Directive giant upconversion by supercritical bound states in the continuum

Photonic bound states in the continuum (BICs), embedded in the spectrum of free-space waves1,2 with diverging radiative quality factor, are topologically non-trivial dark modes in open-cavity resonators that have enabled important advances in photonics3,4. However, it is particularly challenging to achieve maximum near-field enhancement, as this requires matching radiative and non-radiative losses. Here we propose the concept of supercritical coupling, drawing inspiration from electromagnetically induced transparency in near-field coupled resonances close to the Friedrich-Wintgen condition2. Supercritical coupling occurs when the near-field coupling between dark and bright modes compensates for the negligible direct far-field coupling with the dark mode. This enables a quasi-BIC field to reach maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. Our experimental design consists of a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, in which a coherent upconversion luminescence enhanced by eight orders of magnitude is observed. The emission shows negligible divergence, narrow width at the microscale and controllable directivity through input focusing and polarization. This approach is relevant to various physical processes, with potential applications for light-source development, energy harvesting and photochemical catalysis.

Probing Denaturation of Protein A via Surface-Enhanced Infrared Absorption Spectroscopy

We apply surface-enhanced infrared absorption (SEIRA) spectroscopy to monitor the denaturation process of a surface-bound protein A monolayer. Our proposed platform relies on a plasmonic metasurface comprising different spatial subregions (“pixels”) that are engineered to exhibit different resonances covering the infrared region of the electromagnetic spectrum that is matched to the vibrational modes of the Amide groups. Specifically, we are able to determine changes in the Amide I and Amide II vibration coupled modes, by comparing the SEIRA reflectance spectra pertaining to the native state and a denatured state induced by a pH variation. In particular, we observe some evident red-shifts in the principal Amide I mode and the Amide II vibration coupled modes (attributable to the breaking of hydrogen bonds), which result in insurmountable barriers for refolding. Thanks to the strong field localization, and consequent enhancement of the light-matter interactions, our proposed sensing platform can operate with extremely small amounts of an analyte, with an estimated detection limit of about 3 femtomoles of molecules.

Reduced graphene oxide on silicon-based structure as novel broadband photodetector

Heterojunction photodetector based on reduced graphene oxide (rGO) has been realized using a spin coating technique. The electrical and optical characterization of bare GO and thermally reduced GO thin films deposited on glass substrate has been carried out. Ultraviolet-visible-infrared transmittance measurements of the GO and rGO thin films revealed broad absorption range, while the absorbance analysis evaluates rGO band gap of about 2.8 eV. The effect of GO reduction process on the photoresponse capability is reported. The current-voltage characteristics and the responsivity of rGO/n-Si based device have been investigated using laser diode wavelengths from UV up to IR spectral range. An energy band diagram of the heterojunction has been proposed to explain the current versus voltage characteristics. The device demonstrates a photoresponse at a broad spectral range with a maximum responsivity and detectivity of 0.20 A/W and 7 × 1010 cmHz/W, respectively. Notably, the obtained results indicate that the rGO based device can be useful for broadband radiation detection compatible with silicon device technology.

Advanced DNA Detection via Multispectral Plasmonic Metasurfaces

We propose and demonstrate a sensing platform based on plasmonic metasurfaces for the detection of very low concentrations of deoxyribo-nucleic acid (DNA) fragments. The platform relies on surface-enhanced infrared absorption spectroscopy, implemented via a multispectral metasurface. Specifically, different regions (“pixels”) are engineered so as to separately cover the medium-infrared range of the electromagnetic spectrum extending from the functional-groups to the fingerprint region of a single analyte. In conjunction with a suitable bio-functionalization, this enables univocal and label-free recognition of specific molecules. For experimental validation, we fabricate a large-area gold metasurface on a silicon chip, and functionalize it with a recognition layer of peptide nucleic acid (PNA). Our experimental results indicate the possibility to detect complementary DNA fragments in concentrations as low as 50 fM, i.e., well below the value attained by standard methods, with additional advantages in terms of processing time, versatility and ease of implementation/operation.

