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The detection of Raman signals from diluted molecules or biomaterials in complex media is still a challenge. Besides the widely studied Raman enhancement by nanoparticle plasmons, interference mechanisms provide an interesting option. A novel approach for amplification platforms based on supported thin alumina membranes was designed and fabricated to optimize the interference processes. The dielectric layer is the extremely thin alumina membrane itself and, its metallic aluminum support, the reflecting medium. A CVD (chemical vapor deposition) single-layer graphene is transferred on the membrane to serve as substrate to deposit the analyte. Experimental results and simulations of the interference processes were employed to determine the relevant parameters of the structure to optimize the Raman enhancement factor (E.F.). Highly homogeneous E.F. over the platform surface are obtained, typically 370 ± (5%), for membranes with ~100 nm pore depth, ~18 nm pore diameter and the complete elimination of the Al2O3 bottom barrier layer. The combined surface enhanced Raman scattering (SERS) and interference amplification is also demonstrated by depositing ultra-small silver nanoparticles. This new approach to amplify the Raman signal of analytes is easily obtained, low-cost and robust with useful enhancement factors (~400) and allows only interference or combined enhancement mechanisms, depending on the analyte requirements.

Europium-doped La2O3 nanocrystalline powders with sizes ranging from 4 nm to 300 nm have been obtained by the modified Pechini, hydrothermal with conventional furnace, hydrothermal with microwave furnace, and precipitation with ultrasonic bath methods. X-ray diffraction techniques were used to study the evolution of the prepared gels towards the desired crystalline phase. We determined the size and the morphology of the nanoparticles by electronic microscopy. Finally, we studied and analyzed the luminescence properties of the trivalent europium in the hexagonal La2O3 nanocrystals by photoluminescence and cathodoluminescence.

[EN] Silicon is the material of choice for visible light photodetection and solar cell fabrication. However, due to the intrinsic band gap properties of silicon, most infrared photons are energetically useless. Here, we show the first example of a photodiode developed on a micrometre scale sphere made of polycrystalline silicon whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the photocurrent response, extending it into the infrared region well beyond the absorption edge of bulk silicon. It opens the door for developing solar cells and photodetectors that may harvest infrared light more efficiently than silicon photovoltaic devices that are so far developed.

Silicon is the material of choice for visible light photodetection and solar cell fabrication. However, due to the intrinsic band gap properties of silicon, most infrared photons are energetically useless. Here, we show the first example of a photodiode developed on a micrometre scale sphere made of polycrystalline silicon whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the photocurrent response, extending it into the infrared region well beyond the absorption edge of bulk silicon. It opens the door for developing solar cells and photodetectors that may harvest infrared light more efficiently than silicon photovoltaic devices that are so far developed.

10.1186/1556-276X-7-228, We report on an exhaustive and systematic study about the photoluminescent properties of nanoporous anodic alumina membranes fabricated by the one-step anodization process under hard conditions in oxalic and malonic acids. This optical property is analysed as a function of several parameters (i.e. hard anodization voltage, pore diameter, membrane thickness, annealing temperature and acid electrolyte). This analysis makes it possible to tune the photoluminescent behaviour at will simply by modifying the structural characteristics of these membranes. This structural tuning ability is of special interest in such fields as optoelectronics, in which an accurate design of the basic nanostructures (e.g. microcavities, resonators, filters, supports, etc.) yields the control over their optical properties and, thus, upon the performance of the nanodevices derived from them (biosensors, interferometers, selective filters, etc.)

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO4 crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO4 crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO4 (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented., Peer Reviewed, Postprint (published version)

The deposition of specific materials into ordered pores arranged in a regular lattice allows the fabrication of ordered fiber arrays. Polymer micro- and nanofibers using macroporous silicon and nanoporous alumina templates were fabricated employing a vacuum infiltration method for Poly(methyl methacrylate) (PMMA) structures or a melt-assisted template wetting into the pores for Poly(3-hexylthiophene) (P3HT) structures. We have studied how the aspect ratio of the micro- and nanopores and the kind of polymer influence the structures fabrication., Peer Reviewed, Postprint (published version)

A study was conducted to demonstrate the infiltration of polystyrene (PS) spheres inside the pores of silicon membrane. The study conducted significant investigations of the particle arrangement for D values between 1 and 3. The study employed charge-stabilized colloidal particles to conduct investigations. It was demonstrated that sulphate-stabilized PS spheres were infiltrated into the silicon membrane. A combination of PS spheres, with different diameters and high-quality porous silicon membranes were used to cover the values of D and conduct investigations. The silicon membrane was manufactured by electrochemical etching procedure, which allows very long pores with vertical walls. It was also observed that the pore diameter decreases to a value around 50% or less at the bottom from a normal value at the surface.

