We study the presence of dark and bright modes in a planar metamaterial with a double rod unit cell introducing geometric asymmetry in rod lengths. The dark mode displays a Fano-type resonance with a sharp asymmetric profile, rendering it far more sensitive than the bright mode to slight variations of the dielectric environment. This peculiar feature may envisage the possible application of the asymmetric dimer metamaterial as an optical sensor for chemical or biological analysis, provided that the effect of material losses on the dark mode quality factor is properly taken into account.

We studied the waveguiding properties of a photonic quasicrystal based on an octagonal tiling. The structure exhibits intrinsic localisation in the band gap region, that can be exploited to manipulate the signal transmission through a linear defect. The electromagnetic characteristics are first numerically analysed using a full wave simulation and then experimentally verified by measurements carried out in the X-band microwave region. Possible photonic applications include tunable notch filters having large attenuation.

We study the sharp Fano-type resonance in a dimer metamaterial based on nanorods with different lengths. Breaking the length symmetry results in the excitation of a dark mode that weakly couples to the free space. Interference between the dark mode and the higher frequency bright mode gives rise to the peculiar asymmetric and sharp profile of the resonance. The steep dispersion and high sensitivity to slight variations of the dielectric environment of this resonance envisage the possible application of the asymmetric dimer metamaterial as an optical sensor for chemical or biological analysis.

Formation and development of the photonic band gap in two-dimensional 8-, 10-, and 12-fold symmetry quasicrystalline lattices of low-index contrast are reported. Finite-size structures made of dielectric cylindrical rods are studied and measured in the microwave region, and their properties are compared with a conventional hexagonal crystal. Band-gap characteristics are investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence are used to investigate the isotropic nature of the band gap.

We report on the growth mode of N,N'-bis (n-octyl)-dicyanoperylenediimide (PDI-8CN(2)) on sexithiophene (T6) thin films, studied with different structural, morphological and optical techniques. We aim to individuate the most favorable conditions for the realization of heterostructure devices. The crystalline quality was established by X-ray patterns and Atomic Force Microscopy (AFM) images, and found to be generally high. The anisotropic optical constants extracted from ellipsometry measurements shed light on the mean molecular orientation in the PDI-8CN(2) film. AFM images evidence two different growth modes: at T6 thickness less than 2 monolayers (ML), the growth of PDI-8CN(2) on T6 is favored with respect to SiO2, while, at higher thickness (2-6 ML), the situation is reversed. An optimum T6 underlayer thickness of approximately 1 ML provides the best quality of PDI-8CN(2) layer corresponding to the highest island dimension, the highest molecular order parameter, and the lowest roughness. Spectrum broadening was observed for extinction coefficient of PDI-8CN(2) in the heterostructures, as compared with a sole material film, and explained by two effects: increase in molecular disorder and formation of charge transfer complexes.

One-dimensional Thue-Morse (ThMo) lattices are examples of self-similar structures that exhibit bandgap phenomena. ThMo multilayers may also possess fractal photonic bandgaps that give rise to large omnidirectional reflectance and light-emission enhancement effects. Two-dimensional (2D) ThMo aperiodic quasicrystals possess interesting properties for photonic applications too. Here we demonstrate the experimental fabrication of large area 2D ThMo lattices into polymeric substrates at nanometre scale by electron beam lithography (EBL). Far field diffraction patterns of the experimental ThMo structures have been measured and compared with the calculated theoretical Fourier spectra. Scanning electron microscopy and far field diffraction are used to characterize the experimental structures.

The optical characterization of liquid crystals, in a wide spectral range, is becoming a very important technical task because of their expanding applications in displays, optical telecommunications and other advanced areas of science and engineering. One of the most versatile, sensitive, and well-established technique for the optical characterization of solid and liquid materials is spectroscopic ellipsometry. In this paper, an outline is presented on the use of ellipsometry for nematic liquid-crystal characterization: anisotropic refractive-indices measurements and their temperature dependence, anchoring energy, and tilt distribution inside cells will be discussed. The paper is an extended version of a previously published paper.

We describe an experimental procedure for accurate measurement of anchoring energy strength of liquid crystal cells. This technique is based on the possibility of gathering a large amount of very precise data about the linear optical response of the cell in different experimental conditions, using spectroscopic ellipsometry. Then, a careful data analysis exploiting data inversion method, supplemented by simulations from the elastic theory, is able to provide the searched information. The technique has been applied to vertical aligned nematic cells, chosen for its widespread use in the present display market. The results obtained in this particular case together with a thorough comparison with existing alternative techniques, suggest that our technique can be an optimum candidate for the industrial implementation of such measurement. A particular example is fully worked out, giving a result with a precision of 1.5% and an accuracy of 10%.

Multiple-beam holography has been widely used for the realization of photonic quasicrystals with high rotational symmetries not achievable by conventional periodic crystals. Accurate control of the properties of the interfering beams is necessary to provide photonic band-gap structures. Here we show, by finite difference time domain (FDTD) simulations of the transmission spectra of 8-fold quasiperiodic structures, how the geometric tiling of the structure affects the presence and properties of the photonic band-gap for low refractive index contrasts. Hence, we show an interesting approach to the fabrication of photonic quasicrystals based on the use of a programmable spatial light modulator encoding computer-generated holograms, that permits an accurate control of the writing pattern with almost no limitations in the pattern design. Using this single-beam technique we fabricated quasiperiodic structures with high rotational symmetries and different geometries of the tiling, demonstrating the great versatility of our technique.

