Efficient coupler between silicon photonic and metal-insulator-silicon-metal plasmonic waveguides

We report the experimental realization of a compact, efficient coupler between silicon waveguides and vertical metal-insulator-silicon-metal (MISM) plasmonic waveguides. Devices were fabricated using complementary metal-oxide-silicon technology processes, with copper layers that support low-loss plasmonic modes in the MISM structures at a wavelength of 1550 nm. By implementing a short (0.5 μm) optimized metal-insulator-silicon-insulator structure inserted between the photonic and plasmonic waveguide sections, we demonstrate experimental coupling loss of 2.5 dB, despite the high optical confinement of the MISM mode and mismatch with the silicon waveguide mode

Irradiation-induced Ag nanocluster nucleation in silicate glasses: analogy with photography

The synthesis of Ag nanoclusters in sodalime silicate glasses and silica was studied by optical absorption (OA) and electron spin resonance (ESR) experiments under both low (gamma-ray) and high (MeV ion) deposited energy density irradiation conditions. Both types of irradiation create electrons and holes whose density and thermal evolution - notably via their interaction with defects - are shown to determine the clustering and growth rates of Ag nanocrystals. We thus establish the influence of redox interactions of defects and silver (poly)ions. The mechanisms are similar to the latent image formation in photography: irradiation-induced photoelectrons are trapped within the glass matrix, notably on dissolved noble metal ions and defects, which are thus neutralized (reverse oxidation reactions are also shown to exist). Annealing promotes metal atom diffusion, which in turn leads to cluster nuclei formation. The cluster density depends not only on the irradiation fluence, but also - and primarily - on the density of deposited energy and the redox properties of the glass. Ion irradiation (i.e., large deposited energy density) is far more effective in cluster formation, despite its lower neutralization efficiency (from Ag+ to Ag0) as compared to gamma photon irradiation.Comment: 48 pages, 18 figures, revised version publ. in Phys. Rev. B, pdf fil

Ion beam-induced quantum dot synthesis in glass

International audienceIon beam synthesis has played a significant role in fabricating metallic or semiconducting nanocrystal arrays in glass for their optical or magnetic properties, but basic questions remain unanswered. What are the microscopic mechanisms that control nanocluster growth, determine their density and size distributions? To what extent can we control these processes in order to tailor the properties? We demonstrate the role of chemistry (redox properties; charge state equilibrium modified by irradiation) in Ag nanocluster nucleation and growth processes in glasses and extend the conclusions to PbS nanocluster synthesis. In the latter case, we show how charge state differences affect diffusion and growth and devise a strategy that produces PbS quantum dots emitting intense photoluminescence at 1.5 μm. In the course of this work, we also showed that the lognormal shape of cluster size distributions signals a loss of information as to the formation process and hence loss of property control

Significance of lognormal nanocrystal size distributions

Metallic or semiconductor nanocrystals produced by very different techniques often display size distributions whose limiting shape (e.g., after long annealing times) is self-preserving and close to lognormal. We briefly survey the diverse microscopic mechanisms leading to this behavior, and present an experimental study of its inception in the case of semiconducting nanocrystals synthesized by ion implantation in silica. This example shows how the ultimate lognormal distribution is related to the system's memory loss of initial nucleation and growth processes

How chemistry affects the ion beam synthesis of PbS nanocrystals

Solid state chemical reactions involving the multiple oxidation states of sulfur are shown to dominate the synthesis of PbS nanocrystals in pure silica via co-implantation and annealing. The formation of PbSO4 as well as PbS nanocrystals and small amorphous clusters is evidenced, and related to atomic transport occurring during annealing. A mechanism is proposed

Diffusion properties of chalcogens (S, Se, Te) into pure silica

The diffusion properties of chalcogens (S, Se, Te) implanted into SiO2 were studied via secondary ion mass spectroscopy (SIMS) profiling between room temperature and the glass transition temperature (800–950 °C). Annealing of Te-containing samples leads directly to precipitation of metallic tellurium nanocrystals within the implantation profile. The S and Se concentration profiles were fitted by using a simple diffusion model in order to provide estimates of the diffusion constant and approximate solubility of these fast moving chemical species. A comparison of their differing diffusion behavior with complementary data on these systems suggests that (i) their oxidation states play a crucial role and (ii) the chalcogen propagation mechanism actually involves complex chemical interactions

Synthesis of Lead Chalcogenide Nanocrystals by Sequential Ion Implantation in Silica

Lead chalcogenide (PbS, PbSe, and PbTe) nanocrystals were synthesized by sequential implantation of Pb and one of the chalcogen species into pure silica. The implantation energy and fluence were chosen so that the implantation profiles practically overlap at a depth $\approx$ 150 nm with a maximum concentration of about 0.3 atom %. Annealing for 1-8 h at 850-900 °C triggers nanocrystal growth, which is monitored by high-resolution (HRTEM) and conventional transmission electron microscopy (TEM), secondary-ion mass spectrometry (SIMS), and Rutherford backscattering spectrometry (RBS). Striking differences are found in the depth distributions and microstructures of the resulting nanocrystals. We show that the differing chemical interactions of Pb and chalcogens (between each other and with silica) play a crucial role in chalcogenide nucleation and growth. Using available information on chalcogen redox states in silicate glass, we propose a nonclassical nucleation and growth mechanism consistent with our experimental results. The complex chemistry involved at the microscopic level is shown to impair control over the nanocrystal size distribution. Finally, PbS nanocrystal-doped silica is shown to emit intense photoluminescence (PL) in the 1.5-2 $\mu$m wavelength range, an effect that we relate to the above nucleation and growth scheme

On the size distribution of Atlantic tropical cyclones

The size of a tropical cyclone is known to vary considerably across storms, though little is understood about the environmental and internal factors that modulate it. Making use of newly available extended tropical cyclone records that include information about storm structure, we examine the size distribution of Atlantic tropical cyclones, using as a metric the radius of vanishing storm winds normalized by the theoretical upper bound given by the ratio of the potential intensity to the Coriolis parameter. We find that the distribution of this normalized outer radius is closely log-normal.National Science Foundation (U.S.) (grant ATM-0630690

Epitaxial Growth of Sputtered Ultra-thin NbN Layers and Junctions on Sapphire

International audienceHigh crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher Tc, higher Jc and lower resistivity have been obtained by growing untwined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 cm at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of untwined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600 to 300 during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb2Ny05-x ( 2.2 nm) at the unprotected NbN surface and of interfacial NbO ( 0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices