Emiliano Descrovi

Vapor-phase self-assembled monolayers of aminosilane on plasma-activated silicon substrates

Aminosilane self-assembled monolayers on silicon substrates have been prepared via a gas-phase procedure based on the consecutive reactions of the aminosilane precursor and water vapor. X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements have been used to characterize the aminosilane layers. For comparison, substrates modified with aminosilane through a liquid-phase procedure have been prepared and characterized by means of the same techniques. The vapor-based procedure was found to yield more uniform layers characterized by fewer and smaller aggregates as compared with liquid-treated substrates. Grazing angles reflection Fourier transform infrared measurements were carried out on the vapor-treated substrates before and after water exposure to investigate the hydrolysis of the alkoxy groups and further reaction to form siloxane bonds. The surface density of amino groups, as estimated through a colorimetric method, is very similar for vapor- and liquid-treated substrates, suggesting a similar reactivity and accessibility of the functional groups on the surface.

Thickness dependence of surface plasmon polariton dispersion in transparent conducting oxide films at 1.55 mu m

We experimentally demonstrate propagation of surface plasmon polaritons in the near-IR window lambda (1.45 microm,1.59 microm) at the interface of indium-tin-oxide films with different thicknesses deposited on glass. Dispersion of such polaritons is strongly dependent on the film thickness, putting into evidence a regime in which polaritons at both filmss interfaces are coupled in surface supermodes. The experimental data are shown to be in good agreement with the analytical model for thin and absorbing conducting films. Measurements on aluminum-doped zinc oxide, characterized by a redshifted plasma resonance, do not show any surface plasmon polariton excitation in the same wavelength window.

Two-dimensional optics on silicon nitride multilayer: Refraction of Bloch surface waves

When properly designed, a dielectric multilayer can sustain Bloch surface waves (BSWs). Using a multiheterodyne scanning near-field optical microscope that resolves phase and polarization, we will show that a thin dielectric structure deposited on the multilayer deflects the BSW propagation according to Snell’s law. Moreover, the mechanism involved in this process is a transfer of energy from the BSW state in the bare multilayer to the new BSW state generated by the presence of the thin dielectric structure. No relevant radiative counterpart occurs. This characteristic validates the treatment of BSWs at the surface of dielectric multilayers as a two-dimensional phenomenon.

Surface label-free sensing by means of a fluorescent multilayered photonic structure

A fluorescent dielectric multilayer is exploited for label-free sensing in aqueous micro-environment. Fluorescence is laser-excited and collected through prism-coupling to a surface electromagnetic mode, also known as Bloch surface waves (BSW) localized at the interface between the multilayer and the outer aqueous medium. By detecting the spectral changes of the BSW-coupled light emission due to an external perturbation of the refractive index (Δn), a sensitivity of ∼2500u2009nm/RIU and a limit of detection down to Δnu2009∼u20093u2009×u200910−6 are obtained.

Temperature stability of Bloch surface wave biosensors

We report on the experimental characterization of the thermal sensitivity of biosensors based on the coupling of Bloch surface waves (BSW) on amorphous silicon nitride/silicon one dimensional photonic crystals (1DPC). The results show that the silicon alloys compensate the thermo-optic effect taking place in the external medium and indicate that a class of temperature insensitive biosensors can be fabricated by properly designing the layout of the 1DPC. The experimental results are in very good agreement with numerical simulations based on a transfer matrix approach. Moreover, the BSW biosensors show a resolution of 0.03u2009°C for the measurement of temperature.

Smart detection of microRNAs through fluorescence enhancement on a photonic crystal

The detection of low abundant biomarkers, such as circulating microRNAs, demands innovative detection methods with increased resolution, sensitivity and specificity. Here, a biofunctional surface was implemented for the selective capture of microRNAs, which were detected through fluorescence enhancement directly on a photonic crystal. To set up the optimal biofunctional surface, epoxy-coated commercially available microscope slides were spotted with specific anti-microRNA probes. The optimal concentration of probe as well as of passivating agent were selected and employed for titrating the microRNA hybridization. Cross-hybridization of different microRNAs was also tested, resulting negligible. Once optimized, the protocol was adapted to the photonic crystal surface, where fluorescent synthetic miR-16 was hybridized and imaged with a dedicated equipment. The photonic crystal consists of a dielectric multilayer patterned with a grating structure. In this way, it is possible to take advantage from both a resonant excitation of fluorophores and an angularly redirection of the emitted radiation. As a result, a significant fluorescence enhancement due to the resonant structure is collected from the patterned photonic crystal with respect to the outer non-structured surface. The dedicated read-out system is compact and based on a wide-field imaging detection, with little or no optical alignment issues, which makes this approach particularly interesting for further development such as for example in microarray-type bioassays.

Surface-relief formation in azo-polyelectrolyte layers with a protective polymer coating

Azobenzene-modified photosensitive polymers offer the possibility to reshape flat films upon UV-Vis irradiation. The photochemical isomerization between trans-cis configurations is the key mechanism for surface relief grating (SRG) formation on the free surface of thin azo-polymeric films illuminated by structured illumination. In the present work, we consider a sandwich configuration wherein an azopolymeric film (PAZO) is coated by a plasma-deposited polyacrylic acid (PPAA). It is demonstrated that the PPAA coating protects the PAZO against dissolution in aqueous environment. More interestingly, light-induced SRG can still be formed in the PAZO film, and the resulting modulation is transferred to the PPAA coating above. Results obtained open the possibility of exploiting water-soluble azopolymers in bio-oriented applications.

Ultrathin waveguides for Bloch surface waves: Near-field analysis of propagation and polarization

We demonstrate that an ultrathin ridge (<; λ/10) can guide Bloch surface waves in a dielectric multilayer. The propagation and polarization properties of the guided modes are investigated with a multi-heterodyne scanning near-field optical microscope.

Visualization of surface electromagnetic waves in one-dimensional photonic crystal by fluorescence dye

The Rhodamine 6G fluorescence enhanced by the surface electromagnetic waves coupled on surface of 1D photonic crystals is studied. The fluorescence-mediated surface electromagnetic waves (SEW) distribution is visualized by means of far-field fluorescence microscopy. The kinetics of Rhodamine 6G bleaching due to SEW is studied. The way of SEW visualization in reflectivity spectra via fluorescence process is shown. The prospective for SEW application in the optical sensors field is tested via direct spectroscopy of the photonic crystal covered by the ethanol and R6G thin film. Spectral flexibility of the SEW excitation depending on the effective photonic crystal dispersion controlled by its design rather than on material dispersion opens prospectives for the application of SEW-enhanced fluorescence microscopy in biocensing with increased spatial and concentration sensitivity and spectral selectivity.

Tunable hydrophobicity assisted by light-responsive surface micro-structures

In this work we investigate new degrees of freedom in controlling the physical properties of structured photo-sensitive materials that can be usefully exploited in many application fields. We employ azopolymers, a class of light responsive materials, which are structured in micro-pillar array. A reversible and controlled change in morphology of a pre-patterned polymeric film under properly polarized illumination is demonstrated to provide the opportunity to engineer surface structures and dynamically tune their properties. We exploit the laser process taking advantage of the light-induced deformation of a micro-textured azopolymeric film in order to modify the surface hydrophobicity along specific direction.