Nanostructured Metallo‐Dielectric Quasi‐Crystals: Towards Photonic‐Plasmonic Resonance Engineering

The first evidence of out‐of‐plane resonances in hybrid metallo‐dielectric quasi‐crystal (QC) nanostructures composed of metal‐backed aperiodically patterned low‐contrast dielectric layers is reported. Via experimental measurements and full‐wave numerical simulations, these resonant phenomena are characterized with specific reference to the Ammann‐Beenker (quasi‐ periodic, octagonal) tiling lattice geometry and the underlying physics is investigated. In particular, it is shown that, by comparison with standard periodic structures, a moderately richer spectrum of resonant modes may be excited, due to the easier achievement of phase‐matching conditions endowed by its denser Bragg spectrum. Such modes are characterized by a distinctive plasmonic or photonic behavior, discriminated by their field distribution and dependence on the metal film thickness. Moreover, the response is accurately predicted via computationally affordable periodic‐approximant‐based numerical modeling. The enhanced capability of QCs to control number, spectral position, and mode distribution of hybrid resonances may be exploited in a variety of possible applications. To assess this aspect, label‐free biosensing is studied via characterization of the surface sensitivity of the proposed structures with respect to local refractive index changes. Moreover, it is also shown that the resonance‐engineering capabilities of QC nanostructures may be effectively exploited in order to enhance the absorption efficiency of thin‐film solar cells.     

Microstructured fibre Bragg gratings: analysis and fabrication

A microstructured fibre Bragg grating that relies on a partial and localised etching of the cladding layer along the grating region is proposed. The main effect is the formation of defect states inside the bandgap depending on surrounding refractive index. This leads to the possibility of realising novel optoelectronic devices for sensing and telecommunication applications.     

A Fiber-Optic Bragg Grating Seismic Sensor

Here we present a fiber-optic seismic waves sensor based on in-fiber Bragg gratings. Fiber-Bragg-grating (FBG) sensors have been demonstrated to have very high sensitivity to dynamical strain and very extended dynamical response. The seismic sensing system is based on the integration of three FBGs' dynamical strain sensors in a mechanical structure acting as an inverse pendulum. Polar symmetry of the mechanical system and 120deg placement of the FBG sensors could enable directional capability of the seismic sensor. Design, manufacturing, and preliminary dynamical testing of the seismic sensor are discussed.     

Contactless optoelectronic technique for monitoring epoxy cure

We describe a novel noninvasive optical technique to monitor the refractive-index variation in an epoxy-based resin that is due to the polymerization process. This kind of resin is widely used in polymer matrix composites. It is well known that the process of fabricating a thermoset-based composite involves mass and heat transfer coupled with irreversible chemical reactions that induce physical changes. To improve the quality and the reliability of these materials, monitoring the cure and optimization of the manufacturing process are of key importance. We discuss the basic operating principles of an optical system based on angle deflection measurements and present typical cure-monitoring results obtained from optical characterization. The method provides a flexible, high-sensitivity, material-independent, low-cost, noninvasive tool for monitoring real-time refractive-index variation.     

With a little help from my friends

In this paper, we propose a content-based method the for semi-automatic organization of photo albums based on the analysis of how different users organize their own pictures. The goal is to help the user in dividing his pictures into groups characterized by a similar semantic content. The method is semi-automatic: the user starts to assign labels to the pictures and unlabeled pictures are tagged with proposed labels. The user can accept the recommendation or made a correction. To formulate the suggestions is exploited the knowledge encoded in how other users have partitioned their images. The method is conceptually articulated in two parts. First, we use a suitable feature representation of the images to model the different classes that the users have collected, second, we look for correspondences between the criteria used by the different users. Boosting is used to integrate the information provided by the analysis of multiple users. A quantitative evaluation of the proposed approach is obtained by simulating the amount of user interaction needed to annotate the albums of a set of members of the flickr^® photo-sharing community.     

Structured Chirped Fiber Bragg Gratings

In this paper, a theoretical and numerical analysis of novel in-fiber photonic devices based on a structured chirped fiber Bragg gratings (CFBGs) for sensing and communication applications is presented. The investigated structure consists in a CFBG with single or multiple defects obtained by a deep and localized stripping of the cladding layer along the grating structure. The thinning of the cladding layer, partial or total, changes the core propagation features and thus leads to a significant modification of the grating spectral features. The effect of the local thinning, properly exploited, basically consists in the formation of one or more passbands within the original grating bandwidth and in one or more stopbands out of the original grating bandwidth. In addition, due to spatial encoding of the Bragg wavelength in CFBGs, the spectral position of each channel exclusively depends on the features of its own defect in a well defined location along the grating. Thus, the spectral properties of each channel are not affected by additional defects located elsewhere along the grating structure, enabling the possibility to develop independent multichannel devices by exploiting a single grating element. The spectral behavior exhibited by the microstructured device has been here numerically analyzed in dependence on the thinned region parameters. In addition a simple theoretical model has been extracted in order to easily design the device according to the desired spectral features for specific applications.     

Experimental modal analysis of an aircraft model wing by embedded fiber Bragg grating sensors

A critical issue in practical structural health monitoring is related to the capability of proper sensing systems integrated within the host structures to detect, identify, and localize damage generation. To this aim, many techniques have been proposed involving dynamic measurements such as modal analysis, acoustic emission, and ultrasonics. This paper relies on the use of embedded fiber Bragg grating sensors for performing an experimental modal analysis on a wing of an aircraft model. Time domain response of the embedded fiber-optic sensors induced by hammer impacts were acquired and transformed into the frequency domain. Using a classical technique based on the frequency transfer function, the first displacement and strain mode shapes of the wing have been retrieved in terms of natural frequencies and amplitudes. Experimental results confirm the excellent performances of this class of sensing devices to determine the modal behavior within complex structures compared with conventional accelerometer-based detection systems.     

The Influence of Surface Dents and Grooves on Traction in Sliding EHD Point Contacts

Changes in traction, caused by dents and grooves on a highly polished ball, are investigated as these defects approach and go through sliding elastohydrodynamic (EHD) point contacts. The contacts are formed with the ball loaded against a transparent disk. The ball and thus the topographical features are held stationary at various locations in the vicinity and within the contact while the disk is rotating. It is shown that these topographical features can cause substantial changes in the traction when compared to traction obtained with smooth surfaces.     

Spectral behavior in thinned long period gratings: effects of fiber diameter on refractive index sensitivity

We report the experimental investigation of the sensitivity characteristics to the surrounding refractive index (SRI) in thinned long period gratings (LPGs) for a wide range of fiber diameters and different low-orders cladding modes. Wet chemical etching combined with microscopic and spectral analysis allow us to experimentally retrieve the SRI sensitivity characteristics of thinned LPGs. The obtained results allow us to identify accurately the dependence of the sensitivity characteristics on the fiber radius, taking into account the SRI range and the order mode. This provides a useful tool to identify the thinned structure able to fulfill the sensitivity requirements by maintaining an acceptable robustness level.