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The significant boost in surface-enhanced Raman scattering (SERS) by the chemical enhancement of semiconducting oxides is a pivotal finding. It offers a prospective path toward high uniformity and low-cost SERS substrates. However, a detailed understanding of factors that influence the charge transfer process is still insufficient. Herein, we reveal the important role of defect-induced band offset and electron lifetime change in SERS evolution observed in a MoO3oxide semiconductor. By modulating the density of oxygen vacancy defects using ultraviolet (UV) light irradiation, SERS is found to be improved with irradiation time in the first place, but such improvement later deteriorates for prolonged irradiation even if more defects are generated. Insights into the observed SERS evolution are provided by ultraviolet photoelectron spectroscopy and femtosecond time-resolved transient absorption spectroscopy measurements. Results reveal that (1) a suitable offset between the energy band of the substrate and the orbitals of molecules is facilitated by a certain defect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.

An inverted pyramidal metasurface was designed, fabricated, and studied at the nanoscale level for the development of a label-free pathogen detection on a chip platform that merges nanotechnology and surface-enhanced Raman scattering (SERS). Based on the integration and synergy of these ingredients, a virus immunoassay was proposed as a relevant proof of concept for very sensitive detection of hepatitis A virus, for the first time to our best knowledge, in a very small volume (2 μL), without complex signal amplification, allowing to detect a minimal virus concentration of 13 pg/mL. The proposed work aims to develop a high-flux and high-accuracy surface-enhanced Raman spectroscopy (SERS) nanobiosensor for the detection of pathogens to provide an effective method for early and easy water monitoring, which can be fast and convenient.

Plasmonic metasurfaces with spatial symmetry breaking are crucial materials with significant applications in fields such as polarization-controlled photonic devices and nanophotonic platforms for chiral sensing. In this paper, we introduce planar plasmonic metasurfaces, less than one-tenth of a wavelength thick, featuring nanocavities formed by three equilateral triangles. This configuration creates uniform, thin metasurfaces. Through a combination of experimental measurements and numerical modeling, we demonstrate the inherent superchirality of these plasmonic metasurfaces. We address the challenge of achieving a strong enhancement of optical chirality in the visible spectrum, reaching levels comparable to those of 3D chiral metasurfaces.

Understanding binding of DNA bases with gold surfaces is crucial for designing reliable tools in biosensing technology and organic-based electronics. Herein, we discuss an application of quasi periodic plasmonic nanocavities fabricated by a top-down method to study adenine interactions with gold surfaces. We report plasmon-mediated homogenous catalysis of adenine–gold complex formation, when adenine is deposited onto surfaces in different ways. In particular, we elucidate how additional information can be obtained by adsorbate irradiation with laser far from resonant conditions with a low-localized, high-integral field substrate. A comparative study between mono- and multilayers on gold is proposed, offering a solution to the controversial question of adenine orientation on gold surfaces by revealing charge transfer complex formation during real-time surface enhanced Raman scattering tracking. Additionally, the designed nanocavity-based plasmonic substrates exhibit a high sensitivity with a limit of detection ≈ 25 nM.

Based on surface-enhanced Raman scattering (SERS) technology, Au@Ag@4MBA@5′-NH2-ssDNA probes and a Ag/TiO2@3′-NH2-ssDNA substrate were prepared and constructed into a sandwich structure to develop a high sensitivity bioassay of miRNA-21. The Au@Ag@4MBA@5′-NH2-ssDNA probes were prepared by immobilizing 5′-NH2-ssDNA onto the surfaces of 4MBA-labelled Au@Ag core–shell nanoparticles, and the Ag/TiO2@3′-NH2-ssDNA substrate was prepared by immobilizing 3′-NH2-ssDNA on the surface of Ag/TiO2nanowires SERS-active substrates. The experimental results showed that the SERS-based sandwich bioassay of miRNA-21 presented a low limit of detection of 0.75 fM and a broad dynamic range from 1.0 fM to 1.0 nM. Also, the test data for the SERS-based sandwich bioassay were not only consistent with that of the real-time fluorescence quantitative polynucleotide chain reaction (RT-qPCR) method but also displayed higher detection sensitivity. It was shown that the SERS-based sandwich bioassay of miRNA-21 has importance for use in potential applications involving diagnosing clinical cancer patients.

