UNIVERSITA’ DEGLI STUDI DEL SANNIO
Dipartimento di Ingegneria
1. OPTICAL FIBER SENSORS, OPTOELECTRONICS AND NANOPHOTONICS
A. Cutolo, A. Cusano, G.V. Persiano, M. Consales, M. Pisco, D. Paladino
Collaborations: CNR-Istituto per i materiali compositi e biomedici (IMCB), Portici (NA), CNR-Istituto di cibernetica "Edoardo Caianiello" (ICIB), Pozzuoli (NA), CNR-Istituto per la microelettronica e microsistemi (IMM), Lecce, CNR-Istituto di biostrutture e bioimmagini (IBB), Napoli, CNR-Istituto per l'ambiente marino costiero (IAMC), Napoli, CNR- Istituto per lo studio delle macromolecole (ISMAC), Milano, ENEA - Ente Per Le Nuove Tecnologie, l'Energia e l'Ambiente, INGV - Istituto Nazionale di Geofisica e Vulcanologia, Department of Biomedical, Electronic and Telecommunication Engineering, University of Naples Federico II, Napoli, Dipartimento per le Tecnologie, Università di Napoli “Parthenope”, Istituto Nazionale di Fisica Nucleare, Napoli, Chemistry Department and INSTM Research Unit, University of Salerno, Fisciano, Italy, European Organization for Nuclear Research (CERN), Geneve, Switzerland, Centre de recherche en photonique, Université du Québec en Outaouais, Gatineau, Canada, Electromagnetism and Telecommunication Unit, Faculty of Engineering Mons, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Scotland, Electromagnetism and Telecommunication Unit, Faculté Polytechnique de Mons, Mons, Belgium, Department of Electrical and Electronic Engineering, Public University of Navarra, Pamplona, Spain, College of Optical and Electronic Technology, China Jiliang University, Hangzhou, China, Department of Chemistry, Carleton University, Ottawa, Canada.
OPTICAL FIBER SENSORS, OPTOELECTRONICS AND NANOPHOTONICS
A. Cutolo, A. Cusano, G.V. Persiano, M. Consales, M. Pisco, D. Paladino
The Lab-on-Fiber concept essentially envisages the integration of highly functionalized materials at nano and micro scale within a single optical fiber, aimed to the development of a novel generation of miniaturized and advanced “all-in-fiber” devices for both communication and sensing applications . Multifunctional “labs” integrated into a single optical fiber, exchanging information and fusing sensorial data, could provide effective auto diagnostic features as well as new photonic and electro-optic functionalities useful in many strategic sectors such as optical processing, environment, life science, safety and security.
The device design involves the exploitation of all those phenomena that provide light manipulation and control such as trapping and guiding effects in photonic crystals and quasicrystals along with plasmonic nanostructures eventually combined all together in hybrid metallo-dielectric devices. However, the realization of highly integrated optical fiber devices requires that several micro and nanostructures be fabricated, embedded and connected all together in order to achieve the necessary light-matter interaction and physical connection. As a consequence, a critical issue to be addressed consists in the definition of a reliable fabrication procedure able to integrate and process, at micro- and nano-scale, several materials with the desired physical, mechanical, magnetic, chemical and biological properties onto unconventional the optical fiber tip.
Promising approaches in this direction were recently introduced [2, 3], mainly relying on the preventive fabrication of metallic nanostructures on planar silicon wafers and their successive transfer to the fiber tip. Although, the methods so far presented rely on well assessed fabrication process on planar substrates, the last transferring step plays a fundamental role in determining both the fabrication yield and the performance of the final device. To overcome these limitations alternative approaches based on direct-write patterning of the fiber tip have been explored. The key aspect of these methodologies is to adapt the standard fabrication processes, typically used for planar devices, to operate on the optical fiber tip. Thus far, only few direct-writing attempts have been performed  in particular for creating metallic nanostructures, giving rise to LSPR effects exploited for chemical and biological sensing.
