1,930 research outputs found
Four-dimensional dynamic flow measurement by holographic particle image velocimetry
The ultimate goal of holographic particle image velocimetry (HPIV) is to provide space- and time-resolved measurement of complex flows. Recent new understanding of holographic imaging of small particles, pertaining to intrinsic aberration and noise in particular, has enabled us to elucidate fundamental issues in HPIV and implement a new HPIV system. This system is based on our previously reported off-axis HPIV setup, but the design is optimized by incorporating our new insights of holographic particle imaging characteristics. Furthermore, the new system benefits from advanced data processing algorithms and distributed parallel computing technology. Because of its robustness and efficiency, for the first time to our knowledge, the goal of both temporally and spatially resolved flow measurements becomes tangible. We demonstrate its temporal measurement capability by a series of phase-locked dynamic measurements of instantaneous three-dimensional, three-component velocity fields in a highly three-dimensional vortical flow--the flow past a tab
A micromachined zipping variable capacitor
Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and
are found in a wide range of consumer products. At present, MEMS technology for
radio-frequency (RF) applications is maturing steadily, and significant improvements
have been demonstrated over solid-state components.
A wide range of RF MEMS varactors have been fabricated in the last fifteen years.
Despite demonstrating tuning ranges and quality factors that far surpass solid-state
varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance
values while preserving a small device footprint. Secondly, many highly-tunable MEMS
varactors include complex designs or process flows.
In this dissertation, a new micromachined zipping variable capacitor suitable for
application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that
zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown
electrode. Shaping the cantilever using a width function allows stable actuation
and continuous capacitance tuning. Compared to existing MEMS varactors, this device
has a simple design that can be implemented using a straightforward process flow. In
addition, the zipping varactor is particularly suited for incorporating a highpermittivity
dielectric, allowing the capacitance values and tuning range to be scaled
up. This is important for portable consumer electronics where a small device footprint
is attractive.
Three different modelling approaches have been developed for zipping varactor
design. A repeatable fabrication process has also been developed for varactors with a
silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning
range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and
0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and
fit within an area of 500 by 100 μm
Emergence of Topological and Strongly Correlated Ground States in trapped Rashba Spin-Orbit Coupled Bose Gases
We theoretically study an interacting few-body system of Rashba spin-orbit
coupled two-component Bose gases confined in a harmonic trapping potential. We
solve the interacting Hamiltonian at large Rashba coupling strengths using
Exact Diagonalization scheme, and obtain the ground state phase diagram for a
range of interatomic interactions and particle numbers. At small particle
numbers, we observe that the bosons condense to an array of topological states
with n+1/2 quantum angular momentum vortex configurations, where n = 0, 1, 2,
3... At large particle numbers, we observe two distinct regimes: at weaker
interaction strengths, we obtain ground states with topological and symmetry
properties that are consistent with mean-field theory computations; at stronger
interaction strengths, we report the emergence of strongly correlated ground
states.Comment: 14 pages, 9 figure
A micromachined zipping variable capacitor
Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components.A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows.In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive.Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 µm
Probing anisotropic superfluidity of rashbons in atomic Fermi gases
Motivated by the prospect of realizing a Fermi gas of K atoms with a
synthetic non-Abelian gauge field, we investigate theoretically a strongly
interacting Fermi gas in the presence of a Rashba spin-orbit coupling. As the
two-fold spin degeneracy is lifted by spin-orbit interaction, bound pairs with
mixed singlet and triplet pairings (referred to as rashbons) emerge, leading to
an anisotropic superfluid. We show that this anisotropic superfluidity can be
probed via measuring the momentum distribution and single-particle spectral
function in a trapped atomic K cloud near a Feshbach resonance.Comment: 4 pages, 5 figure
Probing Majorana fermions in spin-orbit coupled atomic Fermi gases
We examine theoretically the visualization of Majorana fermions in a
two-dimensional trapped ultracold atomic Fermi gas with spin-orbit coupling. By
increasing an external Zeeman field, the trapped gas transits from
non-topological to topological superfluid, via a mixed phase in which both
types of superfluids coexist. We show that the zero-energy Majorana fermion,
supported by the topological superfluid and localized at the vortex core, is
clearly visible through (i) the core density and (ii) the local density of
states, which are readily measurable in experiment. We present a realistic
estimate on experimental parameters for ultracold K atoms.Comment: 4 pages, 4 figure
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