Ultrasensitive Surface Refractive Index Imaging Based on Quasi-Bound States in the Continuum

Herein, we demonstrate a cavity-enhanced hyperspectral refractometric imaging using an all-dielectric photonic crystal slab (PhCS). Our approach takes advantage of the synergy between two mechanisms, surface-enhanced fluorescence (SEF) and refractometric sensing, both based on high-Q resonances in proximity of bound states in the continuum (BICs). The enhanced local optical field of the first resonance amplifies of 2 orders of magnitude the SEF emission of a probe dye. Simultaneously, hyperspectral refractometric sensing, based on Fano interference between second mode and fluorescence emission, is used for mapping the spatially variant refractive index produced by the specimen on the PhCS. The spectral matching between first resonance and input laser is modulated by the specimen local refractive index, and thanks to the calibrated dependence with the spectral shift of the Fano resonance, the cavity tuning is used to achieve an enhanced correlative refractometric map with a resolution of 10^-5 RIU within femtoliter-scale sampling volumes. This is experimentally applied also on live prostate cancer cells grown on the PhCS, reconstructing enhanced surface refractive index images at the single-cell level. This dual mechanism of quasi-BIC spatially variant gain tracked by quasi-BIC refractometric sensing provides a correlative imaging platform that can find application in many fields for monitoring physical and biochemical processes, such as molecular interactions, chemical reactions, or surface cell analysis.

Tuning the exponential sensitivity of a bound-state-in-continuum optical sensor

In this work, we investigate the evanescent field sensing mechanism provided by an all-dielectric metasurface supporting bound states in the continuum (BICs). The metasurface is based on a transparent photonic crystal with subwavelength thickness. The BIC electromagnetic field is localized along the direction normal to the photonic crystal nanoscale-thin slab (PhCS) because of a topology-induced confinement, exponentially decaying in the material to detect. On the other hand, it is totally delocalized in the PhCS plane, which favors versatile and multiplexing sensing schemes. Liquids with different refractive indices, ranging from 1.33 to 1.45, are infiltrated in a microfluidic chamber bonded to the sensing dielectric metasurface. We observe an experimental exponential sensitivity leading to differential values as large as 226 nm/RIU with excellent FOM. This behavior is explained in terms of the physical superposition of the field with the material under investigation and supported by a thorough numerical analysis. The mechanism is then translated to the case of molecular adsorption where a suitable theoretical engineering of the optical structure points out potential sensitivities as large as 4000 nm/RIU.

Near-infrared modulation by means of GeTe/SOI-based metamaterial

Today, nanophotonics still lacks components for modulation that can be easily implementable in existing silicon-on-insulator (SOI) technology. Chalcogenide phase change materials (PCMs) are promising candidates for tuning in the near infrared: at the nanoscale, thin layers can provide enough contrast to control the optical response of a nanostructure. Moreover, all-dielectric metamaterials allow for resonant behavior without having ohmic losses in the telecom range. Here, a novel hybridization of a SOI-based metamaterial with PCM GeTe is experimentally investigated. A metamaterial based on Si nanorods, covered by a thin layer of GeTe, is designed and fabricated. Switching GeTe from amorphous to crystalline leads to a rather high resonance-governed reflection contrast at 1.55 μm. Additional confocal Raman imaging is done to differentiate the crystallized zones of the metamaterials' unit cell. The findings are in good agreement with numerical analysis and show good perspectives of all-dielectric tunable near-infrared nanophotonics.

Internalization kinetics and cytoplasmic localization of functionalized diatomite nanoparticles in cancer cells by Raman imaging

Porous biosilica nanoparticles obtained from diatomites (DNPs) have been recently demonstrated to be non-toxic nanovectors of therapeutic agents in cancer cells. In this work, the internalization kinetics and intracellular spatial distribution of functionalized DNPs incubated with human lung epidermoid carcinoma cell line (H1355) up to 72 hours are investigated by Raman imaging. The label-free Raman results are compared with confocal fluorescence microscopy and photoluminescence (PL) data. Raman bands specifically assigned to DNPs and cellular components provide evidence that the nanovectors are internalized and co-localize with lipid environments. A considerable DNPs uptake in cells is observed within 6 hours, with equilibrium being achieved after 18 hours. The obtained data show the presence of DNPs up to 72 hours, without damage to cell viability or morphology. The PL measurements performed on DNPs not penetrating the cells at different incubation times are strongly correlated with the results obtained by Raman imaging and confocal microscopy analyses.

