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Stefanovsky S.V.,Moscow Scientific and Industrial Association | Purans J.J.,Institute of Solid State Physics
Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B | Year: 2012

Cesiums ion speciation in sodium cesium borosilicate and sodium aluminophosphate glasses potentially suitable for immobilization of Cs-containing radioactive wastes or ionizing radiation sources was studied using x-ray absorption spectroscopy (XAS). At relatively low Cs2O content (~1-2 mol%) in Na-Cs borosilicate glasses, Cs ions are present in a slightly distorted twelve-fold coordinated oxygen environment where nine oxygens are positioned at a distance of 3·22 Å and three - at a distance of 3·37 Å. The closest Si atoms are positioned at a distance of ~3·31 Å. With the increase of Cs2O concentration in the glass both the coordination number (CN) and the average Cs-O distances reduce, whereas, the Cs-Si distances increase. Both the first and the second coordination shells are split into two subshells pointing to distortion of cesium-oxygen polyhedra.

Pan S.S.,Hong Kong Polytechnic University | Lu W.,Hong Kong Polytechnic University | Zhao Y.H.,Hong Kong University of Science and Technology | Tong W.,Chinese Academy of Sciences | And 7 more authors.
ACS Applied Materials and Interfaces | Year: 2013

Detection of H2O2 is important for the applications in environmental protection, pharmaceutical industries, food production, and clinical control. Current colorimetric assay of H2O2 based on enzyme or nanomaterials always needs TMB or other peroxidase substrate as coloration species. Furthermore, the corresponding response time including incubation process is in order of minute. In this study, we report on the synthesis of heavily Ti3+-doped TiO2 composed of spherelike nanoparticles by pulsed laser ablation method. This TiO2 can directly detect H2O2 without using TMB or any other peroxidase substrate and is free from incubation process. In addition, the detection sensitivity is compatible with or better than that of the natural enzyme or other nanomaterials. Hence, the self-doped TiO2 nanoparticles provide a novel, direct, ultrafast approach for H 2O2 assay application. © 2013 American Chemical Society.