Recently important efforts have been dedicated to the realization of a new kind of photonic crystals, known as photonic quasicrystals, in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. Here we show a novel approach to their fabrication based on the use of a programmable Spatial Light Modulator encoding Computer-Generated Holograms. Using this single beam technique we fabricated Penrose-tiled structures possessing rotational symmetry up to 23-fold, and a two-dimensional Thue-Morse structure, which is an aperiodic structure not achievable by multiple beam holography

We present the simulated distribution of the local director of a nematic liquid crystal inside cylindrical macropores under the influence of an electric field. The Frank free energy approach is used to describe the nematic behavior. The influence of both molecular anchoring strength and pore radius is investigated. The results of this analysis are applied for simulation of an electrically tunable microcavity based on porous silicon infiltrated with a liquid crystal. The Bruggeman approximation is used while calculating the effective refractive index of each layer in the porous silicon multilayer structure. The reflectivity spectrum of the latter is simulated using the transfer matrix approach. The electrical tuning range of a microcavity designed for near-infrared waves is found to vary from 10.5 up to 23 nm for weak and strong surface anchoring conditions, respectively.

In this paper, we present our results for the anisotropic refractive index measurements of commonly used liquid crystal (LC) materials (E7 and 5CB) in the whole visible-near infrared range. In order to achieve a high accuracy in the obtained values, we have employed a combination of two techniques, namely, Mueller matrix spectroscopic ellipsometry in transmission and half-leaky-guided-mode (HLGM). Measurements with the HLGM technique are usually performed at a single wavelength. In order to obtain a spectroscopic measurement based on the HLGM technique in a wide wavelength range, we modified our commercial VASE((R)) ellipsometer. In particular, we designed a sample holder by means of which measurements in guiding structures are also possible. Thus, we take advantage of the wide-spectrum light, emitted from a standard Xe lamp of the ellipsometer, also in the HLGM setup. The complementary and in-one-setup utilization of both techniques has allowed us to overcome most of the problems previously encountered in applying spectroscopic ellipsometry to the determination of the optical parameters of LC materials and eventually to achieve an accuracy in the obtained results of the order of 10(-4). We discuss how individual elements of the Mueller matrix can be used to this aim. Finally, we compare the dispersion curves for the refractive indices and the birefringence of E7 and 5CB with the data already existing in the literature. (c) 2007 American Institute of Physics.

Porous silicon (PS) based devices are nowadays under an intense and widespread investigation in the optoelectronic and sensing fields. Recently, the range of possible applications has been widened by the use of the liquid crystals (LCs), which can be infiltrated in the PS sponge-like structure. The large changes of the optical properties, as exhibited by LCs under the action of electrical or thermal fields, allow developing a new family of optical devices like tunable PS based multilayer mirrors, microcavities and optical filters. In this work, we have optically characterized the LC-PS heterogeneous composite as a guest-host system by means of the variable angle spectroscopic ellipsometry (VASE). A PS layer, 450nm thick, has been infiltrated with the nematic LC 5CB, and the main optical parameters, anisotropic refractive indices and thickness of both materials, have been estimated below and above the isotropic transition temperature, at 27.0 degrees C and 38.0 degrees C respectively. We have found a clear indication that the LC molecules tend to align parallel to the direction of the pore columns.

A commercial variable angle spectroscopic ellipsometer from J. A. Woollam Company is modified in order to make possible measurements in guiding structures. The half leaky guided mode technique is realised for liquid crystal optical dispersion measurements. Features and problems related to experimental data interpretation and error sources in the refractive index measurements are carefully analyzed, achieving a final accuracy of 0.0002. We present the measured optical dispersion curves in the wavelength range from 0.5 to 1.7 mu m for two widely studied liquid crystals, namely E7 and 5CB.

The design and fabrication of active and passive hybrid photonic devices, with tunable optical properties, based on liquid crystals require a very accurate knowledge of their anisotropic refractive indices up to 100 ppm in the wavelength region of interest. At this aim, the authors have integrated two standard optical techniques commonly used in liquid crystals characterization, the variable angle spectroscopic ellipsometry and the half leaky guided mode spectroscopy, exploiting their best performances and overcoming their limits. The dispersion curves of nematic liquid crystal E7 have been estimated in the 450-1700 nm wavelength interval with both precision and accuracy of 10(-4). (c) 2006 American Institute of Physics.

We have studied the electro-optical and angular behavior of holographic-polymer-dispersed liquid crystal gratings at different wavelengths, in the visible and in the near-infrared range. As usual in these kinds of materials, a strong polarization dependent behavior was observed. Our samples showed very high diffraction efficiency for p-polarized radiation at 1.55 m, which is very interesting for many possible applications in the telecom field. However, we also observed a very unusual behavior for visible p-polarized light and we try to suggest some explanation for that. By analyzing the angular dependence of the diffraction efficiency, we could measure the components of the permittivity modulation tensor and infer important information about the main parameters involved in the grating structure: the degree of phase separation and the anisotropy in the liquid crystal droplet distribution. In our opinion, this simple and nondestructive methodology can be very useful for studying these kinds of materials and getting information on their morphology, in view of optimizing their properties. Finally, we discuss the role of the refractive index optical dispersion in order to describe the behavior of these materials at different wavelengths. These remarks are especially important when properties in the infrared range are extrapolated from measurements in the visible.

Cholesteric liquid crystals (CLCs) are one dimensional photonic band-gap materials. Due to distributed feedback, low threshold mirrorless lasing can occur in dyedoped and pure CLCs. In order to optimize the lasing conditions, we have studied the dependence of the lasing threshold on dye concentration and sample thickness. In particular, we have studied dye concentrations in the range of 0.1-3.0 wt%, and cell thicknesses are in the range of 5-50 ?m. We have found that the system has a shallow lasing threshold minimum and can operate efficiently in the range of dye concentrations of 0.3-2.4wt% and sample thicknesses of 10-50 ?m. We discuss the physical processes responsible for the observed behaviour.