866 research outputs found
High-Efficiency All-Dielectric Metalenses for Mid-Infrared Imaging
Metasurfaces-based flat optics, which can make use of existing foundry planar technology for high-throughput production, allows the arbitrary control of the wavefront and polarization of light within subwavelength thick structures. So far, however, flat optics for the mid-infrared (MIR) has received far less attention than devices operating at visible or near-infrared wavelengths. Here, polarization-insensitive, highly efficient, all-dielectric metalenses operating in the MIR around 4 µm are demonstrated. The metalens is designed using rigorous coupled-wave analysis and is based on hydrogenated amorphous silicon (α-Si:H) nanopillars supported by an MgF2 substrate. The metalenses produce close to a diffraction-limited focal spot and can resolve structures on the wavelength scale where the focusing efficiency reaches 78% at a magnification of 120×. The imaging qualities are comparable with commercial bulk-molded chalcogenide aspheric lenses. These results provide novel solutions for existing MIR technology and nurture new functionalities with the population of miniaturized and planarized optoelectrical devices.The authors acknowledge the facility support from the ANU node of
the Australian National Fabrication Facility (ANFF). This work was
supported by China Scholarship Council (201506310074); Australian
Research Council (ARC) Future Fellowship (FT110100853); and the
ARC Centre of Excellence for Ultrahigh Bandwidth Devices for Optical
Systems (CE110001018)
Random Exchange Disorder in the Spin-1/2 XXZ Chain
The one-dimensional XXZ model is studied in the presence of disorder in the
Heisenberg Exchange Integral. Recent predictions obtained from renormalization
group calculations are investigated numerically using a Lanczos algorithm on
chains of up to 18 sites. It is found that in the presence of strong
X-Y-symmetric random exchange couplings, a ``random singlet'' phase with
quasi-long-range order in the spin-spin correlations persists. As the planar
anisotropy is varied, the full zero-temperature phase diagram is obtained and
compared with predictions of Doty and Fisher [Phys. Rev. B {\bf 45 }, 2167
(1992)].Comment: 9 pages + 8 plots appended, RevTex, FSU-SCRI-93-98 and
ORNL/CCIP/93/1
Finite temperature transport at the superconductor-insulator transition in disordered systems
I argue that the incoherent, zero-frequency limit of the universal crossover
function in the temperature-dependent conductivity at the
superconductor-insulator transition in disordered systems may be understood as
an analytic function of dimensionality of system d, with a simple pole at d=1.
Combining the exact result for the crossover function in d=1 with the recursion
relations in d=1+\epsilon, the leading term in the Laurent series in the small
parameter \epsilon for this quantity is computed for the systems of disordered
bosons with short-range and Coulomb interactions. The universal,
low-temperature, dc critical conductivity for the dirty boson system with
Coulomb interaction in d=2 is estimated to be 0.69 (2e)^2 /h, in relatively
good agreement with many experiments on thin films. The next order correction
is likely to somewhat increase the result, possibly bringing it closer to the
self-dual value.Comment: 9 pages, LaTex, no figure
Use of voltammetric solid-state (micro)electrodes for studying biogeochemical processes: Laboratory measurements to real time measurements with an in situ electrochemical analyzer (ISEA)
Solid-state voltammetric (micro)electrodes have been used in a variety of environments to study biogeochemical processes. Here we show the wealth of information that has been obtained in the study of sediments, microbial mats, cultures and the water column including hydrothermal vents. Voltammetric analyzers have been developed to function with operator guidance and in unattended mode for temporal studies with an in situ electrochemical analyzer (ISEA). The electrodes can detect the presence (or absence) of a host of redox species and trace metals simultaneously. The multi-species capacity of the voltammetric electrode can be used to examine complex heterogeneous environments such as the root zone of salt marsh sediments. The data obtained with these systems clearly show that O2 and Mn2+ profiles in marine sedimentary porewaters and in microbial biofilms on metal surfaces rarely overlap indicating that O2 is not a direct oxidant for Mn2+. This lack of overlap was suggested originally by Joris Gieskes\u27 group. In waters emanating from hydrothermal vents, Fe2+, H2S and soluble molecular FeS clusters (FeSaq) are detected indicating that the reactants for the pyrite formation reaction are H2S and soluble molecular FeS clusters. Using the ISEA with electrodes at fixed positions, data collected continuously over three days near a Riftia pachyptila tubeworm field generally show that O2 and H2S anti-correlate and that H2S and temperature generally correlate. Unlike sedimentary environments, the data clearly show that Riftia live in areas where both O2 and H2S co-exist so that its endosymbiont bacteria can perform chemosynthesis. However, physical mixing of diffuse flow vent waters with oceanic bottom waters above or to the side of the tubeworm field can dampen these correlations or even reverse them. Voltammetry is a powerful technique because it provides chemical speciation data (e.g.; oxidation state and different elemental compounds/ions) as well as quantitative data. Because (micro)organisms occupy environmental niches due to the system\u27s chemistry, it is necessary to know chemical speciation. Voltammetric methods allow us to study how chemistry drives biology and how biology can affect chemistry for its own benefit
Antiferromagnetic Heisenberg chains with bond alternation and quenched disorder
We consider S=1/2 antiferromagnetic Heisenberg chains with alternating bonds
and quenched disorder, which represents a theoretical model of the compound
CuCl_{2x}Br_{2(1-x)}(\gamma-{pic})_2. Using a numerical implementation of the
strong disorder renormalization group method we study the low-energy properties
of the system as a function of the concentration, x, and the type of
correlations in the disorder. For perfect correlation of disorder the system is
in the random dimer (Griffiths) phase having a concentration dependent
dynamical exponent. For weak or vanishing disorder correlations the system is
in the random singlet phase, in which the dynamical exponent is formally
infinity. We discuss consequences of our results for the experimentally
measured low-temperature susceptibility of
CuCl_{2x}Br_{2(1-x)}(\gamma-{pic})_2
Photonic mid-infrared nulling for exoplanet detection on a planar chalcogenide platform
The future of exoplanet detection lies in the mid-infrared (MIR). The MIR
region contains the blackbody peak of both hot and habitable zone exoplanets,
making the contrast between starlight and planet light less extreme. It is also
the region where prominent chemical signatures indicative of life exist, such
as ozone at 9.7 microns. At a wavelength of 4 microns the difference in
emission between an Earth-like planet and a star like our own is 80 dB. However
a jovian planet, at the same separation exhibits 60 dB of contrast, or only 20
dB if it is hot due to its formation energy or being close to its host star. A
two dimensional nulling interferometer, made with chalcogenide glass, has been
measured to produce a null of 20 dB, limited by scattered light. Measures to
increase the null depth to the theoretical limit of 60 dB are discussed.Comment: Was published in SPIE: Optical and Infrared Interferometry and
Imaging VI, Mike Ireland presente
A Prospective Longitudinal Cohort to Investigate the Effects of Early Life Giardiasis on Growth and All Cause Diarrhea
Background. Growth stunting in children under 2 years of age in low-income countries is common. Giardia is a ubiquitous pathogen in this age group but studies investigating Giardia's effect on both growth and diarrhea have produced conflicting results
Chalcogenide glass planar MIR couplers for future chip based Bracewell interferometers
Photonic integrated circuits are established as the technique of choice for a
number of astronomical processing functions due to their compactness, high
level of integration, low losses, and stability. Temperature control,
mechanical vibration and acoustic noise become controllable for such a device
enabling much more complex processing than can realistically be considered with
bulk optics. To date the benefits have mainly been at wavelengths around 1550
nm but in the important Mid-Infrared region, standard photonic chips absorb
light strongly. Chalcogenide glasses are well known for their transparency to
beyond 10000 nm, and the first results from coupler devices intended for use in
an interferometric nuller for exoplanetary observation in the Mid-Infrared L
band (3800-4200 nm) are presented here showing that suitable performance can be
obtained both theoretically and experimentally for the first fabricated devices
operating at 4000 nm.Comment: in Proc. SPIE 9907, Optical and Infrared Interferometry and Imaging
V, 990730 (August 4, 2016
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