Optical Biosensors Based on Photonic Crystals Supporting Bound States in the Continuum

A novel optical label-free bio-sensing platform based on a new class of resonances supported in a photonic crystal metasurface is reported herein. Molecular binding is detected as a shift in the resonant wavelength of the bound states in the continuum of radiation modes. The new configuration is applied to the recognition of the interaction between protein p53 and its protein regulatory partner murine double minute 2 (MDM2). A detection limit of 66 nM for the protein p53 is found. The device provides an excellent interrogation stability and loss-free operation, requires minimal optical interrogation equipment and can be easily optimized to work in a wide wavelength range.

Optically monitored drug delivery patch based on porous silicon and polymer microneedles

Fabrication and characterization of an optically monitored hybrid patch for local administration of drugs, based on polymeric micro-needles and a porous silicon free-standing membrane, are reported. The micro-needles are realized by an innovative photolithographic approach that allows fine tuning of geometrical parameters, using polyethylene glycol and a commercial photo-catalyzer. The porous silicon multilayer not only increases the storage of a relevant amount of the drug, but also offers a continuous, naked-eye monitoring of the drug delivery process. As a proof-of-concept experiment, we report our results on the release of a dye molecule (fluorescein, 332 Da) in a phosphate saline buffer.

Hydrodynamic self-focusing in a parallel microfluidic device through cross-filtration

The flow focusing is a fundamental prior step in order to sort, analyze, and detect particles or cells. The standard hydrodynamic approach requires two fluids to be injected into the microfluidic device: one containing the sample and the other one, called the sheath fluid, allows squeezing the sample fluid into a narrow stream. The major drawback of this approach is the high complexity of the layout for microfluidic devices when parallel streams are required. In this work, we present a novel parallelized microfluidic device that enables hydrodynamic focusing in each microchannel using a single feed flow. At each of the parallel channels, a cross-filter region is present that allows removing fluid from the sample fluid. This fluid is used to create local sheath fluids that hydrodynamically pinch the sample fluid. The great advantage of the proposed device is that, since only one inlet is needed, multiple parallel micro-channels can be easily introduced into the design. In the paper, the design method is described and the numerical simulations performed to define the optimal design are summarized. Moreover, the operational functionality of devices tested by using both polystyrene beads and Acute Lymphoid Leukemia cells are shown.

A new strategy for label-free detection of lymphoma cancer cells

In this paper, a new strategy for highly selective and sensitive direct detection of lymphoma cells by exploiting the interaction between a peptide and its B-cell receptor, has been evaluated. In particular, an idiotype peptide, able to specifically bind the B-cell receptor of A20 cells in mice engrafted with A20 lymphoma, has been used as molecular probe. The new detection technique has been demonstrated on a planar crystalline silicon chip. Coverage of 85% of silicon surface and detection efficiency of 8.5 × 10(-3) cells/μm(2) were obtained. The recognition strategy promises to extend its application in studying the interaction between ligands and their cell-surface receptors.

Biologically enabled sub-diffractive focusing

Evolution shows that photonic structures are a constituent part of many animals and flora. These elements produce structural color and are useful in predator-prey interactions between animals and in the exploitation of light for photosynthetic organisms. In particular, diatoms have evolved patterned hydrated silica external valves able to confine light with extraordinary efficiency. Their evolution was probably guided by the necessity to survive in harsh conditions of sunlight deprivation. Here, we exploit such diatom valves, in conjunction with structured illumination, to realize a biological super-resolving lens to achieve sub-diffractive focusing in the far field. More precisely, we consider a single diatom valve of Arachnoidiscus genus which shows symmetries and fine features. By characterizing and using the transmission properties of this valve using the optical eigenmode technique, we are able to confine light to a tiny spot with unprecedented precision in terms of resolution limit ratio, corresponding in this case to 0.21λ/NA.

