Search

Silicon-rich silicon oxide films deposited by plasma enhanced chemical vapor deposition with different total Si contents (from 39 to 46 at. %) have been annealed at increasing temperature (up to 1250 °C) in order to study the Si nanocrystal (Si-nc) nucleation as well as the structural changes induced in the amorphous embedding matrix. The comparison between x-ray absorption measurements in total electron yield mode, Raman spectroscopy, and photoluminescence spectra allowed us to gain insight about the Si nanocrystal formation, while the chemical composition and the nature of chemical bonds into the oxidized matrix was studied by Fourier transform infrared spectroscopy. A comprehensive picture of the nucleation process has been obtained, demonstrating the active role played by the hydrogen and nitrogen atoms in the formation of Si-nc and in the thermally induced evolution of the deposited films. [ABSTRACT FROM AUTHOR]

Abstract: Si-rich silicon oxide and SiO2 (SRSO)/SiO2 multilayer (ML) samples were grown by reactive magnetron sputtering and then annealed at high temperature to induce the formation of Si-nc with mean size of 3–4nm and density of about 3.5×1018 cm−3 as deduced from high resolution TEM micrographs. Refractive index and thickness have been determined by m-line measurements, which have shown a birefringence of about 1.5% due to the ML structure. Rib-loaded waveguides have been fabricated to measure propagation losses in the visible–infrared range. The analysis of the different contributions to optical losses such as Mie scattering and scattering due to waveguide roughness has allowed us to isolate the contribution due to the absorption losses and thus to extract the absorption cross-section at different wavelengths. Values of about 3.5×10−18 cm2 have been found at 830nm, increasing with decreasing of the wavelength. [Copyright &y& Elsevier]

A Fabry-Perot optical microcavity based on liquid crystal infiltration integrated in a waveguide has been designed and fabricated. Careful optimization of the cavity geometry allows designing a device with a simulated resonance peak transmission of 95% with a linewidth of about 1.7 nm. Based on this simulation, a first prototype has been fabricated and tested. [ABSTRACT FROM AUTHOR]

Abstract: In view of the integration within Si-based optical devices, LPCVD (low-pressure chemical vapor deposition) thin-film Si3N4 waveguides have been fabricated on a Si substrate within a CMOS fabrication pilot-line. Different structures (channel, rib and strip-loaded) were designed, fabricated and characterized both optically and structurally to optimize waveguide performances. Geometry, sidewall as well as layer roughness of the waveguides have been investigated by scanning electron microscopy and atomic force microscopy. Optical guided modes have been observed and propagation loss measurements at 632.8 and 780nm have been performed by using the cut-back technique, the insertion loss technique and scattered light collection. The channel waveguides have shown propagation losses of about 0.1–0.2dB/cm. Differences between geometries and lithographic processes have been discussed. Polarization dependence of propagation losses has been investigated too. Optical guided modes have also been measured in the near-infrared range (at 1544nm), where propagation losses are about 4.5–5dB/cm, quite larger with respect to the visible, because of the poorer confinement factor of the optical modes. [Copyright &y& Elsevier]

X-ray absorption spectra obtained by total electron yield (TEY) at the Si absorption K-edge have been measured to have chemical and structural information about Si nanocrystals (Si-nc) produced by plasma-enhanced chemical vapour deposition (PECVD). The TEY technique has been employed to investigate the formation of Si-nc and the modification of the silica matrix as a function of annealing temperature (500–1250°C) and of silicon content in the film (35–46 at%). The amount of silicon present in the Si-nc has been evaluated by TEY. Thanks to Rutherford backscattering spectrometry measurements, the amount of Si atoms bonded to oxygen and to nitrogen, incorporated by PECVD, has been assessed. A compositional model that interprets the experimental findings is presented. [Copyright &y& Elsevier]

