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.
Photonic quasicrystals (PQCs) have neither true periodicity nor translational symmetry, however they can exhibit
symmetries that are not achievable by conventional periodic structures. The arbitrarily high rotational symmetry of these
materials can be practically exploited to manufacture isotropic band gap materials, which are perfectly suitable for
hosting waveguides or cavities. In this work, formation and development of the photonic bandgap (PBG) in twodimensional
8-, 10- and 12-fold symmetry quasicrystalline lattices of low dielectric contrast (0.4-0.6) were measured in
the microwave region and compared with the PBG properties of a conventional hexagonal crystal. Band-gap properties
were also investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of
incidence from 0° to 30° were used in order to investigate the isotropic nature of the band-gap.
We present a survey on organic composite and hybrid materials suitable for a new family of optical devices like thermo-
and electro-tunable multilayer mirrors, microcavities and optical filters. The main component is a tunable and/or
switchable Bragg grating. Two different classes of materials have been realized, characterized, and designed in form of
prototype devices, namely a) polymer-liquid crystal composite and b) liquid crystal infiltrated porous silicon. Different
geometries and patterns can be envisaged, leading to different physical properties and a large number of device
potentialities, most of them yet unexplored.
Obtained results look promising for both material families, especially for the porous silicon one, which includes an
inherent immediate integration with the electronic fabric technology.
A new method of light propagation control in planar waveguides filled by nematic liquid crystals is reported. Permanent deviation has been achieved by light induced reorientation of a selected area of dye-doped liquid crystal cells. Reversible deviation has been achieved by inducing Bragg reflection on the excited mode of the waveguide. The Bragg diffraction grating is produced by the liquid crystal itself, using short-pitch interleaved electrodes.
In this paper an integrated electro-optical switch, designed to operate at the infrared communications wavelength of 1550 nm, is presented. The device is based on a Bragg grating in planar waveguide with a bistable smectic C* liquid crystal overlayer, which allows to change the spectral behavior of the device. The principal advantage of this device is its spectral signature, which allows overcoming the problems of intensity dependent switches.
Recently an ever-increasing activity in the area of coupling between optical waveguide and ring or micro resonator has been developed. Devices based on this coupling held the promise of a new modality of light switching, amplifications and modulation. In this paper, the feasibility of an all optical switch based on the integration of the potentiality of microcavity resonator and organic materials having large nonlinearities, i.e. liquid crystals (LC's), is discussed. The device is based on silicon technology with hybrid integration of liquid crystals as a nonlinear material.
Integrated optical devices based on liquid crystal's (LC) features have been generally discarded for a long period mainly due to very high scattering losses. However, in the 90's this situation is rapidly changing. A better understanding of the physical phenomena underlying the observed effects, the use of new materials with improved performances, the discovery of new electro-optical and nonlinear optical effects make the applications of LC in integrated optics more and more attractive. We present here some recently obtained results in this field. In particular, we have designed and realized an integrated device in a three-stage planar waveguide, having as middle stage a nematic liquid crystal (NLC) film. We studied the device performance in different geometries using TE polarized light. By a proper choice of the material parameters we measured time responses in the microsecond rage. Our experimental results confirm the possibility of employing such a device working as an optical switch and/or beam deflector.
A guided light wave can be modulated by coupling it into a ferroelectric liquid crystal (FLC) waveguide. The FLC, which changes its optical properties under the application of an electric field, acts as active medium. With a carefully chosen aligned configuration, the large change in refractive index induced by the electric field, gives a good contrast ratio in the optical response. The optical and electro- optical properties of liquid crystals can be modified by the propagating light and these effects can be enhanced by the addition of a suitable absorbing dye in the liquid crystal. Thus changes in the guiding conditions of the light can be induced also by the guided light itself.
The integrated optical devices represent the most innovative result of the research in the photonics field. New horizons can be open by introduction of new materials and advanced techniques. The applications depend on the properties of the considered materials and their response times to external applied field. In this article we present an integrated device in a three-stage planar waveguide, having as middle stage a nematic liquid crystal film. We studied the device performance with TE polarized input light. By a proper choice of the material parameters and of the driving voltages we got time response in the microsecond range. Our experimental results confirm the possibility of employing such a device working as optical switch and/or beam deflector. A further analysis is in progress in order to explain an anomalous electro-optical behavior occurred for higher applied voltages.
We present a theoretical and numerical model of light beam propagation in anisotropic and inhomogeneous dielectric structures, obtained as an extension of the scalar FFT-BPM. We solve Maxwell's equations in the Generalized Geometric Optics Approximation (GGOA), which neglects the reflected fields. This is a full-vectorial model because it takes into account the polarization effects due both to the anisotropy and to inhomogeneity of the medium. We have implemented a first version of a numerical code exploiting this method. Some preliminary tests are presented.
An integrated electro-optical switch based on a planar nematic liquid crystal waveguide has been realized and tested. To achieve this goal, we fabricated and measured a number of step-index waveguides, using RF sputtering and sol-gel deposition techniques. A summary of their characteristics is presented. The design of electro-optical switch is reported, together with the analysis of the problems met in order to optimize the manufacturing process. Finally, we present the experimental results obtained with a three-stages device, having as middle stage a thin nematic LC film and two glass waveguides as other stages. The electro-optical behavior and the response times have been studied for different configurations. The experimental findings show that an additional bias voltage can improve both the transmittivity and the response time of the device, leading to very promising results in the frame of new integrated electro-optical switches.
The design procedure is described of integrated optical switches to be fabricated by inserting a liquid-ciystal cell into a glass planar waveguide. Two configurations are analysed, corresponding to ON-OFF and OFF-ON operation, respectively. Preliminary experimental data on the waveguide characteristics and device fabrication process are also presented.
The problem of propagation of guided light in hybrid integrated planar waveguides is afforded in terms of Local Normal Modes. The approach is then generalized taking into account reflected modes arising when index discontinuity between adjacent guiding steps is significant. This is crucial for liquid crystal based waveguide devices. Transmission features in homogeneous as well as inhomogeneous and anisotropic waveguides have been considered in numerical computations. One of the worked examples can well explain previously reported experimental results.
A novel polymeric material a poly (methylmetacrylate) (PMMA) with 7 by wt. of poly (ethylenecovinylacetate) (EVA) blend is presented. The PMMAIEVA thermograph in the temperature range -50 to 220 C exhibits a phase transition at 50 C. During this transition the typical hysteresis of the polymeric transitions has been noted. We show the behaviour of the optical transmission as a function of the temperature and present some physical properties of this material. In the frame of a research on the preparation and characterization of high mechanical performances PMMA-based blends we conducted several optical tests in order to verify the potential applications of such blends in devices of integrated optics. Tough transparent PMMA is industrially produced by a multistage process of incorporation of rubbery particles in a methacrylic matrix. The rubbers are obtained by a suspension process in which a shell of cross-linked rubbery buthyl acrylate-styrene copolymer is polymerized around a PMMA matrix. The core-shell structure (fig. 1) is necessary in order to assume a good stress transfer between the phases and at the same time a better matching of the refractive indices of the two phases. To achieve the desired level of toughening high amounts (20-30 by wt) of rubber are necessary with an unavoidable diminution of elastic modules. In our procedure an amorphous ethylene-co-vinylacetate copolymer (EVA 20 by wt of VA) is used as rubbery component 1 The synthetic process
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