On this line of argument, in a paper in the March 8, 2012 online edition of the high impact factor (9.865) journal ACS Nano, our group, in collaboration with the ICIB-CNR, proposes a reliable fabrication process that enables the integration of dielectric and metallic nanostructures on the tip of optical fibers, thus representing a further step in the lab-on-fiber technology roadmap . In particular, we introduce and validate an innovative fabrication process based on a 'direct writing' approach taking advantage of conventional deposition and nanopatterning techniques, typically used for planar substrates and suitably adapted to directly operate on optical fiber tip. Following this approach we focused our attention on the fabrication of a first technological platform based on the integration onto the optical fiber facet of a 2D hybrid metallo-dielectric photonic crystal nanostructures supporting localized surface plasmon resonances. Moreover, with a view towards possible applications, we presented some preliminary results demonstrating how the proposed technological platform can effectively work as an optical probe for label free chemical and biological sensing as well as a microphone for acoustic wave detection.
Optical Hydrophone based on Coated Fiber Bragg Gratings (FBGs)
Since the first demonstration in the 1970s, fiber-optic acoustic sensors have been extensively studied for several application areas, such as structural health, marine monitoring, as well as military and medical detections. By comparison with conventional piezoelectric hydrophones, fiber-optic sensors have the advantages of providing high sensitivity, large dynamic range, convenience for multiplexing, immunity to electromagnetic interference, absence of electrical parts, especially important for underwater applications.
Although fiber-optic hydrophones based on various principles of operation have been demonstrated, a promising hydrophone scheme relies on the use of the well known FBGs. Besides the above mentioned advantages, common to all fiber-optic sensors, FBGs allow easy configurations, can work in reflectivity mode, and, most importantly, offer excellent multiplexing capabilities.
Acoustic monitoring for underwater applications is also an important area of interest where passive FBG-based hydrophones have gained large popularity. Takahashi et al. firstly proposed the use of FBGs as underwater acoustic sensors . The operating principle relies on the shift in the reflected/transmitted spectrum of the grating caused by the sound pressure. Unfortunately, in the case of uncoated FBGs, the mechanical deformation of the fiber due to the incident pressure is limited by the high Young’s modulus, leading to a poor responsivity at very low pressure levels. An effective strategy to overcome this limitation relies on the use of low-elastic-modulus coatings able to yield significant pressure responsivities . Although the theoretical/numerical analysis carried out in  was limited to the hydrostatic case and cannot rigorously be extended to the acoustic case, experimental evidence of acoustic responsivity optimization in the case of coated FBGs was recently demonstrated by our group . To clearly understand to what extent the static analysis could be extended also to acoustic scenarios, we recently carried out a rigorous numerical study of FBGs coated with cylindrically-shaped polymeric materials, via numerical (finite-element) solution of the complex opto-acousto-mechanical problem in the acoustic wave range (up to 30kHz) . We found that also in the acoustic range a polymeric coating is actually able to enhance the sensor responsivity. However, unlike the static case, free vibration modes of the composite structure may be excited at certain acoustic wavelengths, depending on the coating properties (e.g., Young’s modulus, acoustic impedance, Poisson ratio, and acoustic damping). As a result, the acoustic response of coated FBG hydrophones exhibits a resonant behavior at the frequencies corresponding to these longitudinal symmetric vibration modes .
Our group, in collaboration with the Finmeccanica company Whitehead Alenia Sistemi Subacquei and with the University of Naples “Parthenope”, also recently reported a systematic and accurate experimental characterization of coated FBG hydrophones . In particular, we designed a set of FBG hydrophones with ring-shaped coatings characterized by different size and mechanical properties, in order to elucidate their impact on the acoustic response of the final device. Our experimental data revealed the resonant behavior of these optical hydrophones, as well as its dependence on the coating size and type of material. These data are also in good agreement with numerical results . By comparison with bare FBGs, responsivity enhancements of up to three orders of magnitude were found. Our results also revealed the possibility to properly tailor the FBG hydrophones responsivity via suitable choice of the coating material and geometry, thereby opening up new venues for the optimization and tailoring of acoustic performance of optical hydrophones for specific applications.
