Emilia Giorgetti

Optical fiber-polymer guide coupling by a tapered graded index glass guide

A new four layer tapered coupler (FLTC) has been studied and tested, able to transfer light efficiently from a monomode fiber to a polymer film guide by means of an auxiliary graded index glass waveguide. A simple theory has been developed to calculate the maximum efficiency of these structures. Experiments have been performed, at /spl lambda/=1.321 /spl mu/m, on several FLTC samples, containing K/sup +/-Na/sup +/ ion-exchanged glass waveguides and spin coated poly-3BCMU films. Insertion losses in the range 3-6.5 dB have been measured, depending on the taper configuration. The agreement between theoretical and experimental data is good, provided that a suitable ripple of the taper profile is assumed. The application of the new coupler to future integrated waveguide systems, including passive glass components and polymer based all optical devices, is assessed. >

All optical switches based on the coupling of surface plasmon polaritons

We studied the potentials of All Optical Switches (AOS) based on the intensity-dependent coupling and decoupling of light into the SPP modes (Surface Plasmon Polaritons) of a sinusoidally corrugated thin metal film (TMF), due to Kerr induced refractive index changes of the surrounding dielectrics. The ideal device has two spatially separated outputs, collecting the reflected and transmitted light and the active volume can be as small as 10(-2) mm(3). Gold and PTS (poly-(2,4-hexadiyne-1,6-diol bis(p-toluene sulfonate) are the materials considered. Losses are limited to 1.5 dB,while a 20 dB extinction ratio per gate has been theoretically demonstrated with signal pulsewidths of 5-10 ps , using a maximum optical switching peak power of 11 kW.

Nonlinear characterization of nanometer-thick dielectric layers by surface plasmon resonance techniques

The use of surface plasmon resonance as a powerful tool for the nonlinear characterization of ultrathin dielectric layers is investigated and experimentally demonstrated. The off-resonant intensity-dependent refractive index of 10–200-nm-thick films of the soluble polycarbazolyldiacetylene 1,6-bis-(3,6-dihexadecyl-N-carbazolyl)-2,4-hexadiyne deposited upon silver was measured at 1064 nm and with picosecond pulses.

Optical characterization of quasi-monomolecular layers of the nonlinear polymer polyDCHD-HSPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

The nonlinear optical properties of quasi-monomolecular layers of polyDCHD-HS spun on silver-coated plates were measured using surface plasmon spectroscopy at 1064 nm and with ps pulses. A surprisingly large value of χ(3) was obtained (|χ(3)| > 10−17 m2 V−2). This value was related to the presence of the silver film, whose nanostructured surface provides an electromagnetic mechanism for the enhancement of the nonlinearity through local field effects. This point was confirmed by SERS experiments performed on-resonance (515 nm) and off-resonance (647 nm) on the same samples. It is suggested that the experimental data can be accounted for by the existence of an interaction between the π-conjugated system of the polymer chain and the silver surface.

Multiphoton absorption of solutions of polydiacetylene polyDCHD-HS measured using ps Z-scan at 1064 and 1500 nm

Abstract The nonlinear absorption of benzene and toluene solutions of polydiacetylene polyDCHD-HS was measured at λ=1064 and 1500 nm by using Z-scan and picosecond pulses with a trimmed Airy beam configuration. In the data analysis, we took into account both the saturation of the open aperture Z-scan traces occurring for high values of nonlinear absorption and the possible occurrence of cross-talk effects between nonlinear refraction and multiphoton absorption. The polymer exhibits three-photon absorption at both 1064 and 1500 nm. The molecular three-photon absorption coefficient at 1064 nm was σ 3 =1.8×10 −38 cm 6 / W 2 and σ 3 =2.3×10 −38 cm 6 / W 2 in toluene and benzene, respectively, while at 1500 nm it was σ 3 =1.5×10 −39 cm 6 / W 2 in toluene. On this basis, the optical limiting behavior of polyDCHD-HS in the near infrared range is also shown.

A novel processable polydiacetylene for photonics studies

A novel processable polycarbazolyldiacetylene has been prepared, which allows to obtain homogeneous films very promising for waveguiding devices. The electronic spectra of the polymer in different solvents are discussed and preliminary data on the linear optical characterization of the films are reported. It is shown that at 849 nm the visible guided streak is only a few millimeters long, while at 1321 nm it reaches the film end (about 2.5 cm).

