A substitution for Cu by a 3d-metal (Fe, Co, Mn) in the superconducting Bi phases (Bi2Sr2Can−1CunOy; n = 1,2 and 3) has led to the discovery of new phases. These 3d-metal substituted phases are non-superconducting and, in contrast to the Cu-based phases, they exhibit a structural modulation that is commensurate. Single crystal x-ray studies were performed on the Bi2Sr3Fe2Oy, Bi2Sr2CoOy and Bi2Sr2MnOy compounds. A result, in common, is that the modulation is caused by the periodic insertion of a row of oxygen atoms in the Bi layers and this results in a corrugated-like slab structure. The Bi-O layers can be described as composed of alternating rocksalt-type and oxygen deficient perovskite-type blocks. For the Fe (n=2) phase the Bi atoms form ribbons (chains) in the ab plane. This is in contrast to the n=1 Co or Mn phases for which a disorder at the oxygen position is observed. Although the extra oxygen in the Bi-O layer could account for the doping mechanism in the high Tc Bi-phases, cation non-stoichiometry may also beimportant.

Superconductivity at 94 K is exhibited by the compound Ba2YCu307 which crystallizes in an oxygendeficient perovskite-type structure. ' Cation ordering leads to a tripled c axis, with a Ba-Ba-Y-Ba-Ba-Ystacking sequence. Surprisingly, substitution of trivalent rare-earth (R) atoms for Y has little efl'ect on the superconductivity. Also, relatively little change is observed in the cell constants for Pr, Sm, Eu, Gd, Dy, Ho, Er, and Tm substitutions. Among these elements, only Pr suppresses superconductivity. We report here a systematic investigation of the x-ray structures of R substituted compounds in order to find common structural features, and to determine whether the lack of superconductivity in the Pr compound has a structural origin. Previous x-ray studies on Ba2YCu307 crystals were refined in space group Pmmm or P4/mmm. 7 Subsequent neutron-diffraction experiments ' revealed the symmetry to be Pmmm with the oxygen vacancies in z =0 ordered in the ( —, ,0,0) position (setting of origin as in Ref. 5). In addition, these studies also showed that the z coordinates of the oxygen-atoms forming the base of the pyramidal Cu site are almost equal. All the samples used in the present study are multiple domain crystals with both the orientations of the axes in the pseudotetragonal cell, i.e., microtwins describable with [001) 90 rotation or fl IOj twin laws. Assuming that the structural features are preserved through the whole series, it is possible to obtain the single-crystal structure from domain crystals. The domain size observed by electron microscopy" depends on the preparation procedure and can vary from a few 100 to a few 1000 A.. Because of the twinning, these structures would best be determined from powder neutron data. However, because of the large amount of material necessary and because several R compounds are hampered by strong neutron absorption, x rays were deemed appropriate for a comparative study of Ba2RCu307with R =Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm and x=0.0-0.5. With the exception of the nonsuperconducting praseodymium compound, T, is above 90 K. Accurate atomic positions can be obtained for the metal atoms. For the oxygen positions, somewhat larger standard deviations occur. Nevertheless, we can establish trends of the effect of differently sized rare-earth atoms on the crystal lattice of Ba2RCu307 —, . In this paper, we report the x-ray crystal structure determinations of Ba2RCu307 with R =Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm. Small crystallites suitable for x-ray studies were obtained by standard ceramic techniques. The crystallites chosen were always thin platelets with the c axis perpendicular to the platelet plane. The platelets were mounted either on a Nonius CAD4 K-axis diffractometer or a Picker four-circle diffractometer, driven by programs developed at the National Research Council of Canada. ' All measurements were done in co-scan mode, sampling over all the individual grains, using Mo ECa radiation. A Gaussian-integration absorption correction was applied. All calculations were carried out using the NRCvAx structure package. ' The refinement was carried out in space group P4/mmm, using unit weights for all reflections. This choice of space group over Pmmm is appropriate since the twinning renders the crystals pseudotetragonal. As mentioned earlier, the neutron-diffraction studies showed the z coordinates of the atoms in (0, —, ' z) and ( —, , O, z) positions to be equal. This is crucial to the refinement, since these are the atoms that become symmetry related in space group P4/mmm For the oxy. gen atom in (0, 2,0), another in ( 2,0,0) is generated by the fourfold symmetry; therefore the value for the occupation parameter for O(3) is averaged over both orientations and is a good measurement for the overall oxygen stoichiometry. Refinement in Pmmm usually yielded different occupation parameters for the oxygen atoms in (0, —, ' 0) and ( —, ' 0,0), giving an estimate of the relative occurrence of the two twin orientations. The largest difference was observed in Ba2GdCu307 „with a pronounced difference in the a and b axis and approximately a 2:1 ratio of the two possible orientations. Despite the twinning, orthorhombic lattice parameters can be obtained by centering on the maximum peak height of each reAection, given that the separation of the two peaks stemming from the different twin orientation is sufficient. The crystallographic data obtained are given in Tables I-IV. Table I gives the lattice parameters obtained from twinned crystals by centering a minimum of 40 reAections above 20=40 . In Table II, the number