Hybrid Metallo-Dielectric Structures
Currently, there is a great deal of interest in the study of out-of-plane resonances occurring in nanostructured metallic and dielectric films, stimulated by the formidable advances in nanofabrication technologies and the possible disruptive applications to highly strategic fields such as chemical and biological sensing as well as nonlinear optics and thin-film solar cells. It is well known that flat metallic films can support surface plasmon polaritons (SPPs), which are due to coherent oscillations of the surface charge density bound at the metal surface, and may be excited through prism- or grating-based coupling schemes. Furthermore, dielectric photonic crystals (PCs), in the simpler 2-D form of holey slabs, can support guided resonances (GRs) due to the coupling of leaky modes with the continuum of radiative modes of the surrounding environment.
On this line of argument, our group, in collaboration with the ICIB-CNR, recently reported 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.
Via experimental measurements and full-wave numerical simulations, we characterized these resonant phenomena with specific reference to the Ammann-Beenker (quasi-periodic, octagonal) tiling lattice geometry, and investigate the underlying physics. In particular, we showed that, by comparison with standard periodic structures, a 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 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 in order to outperform the periodic counterpart. To assess this aspect, we first focused on label-free biosensing, through the characterization of the surface sensitivity of the proposed structures with respect to local refractive index changes. Moreover, we also showed that the resonance-engineering capabilities of QC nanostructures may be effectively exploited in order to enhance the absorption efficiency of thin-film solar cells.
Backreflectors in thin film solar cells
The ability to engineer the plasmonic and photonic resonances (see Hybrid Metallo-Dielectric Structures) may provide new solutions in a variety of highly strategic fields, such as thin-film solar cells . In this context, it is well known that a serious limitation to the overall efficiency stems from the poor light absorption, especially at longer wavelengths of the solar spectrum, which is in turn attributable to the limited thickness of the semiconductor (active) region with respect to the absorption length of near infrared photons. Accordingly, over the last few years, light-trapping techniques have been proposed in order to increase the optical thickness of the absorbing layer. A particularly promising approach relies on the use of metallic nanogratings on the back surface, which can couple the incoming radiation into both plasmonic (excited at the metal/semiconductor interface) and photonic modes (guided directly in the semiconductor layer). In such configurations, the patterned metallic backreflector is capable of efficiently coupling the incoming light into photonic and plasmonic modes whose field distributions are mainly localized in the active region of the solar cell. Quasicrystals-besed backreflectors may be also exploited to judiciously tailor the number and location of photonic/plasmonic modes within the near-infrared wavelength range, so as to enhance the overall performance.
Optoelectronic Sensors for “Smart” Railways
In the modern society trains are one of the most used means for passengers and goods transportation. For this reason, it is fundamental that the entire railroads system (rails, electrical lines, poles, switches and so on) is continuously monitored, in order to prevent troubles and forced shuts down of the service and optimize maintenance reducing operating costs. At present time, railway applications monitoring requires extensive sensor networks for measuring strain, vibration, temperature, acceleration, etc. In particular, magnetic sensors are used for axles counter, accelerometers for wheel flat detection and strain gauges, load cells, and quartz sensors are used for weigh in motion: consequently for developing a monitoring system for these applications, different technologies are required and this is difficult and cost-prohibitive.
However, in the last decade fiber-optic sensors, and mainly FBG based sensors, have been gaining more and more attention, especially in railways industries. Recently, substantial amount of researches and field tests have been successfully conducted on applying only FBG sensors in railway operations, such as train and track condition monitoring, train identification, train speed and weight measurement and axle counting [1-5]. These studies have demonstrated that the application of FBG sensors in the railway system have the potential to revolutionize condition monitoring in the railway industry and to upgrade the conventional systems into ‘Smart Railways’, thereby providing safe, reliable and vital information to railway operators. A smart condition monitoring system allows real-time and continuous monitoring of the structural and operational conditions of trains, overhead contact lines, as well as monitoring of the structural health of rail tracks and the location, speed and weight of passing trains of the entire rail systems.
On this line of argument, our group has been engaged by several years, in collaboration with OptoSmart s.r.l. (a spin-off company of the University of Sannio), the IMCB-CNR and the Finmeccanica Company Ansaldo Segnalamento Ferroviario, in research activities aimed at demonstrating the capability of FBG technology to be exploited for railway monitoring applications, in particular for axles counter, occupation state of a rail truck, direction of transit, speed and acceleration control, carriage identification, detection of wheel with defects, weight in motion of all the type of trains for monitoring unbalance and overload on rail truc, switch monitoring.