Microtubes and microfibres composed of poly(3-hexylthiophene) (P3HT) were fabricated by melt-assisted templates using ordered macroporous silicon. We have studied the influence of the pore depth and the template type on the microstructure fabrication, where the templates were membranes or structures opened only at one end. Current– voltage (I–V) measurements demostrated that the resistivity of these P3HT microstructures was in the same order as that of homogeneous films, which allows them to be used in electronic devices., Peer Reviewed, Postprint (published version)

This work focuses on the fabrication of three-dimensional (3D) microstructures by electrochemical etching of n-type silicon. We exploit the possibility of modulating the pore diameter along the growing direction to produce quasi-3D structures which exhibit periodicity also in third dimension. The fabricated structures are further processed to convert them into truly 3D networks consisting of interconnected silicon veins embedded in air. In particular, we show how a variety of complex 3D networks can be obtained starting from a simple initial pore arrangement, e.g. square or hexagonal. These 3D structures can be easily fabricated with very high precision and uniformity on a large scale. They constitute ideal hosts for sensing applications as well as perfect molds for casting 3D structures of unusual materials., Peer Reviewed, Postprint (published version)

We report the experimental characterization of two-dimensional (2D) macroporous silicon photonic crystals using angular-dependent reflectance spectroscopy in the mid-IR region. We have investigated different sample structures and we have shown that an adequate post-processing of the measured data is crucial in order to recognize the photonic bands and to achieve a good agreement of the measured data with the theoretical predictions for the studied structures., Peer Reviewed, Postprint (published version)

Macroporous silicon membranes, prepared by photo-electrochemical etching, were subjected to pore widening performed by multiple oxidation/oxide-stripping cycles. We study the dependence of pore cross section and diameter on the oxidation parameters. The anisotropy of the oxidation process allows us to correct the faceted pore shape after the etching and to obtain pores with circular cross section. The shown post-treatment of macroporous silicon extends the possibilities of the electrochemical etching process to produce structures with a desired geometry., Peer Reviewed, Postprint (published version)

In this work, we study the changes in the optical properties of 300-nm-thick hydrogenated amorphous silicon carbide layers after an annealing process. Both intrinsic and phosphorus-doped amorphous silicon carbide layers (a-SiCx:H) were deposited on silicon wafers by plasma enhanced chemical vapour deposition (PECVD) at 400 °C and annealed in a quartz furnace at 800 °C. The presence of randomly oriented silicon nanocrystals was confirmed by X-ray diffraction (XRD) measurements after the partial recrystallization process only in the doped layers. The presence or the absence of the nanocrystals clearly changes the Fourier transform infrared (FTIR) spectra. From the fitting of the experimental curves with the model of Lorentz oscillators, the refractive index and the extinction coefficient of the different layers were obtained., Peer Reviewed, Postprint (published version)

This article describes the benefits of including cross-layer information in the scheduling mechanism of a UMTS downlink channel. In particular, the information obtained from the fast power control algorithm is used to properly schedule transmissions. A prioritization function that exploits the short-term channel variations is proposed. This strategy is shown to be a feasible approach to improve system performance in terms of capacity and delay. This enhancement is obtained as a benefit of intrinsic multi-user diversity. The proposal is applicable within the current UMTS radio resource management framework., Peer Reviewed, Postprint (published version)

This paper presents a novel technique for silicon dioxide (SiO2) microneedle fabrication. Microneedles are hollow microcapillaries with tip diameters in the range of micrometers. They can be used in the fabrication of microsyringes. These structures can be of high interest in medical and biological applications, such as DNA injection, antibody manipulation and drug delivery, and cell manipulation. Fabrication process is based on electrochemical etching of n-type silicon in hydrofluoric acid (HF) solutions. Basic process flow and etching conditions that ensure a stable pore growth are described. These conditions also determine the geometry of the resulting microneedle structure. Microneedle arrays of different dimensions can be fabricated in a single run on the same wafer. In this work, microneedle arrays with pore diameters ranging from 2 to 5 µm, pore lengths from 30 to 140 µm and wall thicknesses in the range of 70–110 nm are reported., Peer Reviewed, Postprint (published version)

This paper reports on the fabrication of two-dimensional macropore arrays by electrochemical etching of silicon. We describe some critical effects accompanying the etching process that can generate imperfections in the etched pore pattern. We also give the solutions to improve the pore uniformity and to produce high-aspect-ratio macropores with good structural characteristics. The obtained macroporous structures can be further processed to produce high-aspect-ratio pillars, instead of pores. By modulating the applied etching current we have also induced periodicity in the third dimension., Peer Reviewed, Postprint (published version)

We investigate the absolute photonic band gap (PBG) formation in two-dimensional (2-D) photonic crystals designed using symmetry reduction approach. The lattice symmetry, shape and orientation of dielectric scatterers affect the photonic gap parameters. We use symmetry reduction, achieved either by including additional rods into the lattice unit cell or by reorienting noncircular scatterers to engineer the photonic band gaps in 2-D square and triangular structures. The case of air rods drilled into silicon background is considered. We show that for square structures symmetry reduction can be an effective way to enlarge the absolute PBG, but for triangular lattices any modification of the crystal structure considerably reduces the absolute PBG width. We also discuss the practical technological feasibility of the proposed structures., Peer Reviewed, Postprint (published version)

We investigate different aspects of the absolute photonic band gap (PBG) formation in two-dimensional photonic structures consisting of rods covered with a thin dielectric film. Specifically, triangular and honeycomb lattices in both complementary arrangements, i.e., air rods drilled in silicon matrix and silicon rods in air, are studied. We consider that the rods are formed of a dielectric core (silicon or air) surrounded by a cladding layer of silicon dioxide (SiO2), silicon nitride (Si3N4), or germanium (Ge). Such photonic lattices present absolute photonic band gaps, and we study the evolution of these gaps as functions of the cladding material and thickness. Our results show that in the case of air rods in dielectric media the existence of dielectric cladding reduces the absolute gap width and may cause complete closure of the gap if thick layers are considered. For the case of dielectric rods in air, however, the existence of a cladding layer can be advantageous and larger absolute PBG’s can be achieved., Peer Reviewed, Postprint (published version)