Results are presented from electrochemical and in situ x-ray diffraction cells of the ternary molybdenum chalcogenides Li„Mo6Se,Ss, {0(z(8, 0(x (1), where the Mo6Se, S8, samples were obtained by chemical removal of Cu from Cu25Mo6Se, SS,. The electrochemical results for the entire series can be accounted for with a lattice-gas model that assumes that the energy of a Li atom depends on the number of S and Se atoms nearby. This energy is dominated by the two nearest-neighbor chalcogens on the rhombohedral 3 axis. The analysis of the electrochemical results gives the fraction of the Se which resides in these sites. This fraction correlates well with the site occupancies refined from single-crystal x-ray diffraction. It is also accounted for by a model of crystal growth. Our models also account for the lattice expansions in some other Chevrel compounds A„Mo6Se,SS, as a function of x and z.

the entro y s of intercalation compounds from W describe the first measurements of changes in ropy e e d 1 of intercalation accurately describes the variation etr of electrochemical cells. A lattice-gas mo e e calorime y rature T with no adjustable parameters. This agreement of (ils/Bx)T with x in Li„Mo6se& at fixed temperature t j L M Se for x & 1 are randomly distributed over a single type of site. implies that the Li atoms in i„o6 8

The ability to control the nucleation site of a single quantum dot will have a profound effect oil the development of quantum dot-based photonic devices. The deterministic approach will provide a truly scalable technology that can take full advantage of conventional semiconductor processing for device fabrication In this review, we discuss the progress towards the integration of deterministically nucleated single quantum dots with top-down quantum optical devices targeting telecommunication wavelengths Advances in site-controlled quantum dot nucleation using, selective-area epitaxy now makes it possible to position quantum dots at predetermined positions oil a substrate in registry with alignment markers This, in turn, has allowed for devices fabricated in subsequent processing steps to be aligned to individual quantum dots The specific devices being targeted are gated-single dots and Coupled dot-cavity systems which are key components of efficient sources of single photons and entangled photon pairs

The spectral characteristics of a ring resonator made of Si photonic wires are modeled using mode expansion of super-modes of the directional coupler. The influence of the coupling coefficient, loss factor and waveguide dispersion on the spectral features are analyzed in detail. The model is then compared with the experimental data of a ring resonator designed for sensing purposes. The model that includes a wavelength dependence on coupling length reproduces the large variations of the envelope of the experimental spectrum, when coupling coefficient cover its full range from 0 to 1. Fitting parameters explain the details of the experimental spectrum and contribute to the sensor optimization, as well as illustrating general guidelines for ring resonator design.

Photoluminescence from individual InAs/InP quantum dots embedded within a planar n-i structure is studied as a function of vertical electric field. We demonstrate control of the electron number and determine the Stark shift and built-in dipole at zero electric field, In contrast to the well studied InAs/GaAs quantum dot material system, we obtain a built-in dipole which indicates that the electron lies above the hole at zero electric field. The magnitude and direction of the measured dipole suggests a uniform quantum dot composition. The gating principles we demonstrate can be applied to pre-positioned InAs/InP quantum dots, such that arrays of initialized single spins can be employed for quantum information applications. (c) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Silicon photonic wire evanescent field (PWEF) waveguide sensors have a very high intrinsic response to molecular adsorption. This response arises from the high refractive index contrast and small size of these waveguides, which in combination cause much of the electric field to be localized to a thin layer adjacent to the waveguide surface. We describe the basic theory of PWEF waveguide sensors and compare the predicted PWEF sensor performance with that of surface plasmon resonance (SPR) and other waveguide systems. Finally, we present experimental results for PWEF sensors incorporated into integrated optical circuits employing Mach-Zehnder interferometers (MZIs), ring resonators, and folded waveguide structures that both amplify and facilitate the interrogation of the PWEF sensor response.

A method is developed for extracting the coupling and loss coefficients of ring resonators from the peak widths, depths, and spacings of the resonances of a single resonator. Although the formulas used do not distinguish which coefficient is coupling and which is loss, it is shown how these coefficients can be disentangled based on how they vary with wavelength or device parameters.

We present the design and numerical simulation results for a silicon waveguide modulator based on carrier depletion in a linear array of periodically interleaved PN junctions that are oriented perpendicular to the light propagation direction. In this geometry the overlap of the optical waveguide mode with the depletion region is much larger than in designs using a single PN junction aligned parallel to the waveguide propagation direction. Simulations predict that an optimized modulator will have a high modulation efficiency of 0.56 V x cm for a 3 V bias, with a 3 dB frequency bandwidth of over 40 GHz. This device has a length of 1.86 mm with a maximum intrinsic loss of 4.3 dB at 0 V bias, due to free carrier absorption.