Optical properties of diatom nanostructured biosilica in Arachnoidiscus sp: micro-optics from mother nature

Some natural structures show three-dimensional morphologies on the micro- and nano- scale, characterized by levels of symmetry and complexity well far beyond those fabricated by best technologies available. This is the case of diatoms, unicellular microalgae, whose protoplasm is enclosed in a nanoporous microshell, made of hydrogenated amorphous silica, called frustule. We have studied the optical properties of Arachnoidiscus sp. single valves both in visible and ultraviolet range. We found photonic effects due to diffraction by ordered pattern of pores and slits, accordingly to an elaborated theoretical model. For the first time, we experimentally revealed spatial separation of focused light in different spots, which could be the basis of a micro-bio-spectrometer. Characterization of such intricate structures can be of great inspiration for photonic devices of next generation.

Synthesis of mixed-sequence oligonucleotides on mesoporous silicon: chemical strategies and material stability

Rapid screening tests in medical diagnostic and environmental analysis are often based on oligonucleotide biochips. In this paper, we studied the stability of functionalized mesoporous silicon supports in the solid-phase synthesis of oligonucleotides, exploiting several chemical procedures. A 19-mer mixed sequence has been successfully synthesized on aminosilane-modified porous silicon photonic structures. The process and the materials have been characterized by optical reflectivity, atomic force microscopy and high-performance liquid chromatography.

Diatomite silica nanoparticles for drug delivery

Diatomite is a natural fossil material of sedimentary origin, constituted by fragments of diatom siliceous skeletons. In this preliminary work, the properties of diatomite nanoparticles as potential system for the delivery of drugs in cancer cells were exploited. A purification procedure, based on thermal treatments in strong acid solutions, was used to remove inorganic and organic impurities from diatomite and to make them a safe material for medical applications. The micrometric diatomite powder was reduced in nanoparticles by mechanical crushing, sonication, and filtering. Morphological analysis performed by dynamic light scattering and transmission electron microscopy reveals a particles size included between 100 and 300 nm. Diatomite nanoparticles were functionalized by 3-aminopropyltriethoxysilane and labeled by tetramethylrhodamine isothiocyanate. Different concentrations of chemically modified nanoparticles were incubated with cancer cells and confocal microscopy was performed. Imaging analysis showed an efficient cellular uptake and homogeneous distribution of nanoparticles in cytoplasm and nucleus, thus suggesting their potentiality as nanocarriers for drug delivery.

PACS

87.85.J81.05.Rm; 61.46. + w.

Asymmetric MSM sub-bandgap all-silicon photodetector with low dark current

Design, fabrication, and characterization of an asymmetric metal-semiconductor-metal photodetector, based on internal photoemission effect and integrated into a silicon-on-insulator waveguide, are reported. For this photodetector, a responsivity of 4.5 mA/W has been measured at 1550 nm, making it suitable for power monitoring applications. Because the absorbing metal is deposited strictly around the vertical output facet of the waveguide, a very small contact area of about 3 µm2 is obtained and a transit-time-limited bandwidth of about 1 GHz is demonstrated. Taking advantage of this small area and electrode asymmetry, a significant reduction in the dark current (2.2 nA at -21 V) is achieved. Interestingly, applying reverse voltage, the photodetector is able to tune its cut-off wavelength, extending its range of application into the MID infrared regime.

Aminosilane functionalizations of mesoporous oxidized silicon for oligonucleotide synthesis and detection

Direct solid phase synthesis of peptides and oligonucleotides (ONs) requires high chemical stability of the support material. In this work, we have investigated the passivation ability of porous oxidized silicon multilayered structures by two aminosilane compounds, 3-aminopropyltriethoxysilane and 3-aminopropyldimethylethoxysilane (APDMES), for optical label-free ON biosensor fabrication. We have also studied by spectroscopic reflectometry the hybridization between a 13 bases ON, directly grown on the aminosilane modified porous oxidized silicon by in situ synthesis, and its complementary sequence. Even if the results show that both devices are stable to the chemicals (carbonate/methanol) used, the porous silica structure passivated by APDMES reveals higher functionalization degree due to less steric hindrance of pores.

Critically coupled silicon Fabry-Perot photodetectors based on the internal photoemission effect at 1550 nm

In this paper, design, fabrication and characterization of an all-silicon photodetector (PD) at 1550 nm, have been reported. Our device is a surface-illuminated PD constituted by a Fabry-Perot microcavity incorporating a Cu/p-Si Schottky diode. Its absorption mechanism, based on the internal photoemission effect (IPE), has been enhanced by critical coupling condition. Our experimental findings prove a peak responsivity of 0.063 mA/W, which is the highest value obtained in a surface-illuminated IPE-based Si PD around 1550 nm. Finally, device capacitance measurements have been carried out demonstrating a capacitance < 5 pF which has the potential for GHz operation subject to a reduction of the series resistance of the ohmic contact.

