1,721,031 research outputs found
Spectroscopic ellipsometry analysis and nanostructure characterization of ferroelectric PZT thin films
Get PDFThe author has granted permission for their work to be available to the general public.Ferroelectric thin films are used as high dielectric constant capacitors, infrared detectors, piezoelectric transducers, optical modulators, optical waveguides, and nonvolatile memory chips for dynamic random access memory (DRAM) etc. While ferroelectric and dielectric properties of these films have been extensively investigated, their optical properties have been comparatively less studied and of limited use in quantitative evaluation of multilayer thin films. In this work we explored the variable angle spectroscopic ellipsometry (VASE) technique for its effectiveness in physical property characterization. The VASE combined with its computer modeling tool enables nondestructive, nonintrusive, and contactless optical means for optical characterization. Crystalline Lead Zirconium Titanate (PZT) thin films, fabricated on layer atop of Si substrates, were characterized using VASE (J.A. Woollam; Lincoln, NE, USA) by determining the ellipsometric parameters Ψ and Δ as a function of wavelengths (200-1000 nm) and incident angles (65 0, 700, 750) at room temperature. A physical representation of the multilayer system was constructed by a six layer model (analysis software WVASE32, J.A. Woollam) through a step-by-step method. Other physical properties characterized by several well-known techniques on structure, morphology and topographical features correspond well with the models developed using VASE alone. The technique and the methodology developed have shown promises in identifying the respective thickness and optical properties of multilayer thin film system, with limited input of processing or composition information's.Electrical and Computer Engineerin
An optical leaky wave antenna with silicon perturbations for electronic control
Get PDFAn optical leaky wave antenna (OLWA) is a device that radiates a light wave into the surrounding space from a leaky wave (LW) guided mode or receives optical power from the surrounding space into a guided optical mode. In this work, we propose and provide a 3D analysis of a novel CMOS compatible OLWA made of a silicon nitride (Si3N4) waveguide comprising periodic silicon perturbations which allow electronic tuning capability. The analysis presented here includes the effect of the number of semiconductor perturbations, the antenna radiation pattern and directivity. We show that the number of the silicon perturbations has to be large to provide a long radiating section required to achieve radiation with high directivity. In other words, the proposed structure allows for a very narrow-beam radiation. Preliminary results are confirmed by exploiting leaky wave and antenna array factor theory, as well as verified by means of two full-wave simulators (HFSS and COMSOL). Our purpose is to ultimately use PIN junctions as building blocks for each silicon implantation for the electronic control of the radiation. In particular, the electronic tunability of the optical parameters of silicon (such as refractive index and absorption coefficient) via current injection renders itself the ideal platform for optical antennas that can facilitate electronic beam control, and boost the efficiency of optoelectronic devices such as light-emitting diodes, lasers and solar cells, and bio-chemical sensors. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)
Tuning the Optical Bandgap of Semiconducting Two-Dimensional Materials by Different Techniques
No full textThe author has granted permission for their work to be available to the general public.The liquid and mechanical exfoliation techniques are merged in a mechanical exfoliation assisted by molecular tweezers technique (MEAMT). Semiconducting 2-dimensional materials (S2DM), on comparing to the typical liquid exfoliation technique, the MEAMT resulted in a higher yield percentage and higher photoluminescence (PL) quantum efficiency which is attributed to the increased surface homogeneity of the sheets and reduced defect density. Polyethylene glycol (PEG) assembled on the surface of two-dimensional tungsten disulfide (WS2) into a limited number of nanoislands (NIs), nanoshell (NS), and granular nanoparticulates (GNPs) depending on its chain length. Deforming the S2DM by PEG NPs assembly decreased the orbital coupling and lowered the electronic direct band gap significantly. Conjugated polymers (CP) are used for tuning the optical band gap of S2DM. The acceptable reason for the modification of the band gap of MoS2 by CPs is the generation of interlayer excitons (ILE) at their interface. The optical band gap of S2DM is further changed by introducing an inert polymer spacer of different thickness to separate MoS2 from CP. This is attributed to the reduction of the efficiency of excitonic interactions and lowering the exciton binding energy, which is induced by the increase of the dielectric function at the CP-S2DM interface. Plasmonic Janus nanoparticles are also used to tune the band gap of S2DM. Due to the large Plasmonic Janus nanoparticles/S2DM Schottky barrier, the plasmon-induced hot-electron transfer (PHET) from Plasmonic Janus nanoparticles. Pauli blocking and phase-space filling are responsible for the blue shift of S2DM assembled with Plasmonic Janus nanoparticles.Biomedical Engineerin
Magnetoelectric measurement and analysis under cryogenic conditions