Plasmonic quasi-periodic structures are well-known to exhibit several surprising phenomena with respect to their periodic counterparts, due to their long-range order and higher rotational symmetry. Thanks to their specific geometrical arrangement, plasmonic quasi-crystals offer unique possibilities in tailoring the coupling and propagation of surface plasmons through their lattice, a scenario in which a plethora of fascinating phenomena can take place. In this paper we investigate the extraordinary transmission phenomenon occurring in specifically patterned Thue–Morse nanocavities, demonstrating noticeable enhanced transmission, directly revealed by near-field optical experiments, performed by means of a scanning near-field optical microscope (SNOM). SNOM further provides an intuitive picture of confined plasmon modes inside the nanocavities and confirms that localization of plasmon modes is based on size and depth of nanocavities, while cross talk between close cavities viapropagating plasmons holds the polarization response of patterned quasi-crystals. Our performed numerical simulations are in good agreement with the experimental results. Thus, the control on cavity size and incident polarization can be used to alter the intensity and spatial properties of confined cavity modes in such structures, which can be exploited in order to design a plasmonic device with customized optical properties and desired functionalities, to be used for several applications in quantum plasmonics.

In this paper, the size- and shape-dependent spectral characteristics of plasmonic nanostructures based on the Thue-Morse (ThMo) sequence are investigated in theory and experiment. We designed, fabricated, and characterized nine different Au nanopillars (NPs) lattices to evaluate their use as nanosensors based on localized surface plasmon resonances (LSPR). The extinction spectra and the bulk refractive index sensitivity (m) are compared to three selected shapes of the NPs (square, circular, and triangular) with different minimum interparticle distance. The maximum mof 275 nm/RIU is obtained for a ThMo pattern with square NPs. Finally, a detection limit of 260 pM (62 pg/ml) of Thiram pesticide has been achieved using an LSPR nanosensor based on an optimized ThMo pattern with triangular NPs employing a phase-sensitive setup to increase the figure-of-merit (FOM) of the sensor.

AbstractSurface enhanced Raman scattering (SERS) on eight-fold quasicrystal arrays with precisely controlled size and spacing fabricated via electron beam lithography was investigated. This SERS substrate shows high efficiency at 785 nm excitation in the detection of p-mercaptoaniline (pMA), and a SERS enhancement factor (EF ) of 107is achieved. SERS behavior of the realized engineered SERS substrate indicates that the present engineered metamaterial may be used as an ultrasensitive Raman probe and could open up interesting new opportunities in biosensing.

A new nanostructure, silica-coated Ag nanorods (NRs) aggregate with 4-mercaptobenzoic acid molecules (4MBA-Ag NRs@SiO2), was prepared by a seed-mediated growth method and a modified Stöber method. In the synthetic process, the optimal 4MBA-Ag NRs@SiO2was obtained by varying conditions such as surfactant concentration and centrifugation cycles. The morphologies and the optical properties of the 4MBA-Ag NRs@SiO2were investigated in detail by transmission electron microscope, UV-Vis spectrometer and Raman spectrometer. The experimental results show that the 4MBA-Ag NRs@SiO2prepared with 0.05 M CTAB has the aggregate morphology of single-crystalline Ag NRs, typical localized surface plasmon resonance (LSPR) characteristics and a high enhancement factor of surface-enhanced Raman scattering (SERS). Based on the SERS activity of 4MBA-Ag NRs@SiO2and Ag NRs, an immune probe and an immune substrate immobilized with anti-prostate specific antigen (anti-PSA) antibody have been fabricated and used to construct a sandwich structure for a PSA immunoassay; the detection limit of PSA reached 0.3 fg ml−1. It is demonstrated that the as-prepared 4MBA-Ag NRs@SiO2has great potential in biosensing applications.