Efficient coupling between annealed K/sup +/-Na/sup +/ ion-exchanged channel waveguides and 10/125 single-mode fibers at /spl lambda/=1.321 /spl mu/m

The process of thermal annealing of K/sup +/-Na/sup +/ ion-exchanged channel waveguides has been studied with the aim of optimizing their coupling efficiency with commercial single-mode fibers at /spl lambda/=1.321 /spl mu/m. Waveguides obtained in soda-lime glass slides, with mask apertures ranging between 13.4 and 2.6 /spl mu/m, were characterized before the annealing by combining nearfield measurements and an etching procedure. The experimental results were successfully compared with a theoretical model based on the variational principle. The refractive index distribution of K/sup +/-Na/sup +/ ion-exchanged channel waveguides supporting one or a low number of modes was given: compared to the corresponding slab case, the refractive index step /spl Delta/n/sub o/ remained constant, while the waveguide depth was lower. The thermal annealing process of the channels was then performed and modeled by means of the standard diffusion theory. As a result, the channel fabrication parameters for optimum guide-fiber coupling could be predicted: 0.23-dB mode mismatch losses were measured between the optimized channel and a commercial 10/125 single-mode fiber, at /spl lambda/=1.321 /spl mu/m.

Antibacterial activity of silver nanoparticles obtained by pulsed laser ablation in pure water and in chloride solution

Summary Silver nanoparticles (AgNPs) have increasingly gained importance as antibacterial agents with applications in several fields due to their strong, broad-range antimicrobial properties. AgNP synthesis by pulsed laser ablation in liquid (PLAL) permits the preparation of stable Ag colloids in pure solvents without capping or stabilizing agents, producing AgNPs more suitable for biomedical applications than those prepared with common, wet chemical preparation techniques. To date, only a few investigations into the antimicrobial effect of AgNPs produced by PLAL have been performed. These have mainly been performed by ablation in water with nanosecond pulse widths. We previously observed a strong surface-enhanced Raman scattering (SERS) signal from such AgNPs by “activating” the NP surface by the addition of a small quantity of LiCl to the colloid. Such surface effects could also influence the antimicrobial activity of the NPs. Their activity, on the other hand, could also be affected by other parameters linked to the ablation conditions, such as the pulse width. The antibacterial activity of AgNPs was evaluated for NPs obtained either by nanosecond (ns) or picosecond (ps) PLAL using a 1064 nm ablation wavelength, in pure water or in LiCl aqueous solution, with Escherichia coli and Bacillus subtilis as references for Gram-negative and Gram-positive bacteria, respectively. In all cases, AgNPs with an average diameter less than 10 nm were obtained, which has been shown in previous works to be the most effective size for bactericidal activity. The measured zeta-potential values were very negative, indicating excellent long-term colloidal stability. Antibacterial activity was observed against both microorganisms for the four AgNP formulations, but the ps-ablated nanoparticles were shown to more effectively inhibit the growth of both microorganisms. Moreover, LiCl modified AgNPs were the most effective, showing minimum inhibitory concentration (MIC) values in a restricted range of 1.0–3.7 µg/mL. An explanation is proposed for this result based on the increased surface reactivity of the metal surface due to the presence of positively charged active sites.

Investigation of non-linear optical effects at conjugated polymer/silver interface by surface plasmon resonance and surface enhanced Raman scattering

SERS experiments performed at 1064 nm with quasi-monomolecular layers of polycarbazolyldiacetylene (polyDCHD-HS: 1,6-bis-(3,6-dihexadecyl-N-carbazolyl)-2,4-hexadiyne) spun on silver-coated plates suggest that the very large value of the intensity dependent refractive index measured in such samples and at the same wavelength by surface plasmon spectroscopy (SPS) tests is related to the nanostructured surface of the silver films, that provides an electromagnetic mechanism for the enhancement of the non-linearity through local field effects at optical frequencies far from the plasmon resonance of the metal.

Efficient UV polymerisation of 3BCMU: Optical and waveguiding properties of the material

Abstract A modified method to synthesize 3BCMU and to polymerize it by ultraviolet (UV) radiation is presented; the polymerization technique gave a maximum yield of 18%. Langmuir-Blodgett and spin coated films of UV/poly-3BCMU have been fabricated. Visible and resonance Raman spectroscopy and waveguide tests were performed: the results are compared with those obtained with y/poly-3BCMU films.