Second-harmonic generation in silicon waveguides strained by silicon nitride

Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks. All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40pm/V at 2.300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light sources spanning the near- to mid-infrared spectrum from 1.2 to 10 micron

Schwartz - Zippelov teorem i neke njegove primjene

U ovom radu izložen je rezultat poznat pod nazivom Schwartz - Zippelova lema ili Schwartz - Zippelov teorem. Tema rada pripada pretežno algebri, ali ima značajne primjene u drugim matematičkim područjima kao što je na primjer teorija algoritama, te kombinatorika. Rad se sastoji od tri poglavlja. U uvodu je ukratko opisana tema i cilj rada. Prvo poglavlje sadrži kratku povijest nastanka teorema, različite oblike rezultata pojedinih autora, te kratki opis pojmova korištenih u samom radu. Drugo poglavlje sastoji se od iskaza i dokaza Schwartz - Zippelovog teorema, a u trećem poglavlju izložene su neke primjene tog teorema na probleme koji se mogu svesti na testiranje jednakosti polinoma. Takvi su, primjerice, problem postojanja savršenog sparivanja u grafu i ispitivanje svojstva asocijativnosti u grupoidu. Uz svaku primjenu navedeni su i prikladni primjeri.In this diploma thesis we present the result usually called the Schwartz-Zippel lemma or the Schwartz-Zippel theorem. The nature of this theorem is basically algebraic, but it has significant applications in other areas of mathematics, such as the theory of algorithms and combinatorial theory. The thesis consists of three chapters. The main theme and objective are briefly described in the introduction. The first chapter contains a short history of the theorem’s origins, various forms of the main results by different authors and some comments of basic concepts related to this topic. The statement and a proof of the Schwartz-Zippel theorem are given in the second chapter, together with the general outline of its applications. The third and final chapter consists of some applications to problems which can be reduced to polynomial identity testing, including the existence of a perfect matching in a graph and testing of the associativity property in a groupoi

Investigation of sp carbon chain interaction with silver nanoparticles by Surface Enhanced Raman Scattering

Surface Enhanced Raman Spectroscopy (SERS) is exploited here to investigate the interaction of isolated sp carbon chains (polyynes) in a methanol solution with silver nanoparticles. Hydrogen-terminated polyynes show a strong interaction with silver colloids used as the SERS active medium revealing a chemical SERS effect. SERS spectra after mixing polyynes with silver colloids show a noticeable time evolution. Experimental results, supported by density functional theory (DFT) calculations of the Raman modes, allow us to investigate the behavior and stability of polyynes of different lengths and the overall sp conversion towards sp2 phase.Comment: 19 pages, 7 figures, 1 table

Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results

Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole

A COMPARATIVE-ANALYSIS OF RAMAN-SPECTRA OF SODIUM BETA''-ALUMINA CRYSTALS STABILIZED BY LI+ OR MG-2+ IONS

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CADMIUM-ION DISTRIBUTION AND MOTION IN BETA-ALUMINA - AN X-RAY-DIFFRACTION AND RAMAN-SCATTERING INVESTIGATION

The structure and dynamics of a mixed cadmium-sodium \u3b2-alumina crystal, with composition (Cd0.79Na0.42O)1.22 c511Al2O3, was investigated by x-ray diffraction and Raman spectroscopy. The conventional crystal-structure determination was carried out at room temperature, attaining R=0.043 in the least-squares refinement. Cadmium ions are close to mid-oxygen sites, but displaced by 0.168(5) A\u30a from the conduction plane. Sodium ions are located in 6h sites near the Beevers-Ross positions, and the bridging oxygen atoms are disorderly distributed slightly off the threefold axes. Al interstitial-vacancy pairs correlated with oxygen interstitials were detected. Polarized Raman spectra, obtained in the low-frequency region at some temperatures from 300 K down to liquid-helium temperature region, are analyzed. The results of the analysis of features connected with cadmium-ion vibrational dynamics in room-temperature spectra are consistent with the model of ionic distribution obtained by refining x-ray-diffraction data. The temperature evolution of some Raman bands is discussed in terms of cadmium-ion rearrangement

RAMAN-SPECTROSCOPY OF LEAD-CONTAINING BETA''-ALUMINA CRYSTALS

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Raman Study of Ion-Molecules Interaction in Poly(Methylmetacrylate)-Based Gel Electrolytes

Gel electrolytes have been obtained, containing LiX (X = ClO(4), N(CF(3)SO)(2), AsF(6)) dissolved in a ethylene carbonate-propylene carbonate mixture and PMMA as polymeric matrix. Ionic conductivity has been measured, for two different lithium salts. The changes in the Raman spectra have been studied as a function of the polymer content, lithium salt concentrations and for different anions. Two satellite bands of the internal bending and stretching modes of ethylene carbonate appear in the spectrum of lithium containing samples, because of the cation-solvent molecule interaction