Novel devices feature improved carrier transport and operate under higher-order modulation schemes to enable increased data transmission rates. Ethernet protocols (i.e., the IEEE 802.3ba 100Gb/s and the forthcoming IEEE P802.3bs 400Gb/s standards) provide the definitions for data transmission systems that are used in long-range (10km) and extended-range (40km) fiber links. These transmission systems include multichannel 25GBd on-off keying (OOK)—the simplest form of intensity modulation in which digital information is represented by low (0) and high (1) amplitude levels—and four-level pulse-amplitude modulation (PAM4). Directly modulated lasers (DMLs) are an attractive option for a light source in such applications because of their low cost, small footprint, and low power consumption. In addition, they allow simple direct detection, while being operated in an intensity-modulation mode. DMLs, however, are strongly limited by their bandwidth. Indeed, much higher data rates are possible with coherent systems in which more advanced modulation schemes and complex transmitters are used. As an alternative to conventional quantum-well-based DMLs, quantum-dot lasers (QDLs) have been studied extensively. QDLs feature several unique properties, e.g., they have temperature-insensitive and ultralow threshold current, and low linewidth enhancement factors.1 Gallium-arsenide-based (GaAs-based) QDLs exhibit at least three confined electronic energy levels, i.e., a ground state (GS) and two very close excited states (ESs). The GS is twofold degenerate, whereas the ES is fourfold degenerate. The larger degeneracy translates to a larger differential gain and smaller nonlinear gain compression.2 Direct modulation of QDLs on the GS, at over 20Gb/s, has been demonstrated for both indium arsenide (InAs)/GaAs lasers at 1.31μm (O-band)3 and for InAs/indium phosphide (InP) devices at 1.55μm (C-band).4 The bandwidth of these structures, however, is limited by several factors, including inhomogeneous broadening (low modal gain),5 the hot-carrier effect, and the slow capture time into the quantum dots (large gain compression).6 To ensure sufficient gain, several quantum dot (QD) layers with wide spacers (limiting carrier transport across the active region) are usually incorporated into QDLs. The holes tend to accumulate on the p-side of the active region because of their short diffusion length. This inhomogeneous distribution of carriers limits the modulation response of the devices.7 In our work,8 we have designed a novel GaAs-based QDL in which we incorporate graded p-doping of spacers to compensate for the hole distribution. We have designed this grading so that there is a small amount of p-doping on the topside of the active region and a large amount of doping on the bottom (i.e., substrate) side. Our lasers feature a larger maximum modulation bandwidth (9.2GHz) compared with standard p-doped samples (7.2GHz). Furthermore, by modifying the reflectivity of one laser facet, our lasers (operating exclusively at the ES) exhibit an increased maximum modulation bandwidth of 11.7GHz. We can also realize InAs/indium gallium arsenide/InP QD structures at 1.55μm, which exhibit a –3dB bandwidth of 12.1GHz, by shortening the distance between the electrical contacts and the active region, and by incorporating seven QD layers. We can thus provide sufficient gain, but do not limit the carrier transport. Our O-band and C-band QDLs exhibit data transmission at a rate of 25Gb/s for direct modulation in the non-return-to-zero OOK scheme. To further increase the digital bandwidth of our devices, we also explored higher-order modulation formats, e.g., PAM4 and eight-level PAM (PAM8). PAM4 results for the O-band and C-band lasers—see Figure 1(a)—show that 17.5GBd (35Gb/s) data transmission was realized with both structures. We thus achieved a 40% increase in the maximum bit rate. Doubling the bit rate, however, was not possible. This is because in PAM schemes (particularly PAM8), the noise level of the lasers becomes the major limiting factor. The 7.5GBd (22.5Gb/s) PAM8 response of a standard p-doped laser structure across 10km of single-mode fiber (SMF) is shown in Figure 1(b). We successfully demonstrated PAM8 for this particular laser structure because it features a strongly damped (but linear) small signal response, an ability to operate at low currents (threshold of 3mA), and a relatively large modulation-current-efficiency factor.8 Figure 1. Eye diagrams and corresponding bit-error ratio (BER) curves of (a) the O-band (1.31μm) and C-band (1.55μm) lasers under 17.5GBd four-level pulse-amplitude modulation (PAM4) in back-to-back configuration and (b) of a standard p-doped laser under 7.5GBd eight-level PAM (PAM8) across 10km of single-mode fiber (SMF). In (a) the aggregated BERs were determined by direct detection with an error analyzer. In (b) the BERs were retrieved using digital signal processing of single-shot traces that were acquired with a real-time oscilloscope. The improved BERs at lower received powers were achieved by means of equalization. In another part of our work, we have packaged a 39.813GHz monolithic two-section mode-locked laser (MLL) that is based on the 1.31μm graded p-doped laser structure into a module. MLLs (which generate optical and electrical pulse trains) are ideal candidates for various applications, e.g., microwave photonics and radio-over-fiber systems. When combined with modulators, MLLs act as transmitters for optical time-division multiplexing (OTDM) systems in optical communication networks. MLLs operate at frequencies far beyond the intrinsic bandwidth of DMLs. In contrast to DMLs, they benefit strongly from an inhomogeneously broadened QD gain spectrum. When all the longitudinal modes are locked, sub-picosecond pulses are emitted. QD MLLs also exhibit ultrafast recovery, which enables pulse generation up to 100GHz. We achieve jitter reduction and frequency tuning through hybrid mode-locking. In addition, dual-tone injection gives rise to a narrow optical linewidth (essential for coherent systems). We observe a pulse width of 2ps and integrated jitter of 340fs, which makes our MLL suitable for OTDM.9 We generated return-to-zero (RZ) differential quadrature phase-shift keying (DQPSK) data signals by superimposing bit sequences on the MLL pulse train, via successively sequenced dual-drive Mach–Zehnder and phase modulators. We have also conducted data transmission experiments at 40GBd (80Gb/s) across 45km of SMF (see inset of Figure 2). Through the use of DQPSK and OTDM we were thus able to quadruple the bit rate (compared with standard OOK). The corresponding 80GBd (160Gb/s) RZ DQPSK bit-error ratio curves and eye diagrams are also shown in Figure 2.10 Figure 2. Illustration of 80GBd return-to-zero (RZ) differential quadrature phase-shift keying (DQPSK) in back-to-back configuration. Eye diagrams of tributary 1 (Tr 1) and tributary 2 (Tr 2) at the maximum received optical power were measured by a differential receiver, based on a delay interferometer. The signal-to-noise ratios are 6.7 and 6.5, respectively. BERs were detected by means of an electrical demultiplexer (DEMUX). The BER curves of both DEMUX output signals (0P and 1P) and tributaries show error-free performance (i.e., without error floor to below 10−10. Inset: Constellation diagram of 40GBd RZ DQPSK across 45km of SMF, for the maximum received power obtained with an O-band optical modulation analyzer. The error-vector magnitude is 10.1%. In summary, in our novel GaAs-based QDL we incorporate graded p-doping of spacers to compensate for carrier transport limitations across the active region, and thus bandwidth limitations, in DMLs. The ES emission has larger differential gain than the GS emission. These improvements lead to higher cut-off frequencies of 1.31μm-InAs/GaAs devices. In addition, our 1.55μm InAs/InP QD lasers, which have a narrowed active region and barriers, exhibit large 3dB bandwidths. We have demonstrated data rates of 35Gb/s using PAM4 for both wavelength bands, and PAM8 reveals that further optimization of the lasers (in terms of their noise performance) is required. This will therefore be the focus of our future research. We have also shown that integration of MLLs as sources in coherent OTDM systems enables 160Gb/s RZ DQPSK data transmission. Funding for this research was provided by the German Research Foundation in the SFB 787 framework. The authors would like to thank Finisar (Germany) for packaging the quantum-dot mode-locked laser module and C. Meuer at Sicoya for assistance with the digital signal processing. Department of Solid-State Physics and Center of Nanophotonics Technische Universität Berlin Dejan Arsenijević received his diploma in physics from the Technische Universität Berlin in 2009. His current research interests include higher-order modulation formats, as well as low-jitter optical and electrical pulse sources for future high-speed data communications. Department of Solid-State Physics and Center of NanophotonicsTechnische Universität Berlin Department of Solid-State Physics and Center of Nanophotonics Technische Universität Berlin Berlin, Germany and King Abdulaziz University Dieter Bimberg received his diploma in physics and PhD from Goethe University, Germany, in 1968 and 1971, respectively. Between 1972 and 1979 he was a principal scientist at the Max Planck Institute for Solid State Research in Germany. He was then appointed as a professor of electrical engineering at the Technical University of Aachen (Germany) and, in 1981, as the chair of the Technische Universität Berlin's Applied Solid State Physics department. From 1990 to 2011 he was the executive director of the Institute of Solid State Physics, and in 2004 he founded the Center of Nanophotonics. In addition, he was the chairman of the board of the German Federal Government's Centers of Excellence in Nanotechnologies between 2006 and 2011. His many honors include the Russian State Prize in Science and Technology (2001), the Max Born Award (2006), the William Streifer Scientific Achievement Award (2010), the United Nations Educational, Scientific, and Cultural Organization's Nanoscience Medal (2012), and the Welker Award (2015). Department of Solid-State Physics and Center of NanophotonicsTechnische Universität BerlinBerlin, GermanyandKing Abdulaziz University 2. D. Arsenijević, A. Schliwa, H. Schmeckebier, M. Stubenrauch, M. Spiegelberg, D. Bimberg, V. Mikhelashvili, G. Eisenstein, Comparison of dynamic properties of ground- and excited-state emission in p-doped InAs/GaAs quantum-dot lasers, Appl. Phys. Lett. 104, p. 181101, 2014. doi:10.1063/1.4875238 5. H. Dery, G. Eisenstein, The impact of energy band diagram and inhomogeneous broadening on the optical differential gain in nanostructure lasers, IEEE J. Quantum Electron. 41, p. 26-35, 2005. doi:10.1109/JQE.2004.837953

