98 research outputs found
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Integrated shadow mask for sampled Bragg gratings in chalcogenide (As2S3) planar waveguides
No full textWe have developed a new approach for producing high
performance sampled Bragg gratings in planar waveguides as a platform for
WDM on-chip signal processing in a compact integrated device. Using this
method we have successfully integrated a shadow mask directly onto a
chalcogenide (As2S3) waveguide using standard semiconductor processing,
eliminating misalignment errors between the mask and waveguide that
otherwise occur. Through this integrated mask we demonstrate a very low
duty cycle sampled Bragg grating with very narrow rejection peaks and
spanning a very broad bandwidth
Note on the thermodynamic Bethe ansatz approach to the quantum phase diagram of the strong coupling ladder compounds
No full textWe investigate the low-temperature phase diagram of the exactly
solved su(4) two-leg spin ladder as a function of the rung coupling J⊥ and
magnetic field H by means of the thermodynamic Bethe ansatz (TBA). In the
absence of a magnetic field the model exhibits three quantum phases, while
in the presence of a strong magnetic field there is no singlet ground state for
ferromagnetic rung coupling. For antiferromagnetic rung coupling, there is
a gapped phase in the regime H < Hc1, a fully polarized gapped phase for
H > Hc2 and a Luttinger liquid magnetic phase in the regime Hc1 < H < Hc2.
The critical behaviour derived using the TBA is consistent with the existing
experimental, numerical and perturbative results for the strong coupling ladder
compounds. This includes the spin excitation gap and the critical fields Hc1 and
Hc2, which are in excellent agreement with the experimental values for the known
strong coupling ladder compounds (5IAP)2CuBr4·2H2O, Cu2(C5H12N2)2Cl4 and
(C5H12N)2CuBr4. In addition we predict the spin gap ∆ ≈ J⊥−1/2J║ for the weak
coupling compounds with J⊥ ∼ J║ , such as (VO)2P2O7, and also show that the
gap opens for arbitrary J⊥/J║
The photocurrent response of human cones is fast and monophasic
No full textBACKGROUND:
The precise form of the light response of human cone photoreceptors in vivo has not been established with certainty. To investigate the response shape we compare the predictions of a recent model of transduction in primate cone photoreceptors with measurements extracted from human cones using the paired-flash electroretinogram method. As a check, we also compare the predictions with previous single-cell measurements of ground squirrel cone responses.
RESULTS:
The predictions of the model provide a good description of the measurements, using values of parameters within the range previously determined for primate retina. The dim-flash response peaks in about 20 ms, and flash responses at all intensities are essentially monophasic. Three time constants in the model are extremely short: the two time constants for inactivation (of visual pigment and of transducin/phosphodiesterase) are around 3 and 10 ms, and the time constant for calcium equilibration lies in the same range.
CONCLUSION:
The close correspondence between experiment and theory, using parameters previously derived for recordings from macaque retina, supports the notion that the electroretinogram approach and the modelling approach both provide an accurate estimate of the cone photoresponse in the living human eye. For reasons that remain unclear, the responses of isolated photoreceptors from the macaque retina, recorded previously using the suction pipette method, are considerably slower than found here, and display biphasic kinetics
A comparison of methods for calculating population exposure estimates of daily weather for health research
No full textBACKGROUND: To explain the possible effects of exposure to weather conditions on population
health outcomes, weather data need to be calculated at a level in space and time that is appropriate
for the health data. There are various ways of estimating exposure values from raw data collected
at weather stations but the rationale for using one technique rather than another; the significance
of the difference in the values obtained; and the effect these have on a research question are factors
often not explicitly considered. In this study we compare different techniques for allocating
weather data observations to small geographical areas and different options for weighting averages
of these observations when calculating estimates of daily precipitation and temperature for
Australian Postal Areas. Options that weight observations based on distance from population
centroids and population size are more computationally intensive but give estimates that
conceptually are more closely related to the experience of the population.
RESULTS: Options based on values derived from sites internal to postal areas, or from nearest
neighbour sites – that is, using proximity polygons around weather stations intersected with postal
areas – tended to include fewer stations' observations in their estimates, and missing values were
common. Options based on observations from stations within 50 kilometres radius of centroids
and weighting of data by distance from centroids gave more complete estimates. Using the
geographic centroid of the postal area gave estimates that differed slightly from the population
weighted centroids and the population weighted average of sub-unit estimates.