Absorption coefficient (αabs) of Er3+ ions coupled to Si nanoclusters (Si-nc) in SiO2 has been determined by optical transmission measurements in rib-loaded waveguides characterized by different refractive indices, thus gauging an Er3+ absorption cross section (σabs) of 0.4–1.2×10-20 cm2 at 1534 nm. Although no significant enhancement due to the presence of Si-nc was observed, a clear dependence on the refractive index has been found. Measurements of the decay lifetime permit one to model the behavior as due to both local and mean field variations caused by the composite nature of the core waveguide layer. [ABSTRACT FROM AUTHOR]

Pump and probe experiments on Er3+ ions coupled to Si nanoclusters have been performed in rib-loaded waveguides to investigate optical amplification at 1.5 μm. Rib-loaded waveguides were obtained by photolithographic and reactive ion etching of Er-doped silica layers containing Si nanoclusters grown by reactive sputtering. Insertion losses measurements in the infrared erbium absorption region allowed to gauge an Er3+ absorption cross section of about 5×10-21 cm2 at 1534 nm. Signal transmission under optical pumping at 1310 nm shows confined carrier absorption of the Si nanoclusters. Amplification experiments at 1535 nm evidence two pump power regimes: Losses due to confined carrier absorption in the Si nanoclusters at low pump powers and signal enhancement at high pump powers. For strong optical pumping, signal enhancement of about 1.2 dB/cm was obtained. [ABSTRACT FROM AUTHOR]

Si3N4/SiO2 waveguides have been fabricated by low pressure chemical vapor deposition within a complementary metal–oxide–semiconductor fabrication pilot line. Propagation losses for different waveguide geometries (channel and rib loaded) have been measured in the near infrared as a function of polarization, waveguide width, and light wavelength. A maximum thickness of single Si3N4 of 250 nm is allowed by the large stress between Si3N4 and SiO2. This small thickness turns into significant propagation losses at 1544 nm of about 4.5 dB/cm because of the poor optical mode confinement factor. Strain release and control is possible by using multilayer waveguides by alternating Si3N4 and SiO2 layers. In this way, propagation losses of about 1.5 dB/cm have been demonstrated thanks to an improved optical mode confinement factor and the good quality of the interfaces in the waveguide. [ABSTRACT FROM AUTHOR]

X-ray absorption measurements in total electron yield mode have been carried out on Si nanodots embedded in amorphous silica produced by plasma enhanced chemical vapor deposition (PECVD). The amount of Si atoms composing the Si nanodots and the chemical composition of the amorphous host matrix has been determined thanks to the comparison with Rutherford backscattering spectrometry data. The influence of nitrogen, incorporated during the PECVD procedure, on the structure of the host silica matrix has been discussed. [ABSTRACT FROM AUTHOR]

A photoluminescent porous silicon microcavity is exposed to saturated vapor of ethanol. The ethanol substitutes the air inside the pores giving rise to a progressive monotonic redshift of the interference pattern of the photoluminescence spectrum. On the other hand, the photoluminescence intensity of the cavity peak oscillates in time. Both effects can be explained in terms of a very simple model based on the progressive change of the effective refractive index of single layers of the cavity. The change is due to the difference between the index of refraction of air and ethanol. The result suggests that a porous silicon microcavity can be a tool to study the dynamics of gas penetration into porous silicon since it allows a monitoring of the depth reached by the ethanol at any given time. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A nickel metal–organic framework (Ni-MOF) was successfully synthesized using ultrasound irradiation. Further to this, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR), Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and nitrogen adsorption [i.e. Brunauer–Emmett–Teller (BET) Surface Area Analysis] techniques were used to characterize the synthesized Ni-MOF. In addition, the effect of sonication on the surface area, pore diameter and pore volume of the final product was systematically studied using Taguchi technique. The experiments ascertained that manufacturing of the Ni-MOF by means of the ultrasonic-assisted technique is feasible at a relatively shorter time compare to the conventional methods. The final product showed more uniform shape distribution and improved BET properties. The obtained results offered that the synthesized Ni-MOF samples could be used in several applications. [ABSTRACT FROM AUTHOR]