This paper presents a novel strategy to fabricate two-dimensional poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) photonic crystals (PCs) combining electron beam lithography (EBL) and plasma etching (PE) processes. The surface morphology of PEDOT:PSS PCs after mild oxygen plasma treatment was investigated by scanning electron microscopy. The effects on light extraction are studied experimentally. Vertical extraction of light was found to be strongly dependent on the geometric parameters of the PCs. By changing the lattice type from triangular to square and the geometrical parameters of the photonic structures, the resonance peak could be tuned from a narrow blue emission at 445 nm up to a green emission at 525 nm with a full width at half-maximum of 20 nm, which is in good agreement with Bragg’s diffraction theory and free photon band structure. Both finite-difference time-domain and plane wave expansion methods are used to calculate the resonant frequencies and the photonic band structures in the two-dimensional photonic crystals showing a very good agreement with the experiment results. A 2D nanopatterned transparent anode was also fabricated onto a flexible polyethylene terephthalate (PET) substrate and it was integrated into an organic light-emitting diode (OLED). The obtained results fully confirm the feasibility of the developed process of micro/nano patterning PEDOT:PSS. Engineered polymer electrodes prepared by this unique method are useful in a wide variety of high-performance flexible organic optoelectronics.

Quasiperiodic crystals (QCs) are a new class of materials that exhibit long-range aperiodic translational order and high rotational symmetries. Unlike periodically arranged photonic crystals (or photonic band-gaps), PQCs possess unique light localization and transport properties related to their complex, multi-fractal energy spectra. Advances in 2D photonic structures are expected in the introduction of non linear and/or active functionality into a 2D PQC. One-dimensional semiconductor nanostructures are likewise promising materials both in fundamental research and in practical applications. CdSe/CdS rods present the appealing characteristics of strong and tunable light emission from green to red, are highly fluorescent and show linearly polarized emission. These characteristics open the way to a new class of hybrid devices based on polymers and colloidal NRs in which the unique optical properties of the inorganic moiety are combined with the processability of the host matrix to develop new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers and non-linear devices. One of the challenges of these applications is the incorporation of inorganic nanoparticles into organic polymer matrices, since this is usually accompanied by phase separation, aggregation of nanoparticles, loss of transparency and luminescence quenching due to exciton energy transfer. In this paper two-dimensional (2D) hybrid PQCs which consist of air rods in a nanocomposite prepared by incorporating CdSe/CdS core/shell nanorods (NR) in a polymer are proposed and experimentally demonstrated. Scanning electron microscopy and far field diffraction are used to characterize the experimental structures.

Electrically Induced Bragg Reflectors can be very attractive to realize programable waveguides networks. Their practical realization is nevertheless intrinsically connected to the capability to make a peculiar comb-structure electrode on the top of the waveguides with typical period of 200 nm (corresponding to the Bragg length) and a tolerance of few nanometers. In this work, the experimental fabrication of these comb-structure electrodes by means of electron beam lithography is described. We fabricated large areas 1D periodic gold structures with nanometer resolution by using a high resolution electron beam lithography (EBL) process and a post-processing technique based on lift-off. These electrodes can be employed as Induced Bragg Reflectors in a multilayer structure for a not permanent periodically modulation of the effective refractive index of the guiding structure. The desired structures are obtained with nanometric resolution by means of EBL, digging furrows of rectangular section in both a polymetilmethacrylate (PMMA) and in -chloromethacrylate and -methylstyrene (ZEP) layer spin-coated on silicon, then evaporating a metal layer (Au) on the top and then by lift-off of metal. The EBL technique allows a very accurate control of the dielectric distribution of the exposed area able to produce feasible, high efficiency periodic and photonic band-gap structures. The resulting 1D gratings are made of metal lines 100 nm wide with a period of 200 nm and, 120 nm wide with a period of 250 nm, respectively. Large area structures (up to 1 mm x 6 m) have been realized with nanometre resolution and they have been characterized by scanning electron microscopy (SEM). These structures will be used in a future work of ours to realize 40 GHz switching speed modulator by inducing a Bragg Reflector with a reverse biased vertical InP/InGaAsP p-i-n diode according to the predictions of the grating reflectivity spectra and of the transient response.