We demonstrate precise control of electron charging within a single deterministically positioned InAs/InP quantum dot emitting in the telecommunications band around 1500 nm. Photolumine-scence emission as a function of vertical electric field is used to monitor the electron number within the dot. From Stark shift spectroscopy, we extract strength and orientation for the built-in dipole moment that suggests a uniform InAs dot composition and a configuration in which the electron lies above the hole at zero electric field. The scalable gating technology we employ to electrically contact individual prepositioned quantum dots promises arrays of initialized single spins that can be used for fiber-based quantum information applications.

In field-effect transistors used to detect charged biomolecules (BioFETs), the biomolecules form a charged membrane on the transistor surface. In this paper, the one-dimensional Poisson–Boltzmann equation is used to calculate the charge sensitivity (the sensitivity of the BioFET to changes in biomolecule charge), ion sensitivity (to changes in ion concentration of the solution), or pH sensitivity (to changes in pH of the solution), both analytically and numerically, and the results are compared to models where the charged molecules are represented as an infinitely thin plane. Complexation of ions with the oxide surface is shown to have a negligible effect on parameters typical of devices, but the layer used to tether the charged molecules to the surface could modify the sensitivity considerably.

A crucial pre-requisite for quantum information applications is the ability to produce a scalable quantum system that can be controllably manipulated. We present a technique that promises the practical realization of a scalable system with arrays of gated quantum dots (QDs) emitting at wavelengths suitable for fibre based quantum cryptography. We show how the random nature of the nucleation site for self-assembled QDs can be overcome by a nanotemplate deposition technique such that further processing around these dots can be performed. By knowing exactly where the QD is, we position electrostatic gates around the dot with better than 30nm accuracy. With these gates and a Si n-doped back gate, we demonstrate from both simulation and experiment that we can control the electron number of the dot via vertical electric fields.

We present the novel design of a silicon modulator with low operation voltage of <= 3 V by employing periodically interleaved pn junctions. Simulations predict that in depletion mode it has a high modulation efficiency of better than 1.5 V.cm.

Field-effect sensors with hafnium silicate gate insulators were fabricated following standard CMOS processes and compatible post-processes. The floating gates were functionalized with glycidoxypropyltrimethoxy silane and amine-terminated oligonucleotide probes. The pH sensitivity of the silicate surface decreased from 31.4 mV/pH to 24.6 mV/pH after functionalization due to passivation of a significant fraction of the amphoteric proton binding sites. A threshold voltage shift of 33 mV was observed as the buffer solution concentration changed from 0.015 M to 0.3 M after DNA probe molecule attachment. The hybridization caused a net increase in this shift of 3mV. With the help of a model which includes the solution of the Poisson-Boltzmann and proton site-binding at the gate, methods of achieving higher sensitivity are discussed.

We demonstrate folded waveguide ring resonators for biomolecular sensing. We show that extending the ring cavity length increases the resonator quality factor, and thereby enhances the sensor resolution and minimum level of detection, while at the same time relaxing the tolerance on the coupling conditions to provide stable and large resonance contrast. The folded spiral path geometry allows a 1.2 mm long ring waveguide to be enclosed in a 150 microm diameter sensor area. The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of approximately 3 pg/mm(2), or a total mass of approximately 5 fg. The real time measurements are used to analyze the kinetics of biotin-streptavidin binding.

The effect of screening and the establishment of the Donnan potential at the floating gate of field-effect transistors (BioFETs) are calculated for a model that includes proton site binding on metal oxides used as gate insulators. Variations of response with pH, electrolyte concentration, and the fixed charge density of oligonucleotides are calculated assuming the charge is distributed homogenously in an ion-permeable membrane of finite thickness. Derived expressions describe the change in threshold voltage with electrolyte concentration and pH. These could be used to predict the sensitivity of the BioFET to hybridization, and provide a means of calibration prior to use.

We examine the effect of a lateral electric field on the optical properties of a single deterministically positioned InAs/InP quantum dot. We show experimentally that the ground-state excitonic Stark shift is significantly reduced in comparison with the single-particle picture and that the lateral electric field introduces a new previously forbidden optical transition. Results of full configuration-interaction calculations show that the Coulomb interactions of electrons and holes are modified by the electric field leading to the compensation of the single-particle Stark shift. The calculations also account for the appearance of the field-activated optical transition as an excitonic recombination event. The comparison of exciton and predicted charged exciton spectra allows us to exclude the presence of charged exciton complexes within the measured emission spectra. The ability to precisely position a single quantum dot and demonstrate control over the electronic properties of such a dot is expected to find application in scalable techniques for quantum information science.