The conversion of SiO 1.5 CH 3 into SiO 2 glass by pyrolysis in air has been studied by combining density measurements, vibrational spectroscopy, positron annihilation spectroscopy, and nuclear magnetic resonance. The results show that a critical transition takes place between 400 and 500 °C and at 600 °C the organic features are completely removed. The conversion of the hybrid structure into a fully inorganic network takes place through the formation of metastable 3-membered rings that progressively evolve into more stable 5+ rings as the pyrolysis temperature increases. The evolution of the organic moieties is coupled with the formation of Si-OH and molecular water trapped in the network which are well visible between 500 and 800 °C. In such temperature regions, a clear evolution of the network density can be detected, though the amount of free volume is nearly constant (due to the presence of trapped H 2 O). Only at 1250 °C, the network is comparable with a melt-derived glass. [ABSTRACT FROM AUTHOR]

Abstract: In this work we present the state of the art of our work to obtain an infrared optical amplifier in which Er3+ doped SiO2 sensitised with Si nanoclusters (Si-nc) act as the active material to provide the signal amplification. As a result of a careful optimisation of the deposition parameters we have achieved that the Er3+ fraction coupled with the Si-nc with respect to the total optically active Er3+ content is about 23%. This result has been determined both by quantitative measurements of the first excited state population and by pump and probe amplification measurements under non-resonant pumping, where 1dBcm−1 of internal gain (reduction of 2dBcm−1 of the initial absorption losses) has been obtained. We will discuss several material issues that are mandatory to address and then overcome in order to optimise the fraction of Er3+ coupled to Si-nc, with the aim of exciting all the ions through indirect transfer. In particular we will address: carrier absorption (CA) in Si-nc, cooperative up-conversion and non-radiative recombination in Er3+, together with the distance dependent interaction and Auger back-transfer processes in the Er/Si-nc coupled system. [Copyright &y& Elsevier]

Abstract: Rib-loaded waveguides containing Er3+ coupled to Si-nc have been produced by magnetron sputtering and successive thermal annealing to investigate optical gain at 1535nm. It has been shown that all Er ions are optically active, whereas the fraction that can be excited at high pump rates under non-resonant excitation is strongly limited by confined carrier absorption (CA), up-conversion processes, and mainly by the lack of coupling to the Si-nc. Er3+ absorption cross-section is found comparable to that of Er3+ in SiO2, but a dependence with the effective refractive index has been found. Although the presence of Si-nc strongly improves the efficiency of Er3+ excitation, it introduces additional optical loss mechanisms, such as CA. These Si-nc losses affect the possibility of obtaining net optical gain. In the present study, they have been minimized by lowering the annealing time of the Er-doped Si-rich oxide. In pump–probe measurements it is shown that signal enhancement of the transmitted signal can be achieved at low pumping rate when the detrimental role of confined CA is attenuated by reducing the annealing time. A maximum signal enhancement of about 1.30 at 1535nm was observed. [Copyright &y& Elsevier]

Observation of optical gain in silicon nanocrystals (Si-nc) is critically dependent on a very delicate balance among the Si-nc gain cross-sections, the optical mode losses and confinement factors of the waveguide structures, the Si-nc concentration and the strongly competing fast non-radiative Auger processes. Here we report on optical gain measurements by variable stripe length (VSL) method on a set of silicon nanocrystals formed by thermal annealing at 1250°C of SiOx films with different silicon contents prepared by plasma-enhanced chemical vapour deposition. Time-resolved VSL has revealed fast component in the recombination dynamics under gain conditions. Fast lifetime narrowing and superlinear emission has been unambiguously observed. To explain our experimental results we propose a four levels recombination model. Within a phenomenological rate equations description including Auger processes and amplified spontaneous emission we obtained a satisfactory agreement with time-resolved experiments and explained the strong competition between stimulated emission and fast non-radiative Auger processes. [Copyright &y& Elsevier]