Kotomin E.A.,Institute of Solid State Physics | Kuzovkov V.N.,Institute of Solid State Physics | Popov A.I.,Institute of Solid State Physics | Vila R.,CIEMAT
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2016

The diffusion-controlled kinetics of the F center annealing in Al2O3 (sapphire, corundum) is simulated theoretically for the two regimes: after neutron irradiation when the immobile F centers are annihilated with complementary defects - mobile interstitial oxygen ions, and in thermochemically reduced (additively colored) crystals where mobile F centers aggregate and create the metal colloids. A comparison of the experimental and theoretical kinetics allowed us to estimate the migration energies for the F centers and interstitial oxygen ions. It is obtained that the pre-exponents in diffusion coefficients for defects in different neutron irradiated samples can vary by two orders of magnitude which is attributed by presence of numerous traps for mobile interstitial oxygen ions. © 2015 Elsevier B.V. All rights reserved.

Gertners U.,Institute of Solid State Physics | Teteris J.,Institute of Solid State Physics
Physics Procedia | Year: 2013

In this report direct photo-induced formation of surface relief gratings (SRG) in thin layers of arsenic sulfide (As2S3) are shown. This anisotropic light-induced mass transfer phenomenon has been discussed with the special attention focused on the polarization and intensity of the corresponding light. The experimental setup for the SRG recording is straight-forward consisting of ∼ 10 m optical slit through which an unfocused beam of light is projected on the surface of sample. The evolution of surface relief in dependence from the recording time and polarization has been investigated in detail. The processes of SRG formation and mass transfer which are based on the photo-induced plasticity have discussed. © 2013 The Authors. Published by Elsevier B.V.

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