Hydrophobin Vmh2-glucose complexes self-assemble in nanometric biofilms

Hydrophobins are small proteins secreted by fungi, which self-assemble into amphipathic membranes at air-liquid or liquid-solid interfaces. The physical and chemical properties of some hydrophobins, both in solution and as a biofilm, are affected by poly or oligosaccharides. We have studied the interaction between glucose and the hydrophobin Vmh2 from Pleurotus ostreatus by spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and water contact angle (WCA). We have found that Vmh2-glucose complexes forms a chemically stable biofilm, obtained by drop deposition on silicon, 1.6 nm thick and containing 35 per cent of glucose, quantified by SE. AFM highlighted the presence of nanometric rodlet-like aggregates (average height, width and length being equal to 3.6, 23.8 and 40 nm, respectively) on the biofilm surface, slightly different from those obtained in the absence of glucose (4.11, 23.9 and 64 nm). The wettability of a silicon surface, covered by the organic layer of Vmh2-glucose, strongly changed: WCA decreased from 90° down to 17°.

A microfluidics assisted porous silicon array for optical label-free biochemical sensing

A porous silicon (PSi) based microarray has been integrated with a microfluidic system, as a proof of concept device for the optical monitoring of selective label-free DNA-DNA interaction. A 4 × 4 square matrix of PSi one dimensional photonic crystals, each one of 200 μm diameter and spaced by 600 μm, has been sealed by a polydimethylsiloxane (PDMS) channels circuit. The PSi optical microarray elements have been functionalized by DNA single strands after sealing: the microfluidic circuit allows to reduce significantly the biologicals and chemicals consumption, and also the incubation time with respect to a not integrated device. Theoretical calculations, based on finite element method, taking into account molecular interactions, are in good agreement with the experimental results, and the developed numerical model can be used for device optimization. The functionalization process and the interaction between DNA probe and target has been monitored by spectroscopic reflectometry for each PSi element in the microchannels.

An extraordinary directive radiation based on optical antimatter at near infrared

In this paper we discuss and experimentally demonstrate that in a quasi- zero-average-refractive-index (QZAI) metamaterial, in correspondence of a divergent source in near infrared (λ = 1.55 μm) the light scattered out is extremely directive (Δθ(out) = 0.06°), coupling with diffraction order of the alternating complementary media grating. With a high degree of accuracy the measurements prove also the excellent vertical confinement of the beam even in the air region of the metamaterial, in absence of any simple vertical confinement mechanism. This extremely sensitive device works on a large contact area and open news perspective to integrated spectroscopy.

Multi-wavelength study of light transmitted through a single marine centric diatom

The characterization of partially coherent light transmission by micrometer sized valves of marine diatoms is an interesting optical challenge and, from the biological point of view, is of outmost relevance in order to understand evolution mechanisms of such organisms. In the present work, we have studied the transmission of light coming from a monochromator through single valves of Coscinodiscus wailesii diatoms. Incoming light is confined by the regular pore pattern of the diatom surface into a spot of few microns, its dimensions depending on wavelength. The effect is ascribed to the superposition of wavefronts diffracted by the pores' edges. Numerical simulations help to demonstrate how this effect is not present in the ultraviolet region of the light spectrum, showing one of the possible evolutionary advantages represented by the regular pores patterns of the valves.

Bioactive modification of silicon surface using self-assembled hydrophobins from Pleurotus ostreatus

A crystalline silicon surface can be made biocompatible and chemically stable by a self-assembled biofilm of proteins, the hydrophobins (HFBs) purified from the fungus Pleurotus ostreatus. The protein-modified silicon surface shows an improvement in wettability and is suitable for immobilization of other proteins. Two different proteins were successfully immobilized on the HFBs-coated chips: the bovine serum albumin and an enzyme, a laccase, which retains its catalytic activity even when bound on the chip. Variable-angle spectroscopic ellipsometry (VASE), water contact angle (WCA), and fluorescence measurements demonstrated that the proposed approach in silicon surface bioactivation is a feasible strategy for the fabrication of a new class of hybrid devices.