Fiber optic sensors for high-energy physics applications at Cern
CERN, the European Organization for Nuclear Research, is one the world’s largest and respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. Particle accelerators and detectors are used to find the basic constituents of matter: by studying what happens when particles collide, physicists learn about the law of Nature. The new flagship research facility of CERN laboratory is LHC, the Large Hadron Collider, located in a 27 km tunnel in circumference and about 100 meters underground, which has been designed to boost two beams of particles travelling in opposite directions to high energies, before they are collided with each other. Four large experiments (ALICE, ATLAS, CMS and LHCb), together with two smaller ones (TOTEM and LHCf), are installed around the LHC ring, with a complex system of detectors to record the hundreds of particles produced in each collision.
Our research group, in collaboration with the University of Naples “Federico II”, the IMCB-CNR and the INFN, is engaged by several years in research activities aimed at analyzing the exploitation capability of Fiber Optic Sensors (FOS) for high-energy physics (HEP) applications at CERN. In particular, starting from the consideration that fiber optic radiation hardness has been widely proven, we have applied the FOS technology to this very relevant field of interest. In particular, regarding the Compact Muon Solenoid (CMS) experiment, we have monitored temperatures and strains in different locations by using Bragg Grating sensors , and we also moved to the development of novel classes of Relative Humidity sensor based on FOS technology [2, 3]. Preliminary results are very encouraging, letting us consider the use of FOS technique as a robust and effective solution for monitoring requirements in HEP detectors for other physical and environmental parameters.
1) Iadicicco, A., Campopiano, S., Cusano, A., Long-Period Gratings in Hollow Core Fibers by Pressure-Assisted Arc Discharge Technique. Photonics Technology Letters, IEEE , 2011, vol.23, no.21, pp.1567-1569.
2) Lanza G, Breglio G, Giordano M, Gaddi A, Buontempo S, Cusano A. Effect of the anisotropic magnetostriction on terfenol-D based fiber bragg grating magnetic sensors. Sens Actuators A Phys 2011;172(2):420-7.
3) Consales M, Buosciolo A, Cutolo A, Breglio G, Irace A, Buontempo S, Petagna P, Giordano M, Cusano A. Fiber optic humidity sensors for high-energy physics applications at CERN. Sensors and Actuators, B: Chemical 2011;159(1):66-74.
4) Moccia M, Pisco M, Cutolo A, Galdi V, Bevilacqua P, Cusano A. Opto-acoustic behavior of coated fiber bragg gratings. Optics Express 2011;19(20):18842-60.
5) Ricciardi A, Pisco M, Cutolo A, Cusano A, O'Faolain L, Krauss TF, Castaldi G, Galdi V. Evidence of guided resonances in photonic quasicrystal slabs. Physical Review B - Condensed Matter and Materials Physics 2011;84(8).
6) Polcari A, Romano P, Sabatino L, Del Vecchio E, Consales M, Cusano A, Cutolo A, Colantuoni A. Electrical and optical characterization of DNA molecules in a water solution and the role of their concentration. Journal of Applied Physics, 2011; 109 (7).
7) Iadicicco A, Paladino D, Campopiano S, Bock WJ, Cutolo A, Cusano A. Evanescent wave sensor based on permanently bent single mode optical fiber. Sensors and Actuators, B: Chemical 2011;155(2):903-8.
8) Manzillo PF, Pilla P, Buosciolo A, Campopiano S, Cutolo A, Borriello A, Giordano M, Cusano A. Self assembling and coordination of water nano-layers on polymer coated long period gratings: Toward new perspectives for cation detection. Soft Materials 2011;9(2-3):238-63.
9) Quero G, Crescitelli A, Paladino D, Consales M, Buosciolo A, Giordano M, Cutolo A, Cusano A. Evanescent wave long-period fiber grating within D-shaped optical fibers for high sensitivity refractive index detection. Sensors and Actuators, B: Chemical 2011;152(2):196-205.
10) Pilla PP, Malachovská V, Borriello A, Buosciolo A, Giordano M, Ambrosio L, Cutolo A, Cusano A. Transition mode long period grating biosensor with functional multilayer coatings. Optics Express 2011;19(2):512-26.