A detailed study of the carrier absorption (CA) mechanism in multilayered silicon-nanocrystals (Si-nc) rib waveguides is reported. A pump (532 nm) and probe (1535 nm) technique is used to assess two loss mechanisms due to optical excitation of the system: one characterized by slow (seconds) dynamics related to heating and the other characterized by fast (microsecond) dynamics associated to CA mechanisms within the Si-nc. CA losses increase with pumping flux of up to 6 dB/cm for 3×1020 photons/cm2 s. By comparing the temporal dynamics of CA losses and time resolved photoluminescence, we suggest that both are determined by exciton generation and recombination. [ABSTRACT FROM AUTHOR]

Quantum confined Ge nanocrystals embedded in high bandgap and high-k Al2O3 dielectric matrix have been synthesized to demonstrate dual functional devices using Si-compatible fabrication technology. Transmission electron microscopy has shown the formation of Ge nanocrystals of varying diameter from 2.5 to 7.5 nm, much lower than the excitonic Bohr radius of Ge. A broad visible electroluminescence band at room temperature has been observed, which is attributed to the recombination of injected electrons and holes in Ge nanocrystals. An anti-clockwise hysteresis in the capacitance-voltage measurement of these devices indicates the charge storage in nanocrystals, useful for floating gate memory devices. [ABSTRACT FROM AUTHOR]

We report a spectroscopic study about the energy transfer mechanism among silicon nanoparticles (Si-np), both amorphous and crystalline, and Er ions in a silicon dioxide matrix. From infrared spectroscopic analysis, we have determined that the physics of the transfer mechanism does not depend on the Si-np nature, finding a fast (<200 ns) energy transfer in both cases, while the amorphous nanoclusters reveal a larger transfer efficiency than the nanocrystals. Moreover, the detailed spectroscopic results in the visible range here reported are essential to understand the physics behind the sensitization effect, whose knowledge assumes a crucial role to enhance the transfer rate and possibly employing the material in optical amplifier devices. Joining the experimental data, performed with pulsed and continuous-wave excitation, we develop a model in which the internal intraband recombination within Si-np is competitive with the transfer process via an Auger electron-'recycling' effect. Posing a different light on some detrimental mechanism such as Auger processes, our findings clearly recast the role of Si-np in the sensitization scheme, where they are able to excite very efficiently ions in close proximity to their surface. [ABSTRACT FROM AUTHOR]

Optical properties of directly excited erbium (Er3+) ions have been studied in silicon rich silicon oxide materials codoped with Er3+. The spectral dependence of the direct excitation cross section (σdir) of the Er3+ atomic 4I15/2→4I11/2 transition (around 0.98 μm) has been measured by time resolved μ-photoluminescence measurements. We have determined that σdir is 9.0±1.5×10-21 cm2 at 983 nm, at least twice larger than the value determined on a stoichiometric SiO2 matrix. This result, in combination with a measurement of the population of excited Er3+ as a function of the pumping flux, has allowed quantifying accurately the amount of optically active Er3+. This concentration is, in the best of the cases, 26% of the total Er population measured by secondary ion mass spectrometry, which means that only this percentage could provide optical gain in an eventual optical amplifier based on this material. [ABSTRACT FROM AUTHOR]

Linear and nonlinear optical properties of silicon suboxide SiOx films deposited by plasma-enhanced chemical-vapor deposition have been studied for different Si excesses up to 24 at. %. The layers have been fully characterized with respect to their atomic composition and the structure of the Si precipitates. Linear refractive index and extinction coefficient have been determined in the whole visible range, enabling to estimate the optical bandgap as a function of the Si nanocrystal size. Nonlinear optical properties have been evaluated by the z-scan technique for two different excitations: at 0.80 eV in the nanosecond regime and at 1.50 eV in the femtosecond regime. Under nanosecond excitation conditions, the nonlinear process is ruled by thermal effects, showing large values of both nonlinear refractive index (n2∼-10-8 cm2/W) and nonlinear absorption coefficient (β∼10-6 cm/W). Under femtosecond excitation conditions, a smaller nonlinear refractive index is found (n2∼10-12 cm2/W), typical of nonlinearities arising from electronic response. The contribution per nanocrystal to the electronic third-order nonlinear susceptibility increases as the size of the Si nanoparticles is reduced, due to the appearance of electronic transitions between discrete levels induced by quantum confinement. [ABSTRACT FROM AUTHOR]