No full textThe author has granted permission for their work to be available to the general public.Interest into multiferroic materials have been increasing in popularity in recent years due to their many technical applications, particularly magnetoelectrics. The concept of using magnetoelectrics for low-field sensor application in the aeronautics and space industry is a novel idea but first must be studied as a function of temperature. Doing so requires a measurement system that can expose a material to either an electric field or magnetic field over a broad temperature range and be able to record the resultant property of this exposure. In this thesis, a systematic approach was taken to establishing such a system as well testing the system and taking measurements of magnetoelectric materials. First, basic instrumentation was set up in the form of large DC powered electro magnets for magnetic field exposure and lock-in amplifier for voltage measurement. Secondly, a temperature controller and vacuum pump and chamber were used for cryogenic cooling of the material sample so that measurements could be recorded as both a function of magnetic field and temperature. A custom designed and built Helmholtz coil was established for the exposure of a dynamic AC magnetic field. A magnetoelectric composite material composed of layered magnetostrictive and piezoelectric was then used to test the measurement system and the results were compared to experimental data found in literature. The data was in good agreement with literature. Subsequently, measurements were done on the effective permittivity and tan(ä) of NiFe2O4/Pb(Zr-Ti)O3/NiFe2O4 magnetoelectric sample as a function of field and temperature that demonstrated the magnetodielectric effect in the material. Lastly, the orientation dependence of the material was measured at room temperature and exhibited results that agreed with the literature.Electrical and Computer Engineerin
Verification and modification a program for calculating the area of intersection of a cone and voxel
Get PDFThe author has granted permission for their work to be available to the general public.Previously, to investigate the informational content of Compton camera data, a program was written to calculate the area of intersection of a cone with a voxel. In writing the program, an existing subroutine was modified and used. However in doing this modification, no effort was made to improve the portion of the original code that was unchanged. Moreover, little effort was made to verify that the overall program worked correctly. The main objective of this thesis is to verify the performance of the overall program and improve the unchanged code of the modified subroutine. To verify the performance of the overall program, a new program was written to display a voxel-wise image of the cones created by the overall program. These images verified that the overall program was, in fact, working correctly. To improve the subroutine, the logic of the unchanged code was improved. The new logic was verified and was seen to be working correctly. However, it was determined that this improvement had a negligible impact on the performance of the overall program.Electrical and Computer Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Enhancing radiation control of an optical leaky wave antenna in a resonator
Get PDFWe analyze the theoretical and physical properties of a CMOS compatible optical leaky wave antenna (OLWA) integrated into a Fabry-Pérot resonator (FPR) at 193.4 THz (wavelength ?0 = 1550 nm). The presented OLWA design is composed of a silicon (Si) dielectric waveguide sandwiched between two silica glass (SiO2) domains, and it comprises periodic perturbations (cavities of vacuum). We first describe the radiation of the isolated OLWA whose radiation pattern is due to the excitation of a leaky wave, slowly decaying while traveling. The perturbations are indeed designed to obtain a leaky wave harmonic with very low attenuation and phase constants. Then, we integrate the same OLWA into a FPR where two leaky waves with the same wavenumber are travelling in opposite directions inside the resonator. We show that the radiation level at the broadside direction can be effectively controlled by modifying the optical properties of the Si waveguide through electron-hole excess carrier generation (found to be highly enhanced when it is integrated into a FPR). The design of the integrated OLWA is properly set to guarantee the constructive interference of the two radiated beams provided by the two leaky waves in the FPR. The modal propagation constant in the integrated OLWA can be then altered through excess carrier generation in Si, thus the antenna can be tuned in and out of the resonance thanks to the high FPR quality factor, and the LW modal dispersion relation. This allows for enhanced radiation level control at broadside, and preliminary results show up to 13 dB beam modulation. © 2012 SPIE
Numerical predictions of the improvement of solar energy harvesting in inverse opal photonic crystals by tuning the slow-light effect to higher photonic bands
No full textImprovement of solar light harvesting for energy or environmental applications such as photocatalysis can be achieved through the manipulation of light. Inverse opal (IO) photonic crystals have emerged as promising material structures, due to their ability to slow down light and enhance light-matter interactions at specific wavelengths. While previous studies have primarily focused on slow light modes at the lower (i.e., red) and upper (i.e., blue) edges of the first partial photonic band gap (PBG), the potential of higher order, almost flat photonic bands for light conversion has recently been highlighted. Nonetheless, understanding and optimization of this effect require further investigation. In this context, we present a numerical study of anatase TiO2 IOs featuring interpenetrating pores. To this end, we developed a three-dimensional model based on the finite element method (FEM) and benchmarked it against rigorous coupled-wave analysis (RCWA). Excellent agreement was found between the two methods. Light harvesting enhancement was quantified via the ratio of photon fluxes absorbed by the IO structure and a reference homogeneous TiO2 slab of equivalent material volume. By tuning the IO lattice constant (pore diameter), the spectral positions of slow light modes associated with nearly flat bands were controlled. Adjustment of the slow light mode wavelength using a pore diameter of 170 nm enabled tuning to the blue edge of TiO2 electronic band gap, resulting in an enhancement factor equal to 1.62. More notably, we demonstrated that tuning higher photonic flat bands provided even better enhancement with a factor reaching 2.39 for a larger pore diameter of 280 nm. We believe that our FEM-based numerical model could help further understand and optimize slow light effect in IO, opening new perspectives for the exploitation of higher photonic bands in solar light harvesting applications
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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