CONCLUSION: To calculate daily weather exposure values for analysis of health outcome data for
small areas, the use of data from weather stations internal to the area only, or from neighbouring
weather stations (allocated by the use of proximity polygons), is too limited. The most appropriate
method conceptually is the use of weather data from sites within 50 kilometres radius of the area
weighted to population centres, but a simpler acceptable option is to weight to the geographic
centroid
Dispersion extraction with near-field measurements in periodic waveguides
No full textWe formulate and demonstrate experimentally the highresolution
spectral method based on Bloch-wave symmetry properties for
extracting mode dispersion in periodic waveguides from measurements of
near-field profiles. We characterize both the propagating and evanescent
modes, and also determine the amplitudes of forward and backward waves
in different waveguide configurations, with the estimated accuracy of several
percent or less. Whereas the commonly employed spatial Fourier-transform
(SFT) analysis provides the wavenumber resolution which is limited by the
inverse length of the waveguide, we achieve precise dispersion extraction
even for compact photonic structures
U-Pb SHRIMP and Sm-Nd geochronology of the Silvânia volcanics and Jurubatuba granite: juvenile Paleoproterozoic crust in the basement of the Neoproterozoic Brasília Belt, central Brazil
No full textU-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic metavolcanic rocks from the Silvânia Sequence and Jurubatuba Granite in the central part of the Brasília Belt. Zircon grains from a metavolcanic sample yielded 2115 ± 23 Ma and from the granite yielded 2089 ± 14 Ma, interpreted as crystallization ages of these rocks.
Six metavolcanic samples of the Silvânia Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 ± 110 Ma and positive eNd(T) = +3.0 interpreted as a juvenile magmatic event.
Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic TDM model ages between 2.30 and 2.42 Ga and eNd(T) values vary between -0.22 and -0.58. The oldest TDM value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvânia volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasília Belt that implies the presence of arc/suture hidden in reworked basement of the Brasília Belt
Fabrication and characterization of low loss rib chalcogenide waveguides made by dry etching
No full textWe report the fabrication and characterization of rib
chalcogenide waveguides produced by dry etching with CF4 and O2. The
high index contrast waveguides (Δn ~ 1) show a minimum propagation loss
of 0.25 dB/cm. The high refractive nonlinearity of ~ 100 times silica in
As2S3 allowed observation of a Д phase shift due to self-phase modulation of
an 8 ps duration 1573 nm pulse in a 5 cm long waveguide
Nonlinear properties of left-handed metamaterials
No full textWe analyze the properties of microstructured materials with negative refraction, the so-called left-handed metamaterials. We consider a two-dimensional periodic structure created by arrays of wires and split-ring resonators embedded into a nonlinear dielectric, and calculate the effective nonlinear electric permittivity and magnetic permeability. We demonstrate that the hysteresis-type dependence of the magnetic permeability on the field intensity allows changing the material properties from left- to right-handed and back. These effects can be treated as the second-order phase transitions in the transmission properties induced by the variation of an external field
Sharp bends in photonic crystal waveguides as nonlinear Fano resonators
No full textWe demonstrate that high transmission through sharp bends in
photonic crystal waveguides can be described by a simple model of the Fano
resonance where the waveguide bend plays a role of a specific localized
defect.We derive effective discrete equations for two types of the waveguide
bends in two-dimensional photonic crystals and obtain exact analytical
solutions for the resonant transmission and reflection. This approach allows
us to get a deeper insight into the physics of resonant transmission, and it
is also useful for the study and design of power-dependent transmission
through the waveguide bends with embedded nonlinear defects
Slow phase relaxation as a route to quantum computing beyond the quantum chaos border
No full textWe reveal that phase memory can be much longer than energy relaxation in systems with exponentially large dimensions of Hilbert space; this finding is documented by 50 years of nuclear experiments, though the information is somewhat hidden. For quantum computers Hilbert spaces of dimension 2100 or larger will be typical and therefore this effect may contribute significantly to reduce the problems of scaling of quantum computers to a useful number of qubits