We report on the study and modeling of the structural and optical properties of rib-loaded waveguides working in the 600–900-nm spectral range. A Si nanocrystal (Si-nc) rich SiO2 layer with nominal Si excess ranging from 10% to 20% was produced by quadrupole ion implantation of Si into thermal SiO2 formed on a silicon substrate. Si-ncs were precipitated by annealing at 1100 °C, forming a 0.4-μm-thick core layer in the waveguide. The Si content, the Si-nc density and size, the Si-nc emission, and the active layer effective refractive index were determined by dedicated experiments using x-ray photoelectron spectroscopy, Raman spectroscopy, energy-filtered transmission electron microscopy, photoluminescence and m-lines spectroscopy. Rib-loaded waveguides were fabricated by photolithographic and reactive ion etching processes, with patterned rib widths ranging from 1 to 8 μm. Light propagation in the waveguide was observed and losses of 11 dB/cm at 633 and 780 nm were measured, modeled and interpreted. [ABSTRACT FROM AUTHOR]

Stimulated emission from silicon-nanocrystal planar waveguides grown via phase separation and thermal crystallization of SiO/SiO2 superlattices is presented. Under high power pulsed excitation, positive optical gain can be observed once a good optical confinement in the waveguide is achieved and the silicon nanocrystals have proper size. A critical tradeoff between Auger nonradiative recombination processes and stimulated emission is observed. The measured large gain values are explained by the small size dispersion in these silicon nanocrystals. [ABSTRACT FROM AUTHOR]

Complementary metal-oxide-semiconductor-compatible tunable Fabry–Pérot microcavities filled with liquid crystals (LCs) were realized and studied in the near-infrared region. The microcavities were produced by chip bonding technique, which allows one to infill LC between two [SiO[sub 2]/Si][sub 3]λ/4 (λ=1.5 μm) dielectric Bragg reflectors separated by 950-nm-thick SiO[sub 2] posts. Liquid crystals with positive and negative dielectric anisotropy were used, i.e. MerckE7 (Δε=13.8) and Merck-6608 LC (Δε=-4.2). Mirror-integrated electrodes allow an external bias to induce an electric field and to tune the LC properties and, hence, the microcavity resonance. Electric-field-induced shifts of the second-order cavity modes of 127 and 49 nm were obtained for Merck-E7 and Merck-6608 LC, with driving potentials of 5 and 10 V, respectively. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Organically modified silicate thin film and bulk samples were prepared using [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAP-TMOS) as precursor with the addition of different amounts of AEAP-TMOS functionalized C-dots, prepared by reaction of AEAP-TMOS and citric acid at high temperature. The synthesis of surface functionalized C-dots was followed by Fourier Transform Infrared (FTIR) spectroscopy, and the C-dots optical properties were characterized by optical absorption and UV–vis fluorescence. Thin xerogel films and bulk samples were studied by FTIR, Raman and fluorescence spectroscopy. Intense blue-green emission was observed by UV excitation of functionalized C-dots. Carbon quantum dot (CQD) luminescence was preserved also in the xerogel matrices, and the energy transfer from the matrix to CQDs, which is a key characteristic for scintillation detectors, was investigated in the two systems. [Copyright &y& Elsevier]

