing the acoustic wave fields inside the crystal.

Photonic band gap Crystals (PhC) are usually analyzed using the analogy between photon propagation in artificial periodic structures and electron wave propagation in real crystals. The forbidden band of photons is regarded as equivalent to the energy gap that electrons experience in crystals because of the periodic potential.On the other hand, electron propagation and electromagnetic wave diffraction in periodic solids, respectively developed into band-theory and Dynamical Diffraction Theory (DDT), are formally identical. It appears therefore natural to perform an analysis of the features of an electromagnetic phenomenon, as the PBG, in analogy to the most direct antecedent electromagnetic theory, the DDT, that historically has also represented the direct reference for the derivation of the band-theory of electrons.In this communication, we introduce an analysis of the features of PhCs in analogy with the DDT, underlining the differences between DDT classical application to the x-ray diffraction from real crystals and that from artificial crystals at optical wavelengths. In particular, the high contrast of material refractive indices in PhC makes inapplicable some approximations generally used in x-ray diffraction analysis. Moreover, we discuss in which cases DDT has to be generalized in order to overcome such limitations.

Multi-layer structures, such as Bragg reflectors, rugate filters, and optical microcavities are widely used in optical sensing. They are characterised by a periodical modulation of the refractive index so that they can be classified as 1-D photonic crystals. In this communication, the optical features of such a class of sensors are analyzed from the band structure point of view. This general approach is then applied to the case of vapour sensors based on a porous silicon microcavity. A numerical analysis of the photonic bands, when the porous microcavity is exposed at chemical vapours, is presented and discussed for design optimisation purposes. In particular, we investigate how the photonic band gap changes when a volatile substance condensates in the silicon pores inducing a variation of the refractive indices of the layers forming the microcavity. Results are also compared with those obtained by the usual optical transfer matrix method.

The main limitation of Laue geometry for the achievement of a small focus size is the focus broadening caused by the intrinsic Darwin width and the spread of the beam in the Borrmann triangle, resulting from propagation inside the crystal. A method, based on dynamical focusing, is suggested that allows improvement of the quality of high-energy polychromatic focusing by bent crystals in Laue geometry.