Gianluca Ruffato

Nanoporous gold plasmonic structures for sensing applications

The fabrication, characterization and functionalization of periodically patterned nanoporous gold layers is presented. The material shows plasmonic properties in the near infrared range, with excitation and propagation of surface plasmon polaritons. Functionalization shows a marked enhancement in the optical response in comparison with evaporated gold gratings, due to a great increase of the active surface. Due to its superior response, nanoporous gold patterns appear promising for the realization of compact plasmonic platforms for sensing purposes.

Generation of high-order Laguerre–Gaussian modes by means of spiral phase plates

Spiral phase plates for the generation of Laguerre-Gaussian (LG) beam with non-null radial index were designed and fabricated by electron beam lithography on polymethylmethacrylate over glass substrates. The optical response of these phase optical elements was theoretically considered and experimentally measured, and the purity of the experimental beams was investigated in terms of LG modes contributions. The far-field intensity pattern was compared with theoretical models and numerical simulations, whereas interferometric analyses confirmed the expected phase features of the generated beams. The high quality of the output beams confirms the applicability of these phase plates for the generation of high-order LG beams.

The role of polarization on surface plasmon polariton excitation on metallic gratings in the conical mounting

The polarization effects of surface plasmon polaritons (SPP) in an azimuthally rotated gratings have been investigated. Mixed s and p modes can be excited using the polarization angle of the incident light to optimize the SPP coupling. Experimental results and simulations show which component of polarization is effective for the SPP excitation. The optimum angle direction has been calculated analytically. The SPR tunability by polarization may uncover important features in SPP applications.

Diffractive optics for combined spatial- and mode- division demultiplexing of optical vortices: design, fabrication and optical characterization

During the last decade, the orbital angular momentum (OAM) of light has attracted growing interest as a new degree of freedom for signal channel multiplexing in order to increase the information transmission capacity in today’s optical networks. Here we present the design, fabrication and characterization of phase-only diffractive optical elements (DOE) performing mode-division (de)multiplexing (MDM) and spatial-division (de)multiplexing (SDM) at the same time. Samples have been fabricated with high-resolution electron-beam lithography patterning a polymethylmethacrylate (PMMA) resist layer spun over a glass substrate. Different DOE designs are presented for the sorting of optical vortices differing in either OAM content or beam size in the optical regime, with different steering geometries in far-field. These novel DOE designs appear promising for telecom applications both in free-space and in multi-core fibers propagation.

Compact sorting of optical vortices by means of diffractive transformation optics

The orbital angular momentum (OAM) of light has recently attracted a growing interest as a new degree of freedom in order to increase the information capacity of todays optical networks, both for free-space and optical fiber transmission. Here we present our work of design, fabrication, and optical characterization of diffractive optical elements for compact OAM mode division demultiplexing based on optical transformations. Samples have been fabricated with 3D high-resolution electron beam lithography on a polymethylmethacrylate resist layer spun over a glass substrate. Their high compactness and efficiency make these optical devices promising for integration into next-generation platforms for OAM modes processing in telecom applications.

A peptide nucleic acid label-free biosensor for Mycobacterium tuberculosis DNA detection via azimuthally controlled grating-coupled SPR

Grating coupled surface plasmon resonance phenomena under azimuthal control of incident light (φ ≠ 0° GC-SPR) have recently been exploited for the development of biosensing solutions with a sensitivity similar to that of classic prism-coupled SPR sensors, with the advantage of higher miniaturization potential. Here we combined the use of φ ≠ 0° GC-SPR with the use of peptide nucleic acid (PNA) probes and a strategy for maximizing the signal-to-noise ratio for the sensitive detection of Mycobacterium tuberculosis (MT) DNA. We focused our attention on the optimization of the PNA-based sensing layer by controlling the sensing surface composition with the PNA-based probe and a poly(ethylene oxide) (PEO)-based antifouling layer. We tested the sensor response first in the presence of complementary and non-complementary oligonucleotides, and then we applied our strategy for the detection of PCR amplified samples, using the fluorescence-based microarray technology as the control. With the φ ≠ 0° GC-SPR set-up adopted, a limit of detection (LOD 0.26 pM) more than one order of magnitude lower than that obtained by the fluorescence method (LOD 8.9 pM) was observed using a complementary oligonucleotide target. Also when PCR amplicons were analysed on SPR grating surfaces, lower DNA concentrations were detectable with the SPR readout as compared to the fluorescence one, and with an experimental protocol that does not include the need to use expensive fluorophore molecules. The whole approach, involving the sensor fabrication, the sensing surface control and DNA detection, has demonstrated that φ ≠ 0° GC-SPR is a good starting point for a sensitive, versatile and scalable biosensing technique that will be further investigated in future experiments.

