Valerio Pruneri

Widely transparent electrodes based on ultrathin metals

Transparent electrodes made of single-component ultrathin (<10 nm) metal films (UTMFs) are obtained by sputtering deposition. We show that the optical transparency of the deposited films (chromium and nickel) is comparable to that of indium tin oxide (ITO) in the visible and near-infrared range (0.4-2.5 microm), while it can be significantly higher in the ultraviolet (175-400 nm) and mid-infrared (2.5-25 microm) regions. Despite their very small thickness, the deposited UTMFs are also uniform and continuous over the 10 cm substrate, as it is confirmed by the measured low electrical resistivity. The excellent optical and electrical properties, stability, compatibility with active materials, process simplicity, and potential low cost make UTMFs high-quality transparent electrodes for the optoelectronics industry, seriously competing with widely used transparent conductive oxides, such as ITO.

Very low voltage single drive domain inverted LiNbO 3 integrated electro-optic modulator

Domain inversion is used in a simple fashion to improve significantly the performance of a waveguide electro-optic modulator in z-cut LiNbO(3). The waveguide arms of the Mach-Zehnder interferometer are placed in opposite domain-oriented regions under the same, narrower and more efficient electrode, so that opposite phase shifts (push-pull effect) can still be achieved despite the arms being subjected to the same electric field. Switching voltages close to 2 V are obtained, which allow 10Gb/s modulation with inexpensive drivers, such as those used for electro-absorption modulators, which deliver driving voltages well below 3V.

Integrated acousto-optic polarization converter in a ZX-cut LiNbO 3 waveguide superlattice

We report an integrated acousto-optic polarization converter exploiting a novel surface acoustic superlattice (S-ASL) transducer. The S-ASL transducer is made of a ZX-cut periodically poled lithium niobate (PPLN) crystal with uniform coplanar electrodes for surface acoustic wave (SAW) generation. For a PPLN period of 20 microm the SAW is excited at an rf of about 190 MHz, while the phase matching occurs at an optical wavelength of around 1456 nm. The measured mode conversion efficiency of 90% at an input rf power of 1 W and the 3 dB optical bandwidth of 2.5 nm confirm the confinement of the SAW between the electrode gap and the constructive interaction along the whole 10 mm electrode length.

Waveguide electro-optic modulation in micro-engineered LiNbO3

In this paper, after describing the basics of LiNbO3 based integrated electro-optic modulators, we will show how techniques such as etching, domain inversion and thin film processing can be used to realize new geometries which can take the performance to unprecedented levels. In particular we will review recent results on the use of domain inversion on a micron scale to improve the electro-optic response of LiNbO3 waveguide modulators in terms of bandwidth and driving voltage. These applications of domain inversion techniques might be even more important and commercially valuable than those in nonlinear optics (e.g. quasi-phase-matched optical parametric devices). With respect to standard single-domain structures, larger bandwidths and lower driving voltages can be obtained, thus achieving figure of merits for the electro-optic response that are up to 50% larger. As a demonstration, a chirp-free modulator, having ∼2 V switching voltage and bandwidth of 15 GHz, was fabricated by placing the waveguide arms of a Mach–Zehnder interferometer in opposite domain oriented regions. The modulator, as indicated by system measurements, could be driven in a single-drive configuration with inexpensive low-voltage drivers, e.g. SiGe based, typically used for electro-absorption devices.

Low power consumption integrated acousto-optic filter in domain inverted LiNbO3 superlattice

We report on an integrated acousto-optic filter in domain inverted LiNbO3 using a coplanar electrode configuration, which can achieve complete optical switching at electrical powers as low as 50 mW. These values are more than one order of magnitude lower than previously reported results [Opt. Lett. 34, 3205 (2009)]. In order to design the low power consumption devices, we have calculated surface acoustic wave excitation, propagation and acousto-optic interaction in the domain inverted LiNbO3 superlattice using scalar approximation and FEM analysis. Results from both modeling techniques are in good agreement with the experiments, including direct measurement of the acoustic displacement using laser interferometry and acousto-optic performance.

Functional photonic crystal fiber sensing devices

We report on a functional, highly reproducible and cost effective sensing platform based on photonic crystal fibers (PCFs). The platform consists of a centimeter-length segment of an index-guiding PCF fusion spliced to standard single mode fibers (SMFs). The voids of the PCF are intentionally sealed over an adequate length in the PCF-SMF interfaces. A microscopic collapsed region in the PCF induces a mode field mismatch which combined with the axial symmetry of the structure allow the efficient excitation and recombination or overlapping of azimuthal symmetric modes in the PCF. The transmission or reflection spectrum of the devices exhibits a high-visibility interference pattern or a single, profound and narrow notch. The interference pattern or the notch position shifts when the length of the PCF experiences microelongations or when liquids or coatings are present on the PCF surface. Thus, the platform here proposed can be useful for sensing diverse parameters such as strain, vibration, pressure, humidity, refractive index, gases, etc. Unlike other PCF-based sensing platforms the multiplexing of the devices here proposed is simple for which it is possible to implement PCF-based sensor arrays or networks.

Fourier imaging cytometry for optical analysis of phytoplankton and bacteria in ballast water

We introduce a Fourier imaging cytometer for ballast water analysis. We demonstrate detection of Tetraselmis and Nannochloropsis by autofluorescence, and E. faecalis and V. cholerae by brightfield scattering. Achieving sensitivity of ~95% and specificity of ~80%.

Preliminary Telcordia of Packaged Optical Fibre Sensors for High Temperature Applications

We present packaged temperature sensors based on regenerated fibre Bragg gratings for measurements up to 1000°C. The thermo-optical response and mechanical performances of the packaged prototypes have been tested, according to international certification Telcordia CR1221.

Nanostructuring of Glass Surfaces Starting from Ultrathin Metals

Monolithic surface nanostructures can be formed in glass from metal dewetted nanoparticles. This results in antireflection, low scattering, superhydrophobic and mechanically resistant surfaces. The technique is industrially scalable and usable on optical fibre surfaces.

Photonic-crystal and optical micro/nano fiber interferometric sensors

We report on mode interferometers built with photonic crystal fibers (PCFs) and optical micro/nano fibers (MNFs). This type of mode interferometers exploit the beating between two modes, are very compact and highly stable over time for which they are suitable for a myriad of sensing applications. Moreover, their fabrication is simple since it can be carried out by means of cleaving and splicing or tapering techniques. The transmission spectrum of these interferometers typically exhibits truly sinusoidal interference patterns which simplifies their analysis. PCF-based mode interferometers may have niche applications since they are capable of operating at extreme temperatures (up to 1000ºC). To make these interferometers functional and competitive, our group has placed emphasis on the design of the PCF microstructure, minimizing the insertion losses, and on the elaboration of ad-hoc packaging for both harsh environment and biosensing applications. MNF-based interferometers, on the other hand, are extremely compact, require minimal amount of sample and can be combined with microfluidics for which they may be adequate for refractometric or biosensing applications. Adequate protection of the MNFs and ad-hoc microfluidics are being implemented to make MNF interferometers practical.