Abstract: We present a electroluminescence (EL) study of the Si-rich silicon oxide (SRSO) LEDs with and without Er3+ ions under different polarization schemes: direct current (DC) and pulsed voltage (PV). The power efficiency of the devices and their main optical limitations are presented. We show that under PV polarization scheme, the devices achieve one order of magnitude superior performance in comparison with DC. Time-resolved measurements have shown that this enhancement is met only for active layers in which annealing temperature is high enough (>1000°C) for silicon nanocrystal (Si-nc) formation. Modeling of the system with rate equations has been done and excitation cross-sections for both Si-nc and Er3+ ions have been extracted. [Copyright &y& Elsevier]

We studied the effect of rapid thermal processing and furnace annealing on the transport properties and electroluminescence (EL) of SiO2 layers doped with Si and Er ions. The results show that for the same annealing temperature, furnace annealing decreases the electrical conductivity and increases the probability of impact excitation, which leads to an improved external quantum efficiency. Correlations between predictions from phenomenological transport models, annealing regimes and erbium EL are observed and discussed. [ABSTRACT FROM AUTHOR]

We have fabricated Er doped germanium nanowires of different diameters by pulsed laser deposition and chemical methods. Er induced photoluminescence emission due to the intra-4f 4I13/2→4I15/2 transition of Er energy levels at 1.53 µm has been achieved at room temperature using both resonant (980 nm) and non-resonant (325 nm) excitation of Er ions. The observed 1.53 µm photoluminescence signal upon non-resonant 325 nm excitation is attributed to the Ge related oxygen deficiency centers surrounding the Ge core. For direct excitation, the infrared photoluminescence characteristics have been studied as a function of Er concentration, photon flux, and diameter of the nanowires. The Er related emission signal is found to be enhanced with increase in Er concentration, pump flux of 980 nm, and the nanowire diameter. The time resolved characteristics of the Er induced emission peak have been studied as a function of the pump flux as well as the diameter of the Ge nanowires. [ABSTRACT FROM AUTHOR]

Abstract: In this work we will give an overview of the optical properties of Si nanoclusters coupled to Er3+ ions in SiO2 matrices produced by reactive magnetron co-sputtering. We have divided the work into two separate studies realised on the same samples, which are the result of a thorough optimisation work. The first one have been realised in order to get a clear picture of the interaction mechanism. On the second we will show a quantitative evaluation of the potential performances from a material point of view (determination of the whole optically active Er3+ content, excitable by direct or indirect means) and actual performance in a waveguide device (determination of internal gain values). [Copyright &y& Elsevier]

Abstract: We review here our work on the photonic and sensor applications of nanostructured silicon. As we change the dimensionality of silicon very fascinating and new optical properties of the material appear. Light sources, modulators, waveguides, logical gates are a few examples of the various photonic devices which have been developed based on silicon nanocrystals. Needless to say, all these devices rely on quantum confinement and on the interplay between bulk and surface properties. Size effects and surface reconstructions are two critical issues which one has to master to employ silicon nanocrystals. Finally the use of silicon nanocrystals to develop innovative sensing mechanisms to reveal biomolecules or poisoning gasses will be discussed. [Copyright &y& Elsevier]

Recent results on the photoluminescence properties of silicon nanocrystals embedded in silicon oxide are reviewed and discussed. The attention is focused on Si nanocrystals produced by high-temperature annealing of silicon rich oxide layers deposited by plasma-enhanced chemical vapor deposition. The influence of deposition parameters and layer thickness is analyzed in detail. The nanocrystal size can be roughly controlled bymeans of Si content and annealing temperature and time. Unfortunately, a technique for independently fine tuning the emission efficiency and the size is still lacking; thus, only middle size nanocrystals have high emission efficiency. Interestingly, the layer thickness affects the nucleation and growth kinetics so changing the luminescence efficiency. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we present the waveguiding properties of a set of mono-dispersed silicon nanocrystal Si-nc superlattices. The measured modal refractive indices (obtained by the m-line technique) of different samples can only be explained by assuming a negative “form birefringence” of about 1% originating from the particular structure of the active core, i.e., a periodical set of parallel planes of two different materials. By means of a simple model for the ordinary and extraordinary refractive indices in this kind of structures, by using the information from TEM images and the nominal growth parameters, and by considering as free parameters the upper size of the diameter of Si-nc, we are able to fit the m-line measurements in a unique way. We also report gain measurements on this set of samples, showing that, under high power pulsed excitation, positive optical gain is observed in samples that provide both good light confinement inside the waveguide and the right Si-nc dimension. [Copyright &y& Elsevier]

