A. Ricciardi

Broadband Mirrors in the Near-Infrared Based on Subwavelength Gratings in SOI

We describe the design, fabrication, and characterization of high-reflectivity broadband mirrors operating in the near-infrared (700-1000 nm) wavelength range. The mirrors consist of 1-D and 2-D subwavelength resonant gratings (SWGs) fabricated on a silicon-on-insulator (SOI) wafer. A very good agreement between numerical and experimental results is obtained. The mirror response can be tailored by adjusting the geometrical parameters of the gratings, with the grating period as the main parameter. The optimized mirrors reflect strongly (> 95%) over a fractional optical bandwidth Δλ/λ of about 12% and 7.5% for 1-D and 2-D gratings, respectively. The important and somewhat surprising feature of these gratings is that high reflectivities have been achieved, despite the fact that silicon exhibits significant absorption in this wavelength range.

Optical fiber tip templating using direct focused ion beam milling.

We report on a method for integrating sub-wavelength resonant structures on top of optical fiber tip. Our fabrication technique is based on direct milling of the glass on the fiber facet by means of focused ion beam. The patterned fiber tip acts as a structured template for successive depositions of any responsive or functional overlay. The proposed method is validated by depositing on the patterned fiber a high refractive index material layer, to obtain a ‘double-layer’ photonic crystal slab supporting guided resonances, appearing as peaks in the reflection spectrum. Morphological and optical characterizations are performed to investigate the effects of the fabrication process. Our results show how undesired effects, intrinsic to the fabrication procedure should be taken into account in order to guarantee a successful development of the device. Moreover, to demonstrate the flexibility of our approach and the possibility to engineering the resonances, a thin layer of gold is also deposited on the fiber tip, giving rise to a hybrid photonic-plasmonic structure with a complementary spectral response and different optical field distribution at the resonant wavelengths. Overall, this work represents a significant step forward the consolidation of Lab-on-Fiber Technology.

Light trapping efficiency of periodic and quasiperiodic back-reflectors for thin film solar cells: A comparative study

Recently, great efforts have been carried out to design optimized metallic nano-grating back-reflectors to improve the light absorption in thin film solar cells. In this work, we compare the performances of deterministic aperiodic backreflectors in the form of 1-D nanogratings based on the generalized Fibonacci deterministic aperiodic sequence with a standard periodic one. The case of study here analyzed relies on a realistic solar cell model, where light absorption is evaluated only in the intrinsic region of an amorphous silicon P-I-N junction. We found that the results of comparison are strongly influenced by the amorphous silicon extinction coefficient within the near-infrared wavelength range, where most photonic-plasmonic modes (responsible for the light absorption enhancement typically observed when structured metal nanogratings are employed) are excited. In particular, with device-grade hydrogenated amorphous silicon, we demonstrate that Fibonacci-like backreflectors are able to provide an absorpt...

Microgel photonics: a breathing cavity onto optical fiber tip

We experimentally demonstrate a novel multifunctional optical fiber probe resulting from the integration between two rapidly emerging technologies such as Lab-on-Fiber and Microgel Photonics. The device consists of a microgel based cavity formed by metallic slabs supporting plasmonic resonances, directly integrated on the optical fiber tip. By exploiting the multiresponsivity of microgel systems, variations of temperature, PH, ionic strength, as well as molecular binding events, make the cavity to ‘breath’, thus modulating the interference pattern in the reflection spectrum. The microgel layer can be synthetized in such a way to obtain different thicknesses, corresponding to different operating regimes, opening new avenues for the realization of advanced multifunctional nanoprobes.

Lab-on-Fiber biosensing for cancer biomarker detection

This work deals with a novel Lab-on-Fiber biosensor able to detect in real time thyroid carcinomas biomarkers. The device is based on a gold nanostructure supporting localized surface plasmon resonances (LSPR) directly fabricated on the fiber tip by means of electron beam lithography and lift-off process. Following a suitable chemical and biological functionalization of the sensing area, human Thyroglobulin has been detected at nanomolar concentrations. Also, compatibility with full baseline restoration, achieved through biomarkers/bioreceptors dissociation, has been demonstrated.

Lab-on-fiber platforms for ultrasound detection: a comparative study

Sub-wavelength metallo-dielectric gratings integrated on optical fibers tip and supporting plasmonic resonances were numerically investigated in their behavior as acousto-optical transducers. Different configurations have been analyzed and compared among them in terms of sensitivity, defined as reflectivity intensity variation (at fixed wavelength) with respect to dielectric layer thickness changes. Our results demonstrate that the maximum sensitivity is obtained when an interaction between different plasmonic modes occurs. Sensitivity enhancement up to a factor of 3 with respect to Fabry- Perot cavity like transducer with same materials and sizes can be achieved.

Compact tunable Terahertz source: Perspectives on planar configurations

In the last decade there has been an increasing interest for terahertz technology. The reasons of this increasing interest is mainly due to its potential use in security applications with particular reference to drug and explosive detection. In this framework, the appropriate integration of metamaterials with nanotechnologies looks very promising for the construction of new compact low cost devices. Following this line of argument, we are exploring the possibility to combine the use of nanocathodes as sources of electron beams with engineered metamaterial layers in order to deliver coherent tunable terahertz radiation. More exactly, starting from a revised design of microwave sources, we discuss the issues related to manufacture miniaturized planar devices taking advantage of some appropriate integration of post silicon technologies.