Self-collimation of light over millimeter-scale distance in a quasi-zero-average-index metamaterial

Inspired by the concept of complementary media, we experimentally demonstrate that an engineered metamaterial made of alternating, stripe layers of negatively refracting (photonic crystals) and positively refracting (air) materials strongly collimates a beam of near-infrared light. This quasi-zero-average-index metamaterial fully preserves the beam spot size throughout the sample for a light beam traveling through the metamaterial a distance of 2 mm-more than 1000 times the input wavelength lambda=1.55 microm. These results demonstrate the first explicit experimental verification of optical antimatter as proposed by Pendry and Ramakrishna [J. Pendry and S. Ramakrishna, J. Phys. Condens. Matter 15, 6345 (2003)10.1088/0953-8984/15/37/004], using two complementary media in which each n(eff)=-1 layer appears to annihilate an equal thickness layer of air.

Protein conformational changes revealed by optical spectroscopic reflectometry in porous silicon multilayers

The protein-ligand molecular interactions imply strong geometrical and structural rearrangements of the biological complex which are normally detected by high sensitivity optical techniques such as time-resolved fluorescence microscopy. In this work, we have measured, by optical spectroscopic reflectometry in the visible-near-infrared region, the interaction between a sugar binding protein (SBP), covalently bound on the surface of a porous silicon (PSi) microcavity, and glucose, at different concentrations and temperatures. Variable-angle spectroscopic ellipsometric (VASE) characterization of protein-functionalized PSi layers confirms that the protein-ligand system has an overall volume smaller than the SBP alone.

A parametric study of laser induced ablation-oxidation on porous silicon surfaces

We have investigated the laser induced ablation-oxidation process on porous silicon layers having different porosities and thicknesses by non-destructive optical techniques. In particular, the interaction between a low power blue light laser and the porous silicon surfaces has been characterized by variable angle spectroscopic ellipsometry and Fourier transform infrared spectroscopy. The oxidation profiles etched on the porous samples can be tuned as functions of the layer porosity and laser fluence. Oxide stripes of width less than 2 µm and with thicknesses between 100 nm and 5 µm have been produced, depending on the porosity of the porous silicon, by using a 40 × focusing objective.

Lensless light focusing with the centric marine diatom Coscinodiscus walesii

In this work, we report on the light focusing ability exploited by the microshell of a marine organism: the Coscinodiscus wailesii diatom. A 100 microm spot size of a red laser beam is narrowed up to less than 10 microm at a distance of 104 microm after the transmission through the regular geometry of the diatom structure, which thus acts as a microlens. Numerical simulations of the electromagnetic field propagation show a good qualitative agreement with the experimental results. The focusing effect is due to the superposition of the waves scattered by the holes present on the surface of the diatom valve. Very interesting applications in micro-optic devices are feasible due to the morphological and biological characteristic of these unicellular organisms.

Self-assembled biofilm of hydrophobins protects the silicon surface in the KOH wet etch process

The anisotropic wet micromachining of silicon, based on a water solution of potassium hydroxide (KOH), is a standard fabrication process that is extensively exploited in the realization of very complex microsystems, which comprise cantilevers, membranes, and bridges. A nanostructured self-assembled biofilm of amphiphilic proteins, the hydrophobins, was deposited on crystalline silicon by solution deposition and characterized by variable-angle spectroscopic ellipsometry (VASE). This procedure formed chemically and mechanically stable mono- and multilayers of self-assembled proteins. The biomolecular membrane has been tested as masking material in the KOH wet etch of the crystalline silicon. The process has been monitored by VASE and atomic force microscopy measurements. Because of the high persistence of the protein biofilm, the hydrophobin-coated silicon surface is perfectly protected during the standard KOH micromachining process.

Influence of surface termination on negative reflection by photonic crystals

When a wave impinges obliquely to the interface of a Photonic Crystal (PhC), the wave can be completely reflected in counter-propagating direction instead of the usually expected specular direction. However the beam is totally specularly reflected with a simple modification of the surface termination. The analysis of the time average Poynting vector evidences that PhC termination modifies the energy flow and determines the reflection properties.