We present a study that demonstrates the limits for achieving net optical gain in an optimized waveguide where Si nanoclusters in SiO2 codoped with Er3+ are the active material. By cross correlating absorption losses measurements with copropagant pump (λpump = 1.48 µm) and probe (λprobe = 1.54 µm) experiments we reveal that the role of more than 80% of the total Er3+ population present on the material (intended for optical amplification purposes) is to absorb the propagating light, since it is unfeasible to invert it. [ABSTRACT FROM AUTHOR]

Si excess, Er content, and processing parameters have been optimized in a series of cosputtered oxide layers for maximizing Er emission and lifetime. The amount of excited Er as a function of the incident photon flux has been quantified for resonant (488 nm) and nonresonant (476 nm) excitations. Results show that a maximum of 3.5% of Er ions is excitable through the Si nanoclusters (Si-nc). This low value cannot be explained only by cooperative upconversion and/or excited state absorption. A short range (0.5 nm) distance dependent interaction model is developed that accounts for this low Er population inversion. The model points to the low density of Si-nc [(3–5)×1017 cm-3] as the ultimate limiting step for indirect Er excitation in this system. [ABSTRACT FROM AUTHOR]

Experimental and theoretical investigations of photonic crystal (PhC) structures in silicon nitride/silicon dioxide (Si3N4/SiO2) vertical waveguiding geometry are reported. One-dimensional patterns, either periodic or with cavity layers, are carved onto the channel waveguides by using focused ion beam lithography. Broadband transmittance spectroscopy in the visible and near-infrared frequency ranges is employed to show photonic band gap behavior up to the fourth order. For structures with a cavity layer, resonant peaks appear in transmittance spectra within the photonic gaps, in agreement with theory. The results show the interest of Si3N4/SiO2-based PhC waveguides for photonics applications from the infrared up to the visible range. [ABSTRACT FROM AUTHOR]

We investigate the optical properties of planar waveguides where the core layer is formed by a silicon nanocrystals (Si-nc)/SiO2 superlattice. M-line measurements of the different waveguides yield the mode indices, which can be modeled by assuming anisotropic optical properties of the core layer. This anisotropy is related to the superlattice, i.e., it is a form birefringence. By modeling the m-line measurements with the structural data obtained by transmission electron microscopy analysis, we determine for each waveguide the value of the form birefringence, an upper limit of the nanocrystals size and their refractive index. Values of the form birefringence as high as 1% have been found. [ABSTRACT FROM AUTHOR]

The possibility to detect fast neutrons as a distinct signal from that one of γ-rays background is surely of great importance for several topics, spanning from homeland security to radiation monitoring in nuclear physics research plants. Nowadays, Helium-3 based detectors are extremely expensive, while the use of large volume liquid scintillators presents serious concerns related to spillage risks and waste disposal. A very attractive alternative is the use of commercially available solid scintillators, which exploits an aromatic polymer matrix entrapping very high loadings of primary dye, thereby enabling the use of pulse shape analysis (PSA) to discriminate between fast neutrons and γ-rays. In this work, we analyse in detail the optical features of a solid scintillator composed by polymethylphenylsiloxane (PMPS) as base polymer loaded with moderate amounts of 2,5-diphenyloxazole (PPO). Furthermore, fluorescence decay kinetics have been correlated to the observed pulse shape discrimination capabilities of this radiation and thermally resistant scintillator, whose performances have been discussed in terms of conformational features and excimers formation revealed by the optical analyses. [ABSTRACT FROM AUTHOR]