The use of biodegradable polymer films for mulching is a topic of great interest both from the environmental point of view and for the intrinsic properties that the above films, when appropriately designed, might have. In the present article we report on a synthetic approach that, starting from existing biodegradable polymers, leads to a new material characterized by a time-controlled biodegradation. The idea that subsides is to bridge polyvinyl alcohol (PVOH) chains through polycaprolactone (PCL) crosslinks. In such a way, PVOH looses its water sensitivity and can stand on the ground for the time needed for the mulching to occur, while the PCL crosslinks, being sensitive to (slow) fungal attack, will undergo cleavage, followed by the complete bio-assimilation of the residual PVOH chains. The number of crosslinks introduced can control time of biodegradation. The polymers are characterized in their intrinsic chemical-physical properties, while a preliminary evaluation of their efficiency as mulches is obtained through the analysis of their thermal behavior when deposited on an irrigated soil. The results are also compared with a model approach able to predict the thermal behavior of a film in specific environmental conditions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 622–632, 2002

A detailed theimo-optical analysis of dye-doped Polymer Dispersed Liquid Crystal (D-PDLC) systems has been performed. A comparison of the optical properties between D-PDLCs and not doped PDLCs based on the same polymeric mafrix is addressed. For any D-PDLC composition investigated, the light intensity and the sample temperature are used to control or adjust the nonlinear part of its refractive index. We investigated the thermal responses of PDLC samples prepared with three different dyes at two dopant concentrations in order to evaluate the contribute of the dopants as a function also of its percentage in the polymeric matrix. Our experimental results demonstrate that self transparency effect, optical hysteresis and thermo-optical modulation are notably affected by the used dye. We show how the use of dye in preparing PDLC can be useful in order to design thermo-optical devices employing waveguides connected with the considered materials or to realise low cost thermal sensors.

AbstractAn integrated electro-optical switch based on a planar nematic liquid crystal waveguide has been realized and tested. We present the experimental results obtained with a three-stage device, having as middle stage a thin nematic LC film and two glass waveguides as other stages. The electro-optical behavior and the response times have been studied for different configurations. The experimental findings show that an additional bias voltage can improve both the transmittivity and the response time of the device (faster than 100 μs), leading to very promising results in the frame of new integrated electro-optical switches. The effect underlying the latter result is not yet understood and probably need some deep insight into liquid crystal physics and into surface interactions. A further analysis is in progress in order to explain an anomalous electro-optical behavior occurred for higher applied voltages.

AbstractA thermosetting matrix based on a bifunctional epoxy resin was employed to realize a Polymer Dispersed Liquid Crystal (PDLC) system. Two different switching effects from an opaque to a transparent state have been observed: an electro-optical one occurring at a threshold value of the applied electric field and a self-switchi effect of thermal nature, occurring at a critical incident intensity in a PDLC. The experimental results showed promisi switching times with a rise time of 200 μsec and a decay time of 2.2 msec and an exceptionally high contrast ratio up to 430 for the electro-optical switch. The self-transparency effect is taking into account the beam profile variations of the light propagating inside such a medium. A three-dimensional analysis is reported.

AbstractTwo thermosetting matrices based, respectively, on an unsaturated polyester resin and a bifunctional epoxy resin were employed to prepare Polymer Dispersed Liquid Crystal (PDLC) systems. A molecular, morphological and thermo-optical characterisation of these systems is presented. The unsaturated polyester based system was found to be phase-separated at all compositions. Image analysis demonstrated morphological features suitable to achieve interesting thermo-optical properties, particularly for the composition containing 40 wt % of LC. The epoxy based system was found to be miscibie before curing at all compositions. A phase separation occurred upon curing for LC contents higher than 40 wt %. In this composition range also this system behaves as an efficient thermo-optical switch.

AbstractLiquid crystals have effective electro-optic coefficients that are orders of magnitude larger than other materials used in integrated optics. A number of integrated optical devices based on these materials have been recently proposed. We have designed and realized an integrated device in a three-stage planar waveguide, having as middle stage a nematic liquid crystal film. We studied the device performance in different geometries using TE polarized light. By a proper choice of the material parameters we measured time responses in the microsecond range. Our experimental results confirm the possibility of employing such a device working as an optical switch and/or beam deflector.