Enzymes and proteins from extremophiles as hyperstable probes in nanotechnology: the use of D-trehalose/D-maltose-binding protein from the hyperthermophilic archaeon Thermococcus litoralis for sugars monitoring

The D-trehalose/D-maltose-binding protein (TMBP), a monomeric protein of 48 kDa, is one component of the trehalose and maltose (Mal) uptake system. In the hyperthermophilic archaeon Thermococcus litoralis, this is mediated by a protein-dependent ATP-binding cassette system transporter. The gene coding for a thermostable TMBP from the archaeon T. litoralis has been cloned, and the recombinant protein has been expressed in E. coli. The recombinant TMBP has been purified to homogeneity and characterized. It exhibits the same functional and structural properties as the native one. In fact, it is highly thermostable and binds sugars, such as maltose, trehalose and glucose, with high affinity. In this work, we have immobilized TMBP on a porous silicon wafer. The immobilization of TMBP to the chip was monitored by reflectivity and Fourier Transformed Infrared spectroscopy. Furthermore, we have tested the optical response of the protein-Chip complex to glucose binding. The obtained data suggest the use of this protein for the design of advanced optical non-consuming analyte biosensors for glucose detection.

Photonic band gaps analysis of Thue-Morse multilayers made of porous silicon

Dielectric aperiodic Thue-Morse structures up to 128 layers have been fabricated by using porous silicon technology. The photonic band gap properties of Thue-Morse multilayers have been theoretically investigated by means of the transfer matrix method and the integrated density of states. The theoretical approach has been compared and discussed with the reflectivity measurements at variable angles for both the transverse electric and transverse magnetic polarizations of light. The photonic band gap regions, wide 70 nm and 90 nm, included between 0 and 30 degrees , have been observed for the sixth and seventh orders, respectively.

Glutamine-Binding Protein from Escherichia coli Specifically Binds a Wheat Gliadin Peptide Allowing the Design of a New Porous Silicon-Based Optical Biosensor

In this work, the binding of the recombinant glutamine-binding protein (GlnBP) from Escherichia coli to gliadin peptides, toxic for celiac patients, was investigated by mass spectrometry experiments and optical techniques. Mass spectrometry experiments demonstrated that GlnBP binds the following amino acid sequence: XXQPQPQQQQQQQQQQQQL, present only into the toxic prolamines. The binding of GlnBP to gliadin suggested us to design a new optical biosensor based on nanostructured porous silicon (PSi) for the detection of trace amounts of gliadin in food. The GlnBP, which acts as a molecular probe for the gliadin, was covalently linked to the surface of the PSi wafer by a proper passivation process. The GlnBP-gliadin interaction was revealed as a shift in wavelength of the fringes in the reflectivity spectrum of the PSi layer. The GlnBP, covalently bonded to the PSi chip, selectively recognized the toxic peptide. Finally, the sensor response to the protein concentration was measured in the range 2.0-40.0 microg/L and the sensitivity of the sensor was determined.

Porous silicon-based optical microsensor for the detection of l-glutamine

The molecular binding between the glutamine-binding protein (GlnBP) from Escherichia coli and L-glutamine (Gln) is optically transduced by means of a biosensor based on porous silicon nano-technology. The sensor operates by the measurement of the interferometric fringes in the reflectivity spectrum of a porous silicon Fabry-Perot layer. The binding event is revealed as a shift in wavelength of the fringes. Due to the hydrophobic interaction with the Si-H terminated surface of the porous silicon, the GlnBP protein, which acts as a molecular probe for Gln, penetrates and links into the pores of the porous silicon matrix. We can thus avoid any preliminary functionalization process of the porous layer surface, which is also prevented from oxidation, at least for few cycles of wet measurements. The binding of Gln to GlnBP has also been investigated at different concentration of GlnBP.

A polarizing beam splitter using negative refraction of photonic crystals

Light passing through a photonic crystal can undergo a negative or a positive refraction. The two refraction states can be functions of the contrast index, the incident angle and the slab thickness. By suitably using these properties it is possible to realize very simple and very efficient optical components to route the light. As an example we present a passive device acting as a polarizing beam splitter where TM polarization is refracted in positive direction whereas TE component is negatively refracted.