Label-Free Efficient and Accurate Detection of Cystic Fibrosis Causing Mutations Using an Azimuthally Rotated GC-SPR Platform

Plasmonic nanosensors are candidates for the development of new sensors with low detection limits, high sensitivity, and specificity for target detection: these characteristics are of critical importance in the screening of mutations responsible for inherited diseases. In this work, we focused our study on the detection of some of the most frequent mutations responsible for cystic fibrosis (CF) among the Italian population. For the detection of the CF mutations we adopted a recently developed and highly sensitive Grating Coupled-Surface Plasmon Resonance (GC-SPR) enhanced spectroscopy method for label-free molecular identification exploiting a conical illumination configuration. Gold sinusoidal gratings functionalized with heterobifunctional PEG were used as sensing surfaces, and the specific biodetection was achieved through the coupling with DNA hairpin probes designed for single nucleotide discrimination. Such substrates were used to test unlabeled PCR amplified homozygous wild type (wt) and heterozygous samples, deriving from clinical samples, for the screened mutations. Hybridization conditions were optimized to obtain the maximum discrimination ratio (DR) between the homozygous wild type and the heterozygous samples. SPR signals obtained from hybridizing wild type and heterozygous samples show DRs able to identify univocally the correct genotypes, as confirmed by fluorescence microarray experiments run in parallel. Furthermore, SPR genotyping was not impaired in samples containing unrelated DNA, allowing the platform to be used for the concomitant discrimination of several alleles also scalable for a high throughput screening setting.

Coupled SPP Modes on 1D Plasmonic Gratings in Conical Mounting

Plasmonic nanostructures exhibit a variety of surface plasmon polariton (SPP) modes, with different characteristic properties. While a single metal dielectric interface supports a single-interface SPP mode, a thin metal film can support extended long range SPPs and strongly confined short range SPPs. When the coupling between the incident light and the SPP is provided through a diffraction grating, it is possible to azimuthally rotate the grating with respect to the scattering plane, introducing the possibility to propagate the SPP along an arbitrary direction. We present a theoretical and experimental analysis of the coupling conditions for long range and short range SPPs under this configuration. We also investigate the propagation length of the modes depending on the propagation direction with respect to the grating grooves, showing in particular that the long range SPP propagation length can be sensibly enhanced with respect to the null-azimuth case.

Test of mode-division multiplexing and demultiplexing in free-space with diffractive transformation optics

In recent years, mode-division multiplexing (MDM) has been proposed as a promising solution in order to increase the information capacity of optical networks both in free-space and in optical fiber transmission. Here we present the design, fabrication and test of diffractive optical elements for mode-division multiplexing based on optical transformations in the visible range. Diffractive optics have been fabricated by means of 3D high-resolution electron beam lithography on polymethylmethacrylate resist layer spun over a glass substrate. The same optical sequence was exploited both for input-mode multiplexing and for output-mode sorting after free-space propagation. Their high miniaturization level and efficiency make these optical devices ideal for integration into next-generation platforms for mode-division (de)multiplexing in telecom applications.

Near-field numerical analysis of surface plasmon polariton propagation on metallic gratings

Purpose – The purpose of this paper is to simulate and analyze the excitation and propagation of surface plasmon polaritons (SPPs) on sinusoidal metallic gratings in conical mounting. Design/methodology/approach – Chandezons method has been implemented in MATLAB environment in order to compute the optical response of metallic gratings illuminated under azimuthal rotation. The code allows describing the full optical features both in far- and near-field terms, and the performed analyses highlight the fundamental role of incident polarization on SPP excitation in the conical configuration. Findings – Results of far-field polarization conversion and plasmonic near-field computation clearly show that azimuthally rotated metallic gratings can support propagating surface plasmon with generic polarization. Originality/value – The recent papers experimentally demonstrated the benefits in sensitivity and the polarization phenomenology that are originated by an azimuthal rotation of the grating. In this work, numer...