Optical microsensors for pesticides identification based on porous silicon technology

A simple and low cost optical sensor, based on porous silicon nanotechnology, has been used to detect and quantify the presence of atrazine pesticide in water and humic acid solutions. In both cases, a well defined optical signal variation can be registered, even at low concentration as 1 ppm. The phenomenon can be ascribed to the capillary infiltration of liquid into the pores, which changes the average refractive index of the structure. Due to the resonant cavity enhanced operation of the proposed sensors, very low detection limits can be reached.

Smart optical sensors for chemical substances based on porous silicon technology

A simple geometry optical sensor based on porous silicon technology is theoretically and experimentally studied. We expose some porous silicon optical microcavities with different porous structures to several substances of environmental interest: Very large red shifts in the single transmission peak in the reflectivity spectrum due to changes in the average refractive index are observed. The phenomenon can be ascribed to capillary condensation of vapor phases in the silicon pores. We numerically compute the peak shifts as a function of the liquid volume fraction condensed into the stack by using the Bruggeman theory. The results presented are promising for vapor and liquid detection and identification.

Thermo-optic design for microwave and millimeter-wave electromagnetic power microsensors

Rendina et al. recently proposed the original configuration of an electromagnetic power sensor for microwaves and millimeter waves that is based on an optically interrogated all-silicon chip [Electron. Lett. 35, 1748 (1999)]. Here we theoretically analyze and discuss in detail the performances of such a new class of nonperturbing and wideband probe in terms of sensitivity, resolution, intrinsic detectivity, linearity, and response time. Good agreement between theory and experiments is demonstrated. In particular, minimum resolutions of approximately 1 mW/cm2 are obtained at frequencies beyond 10 GHz. The dependence of response on the geometrical and electromagnetic parameters of the sensing element is analyzed, and on this basis the possibility of achieving optimized configurations is discussed.

Amorphous silicon waveguides and light modulators for integrated photonics realized by low-temperature plasma-enhanced chemical-vapor deposition

A new amorphous silicon waveguide is realized by use of amorphous silicon carbon as cladding material. The structure is characterized both experimentally and theoretically, and its application for optical interconnections in photonic integrated circuits on silicon motherboards is proposed. The fabrication process is based on low-temperature (220 degrees C) plasma-enhanced chemical-vapor deposition and is compatible with standard microelectronic processes. Propagation losses of 1.8 dB/cm have been measured at the fiber-optic wavelength of 1.3 microm. A strong thermo-optic coefficient has been measured in this material at this wavelength and exploited for the realization of a light-intensity modulator based on a Fabry-Perot interferometer that is tunable by temperature.

All-silicon Fabry-Perot modulator based on the thermo-optic effect

The operation at 1.5 microm of a silicon Fabry-Perot optical modulator is reported. The electrically driven device, which uses the thermo-optic effect to achieve as much as a 55% intensity modulation depth, has been realized by means of standard silicon microelectronic technology. This demonstrates that this new type of optical modulator can easily be integrated with electronic circuits. An accurate three-dimensional thermal analysis of the device has permitted the setup of a reliable numerical code aimed at the design of optimized integrated versions of it. The simulation outputs therefore predict operation frequencies of hundreds of kilohertz, remarkably superior to those previously reported in thermo-optic-effect-based modulators.

Tapered and inhomogeneous dielectric light concentrators

A detailed analysis of tapered and inhomogeneous dielectric light collectors was worked out for both illumination and solar energy applications. In particular, tapered dielectric guides have been investigated both theoretically and experimentally together with their capability to collect and transmit high fluxes of light energy. Furthermore, GRIN rods are considered as matching devices, to improve the collecting performance of tapered guides.

Optoelectronic technique for the characterization of high concentration gas-solid suspension

We describe a simple technique for measuring high (up to 0.16) time-averaged solids volumetric concentration in a two-phase flow. The technique is based on a properly modified version of the forward scattering of laser light. It is useful in a variety of practical configurations, and, in particular, it is instrumental in the diagnostics of particle flow in the free board of bubbling fluidized beds and in the circulating fluidized beds. A fallout of this work is the measurement of the extinction coefficient of the solid material tested.