146 research outputs found
Topological Insulators by Topology Optimization
An acoustic topological insulator (TI) is synthesized using topology
optimization, a free material inverse design method. The TI appears
spontaneously from the optimization process without imposing requirements on
the existence of pseudo spin-1/2 states at the TI interface edge, or the Chern
number of the topological phases. The resulting TI is passive; consisting of
acoustically hard members placed in an air background and has an operational
bandwidth of 12.5\% showing high transmission. Further analysis
demonstrates confinement of more than 99\% of the total field intensity in the
TI within at most six lattice constants from the TI interface. The proposed
design hereby outperforms a reference from recent literature regarding energy
transmission, field confinement and operational bandwidth.Comment: 6 pages, 5 figure
Designing Photonic Topological Insulators with Quantum-Spin-Hall Edge States using Topology Optimization
Designing photonic topological insulators is highly non-trivial because it
requires inversion of band symmetries around the band gap, which was so far
done using intuition combined with meticulous trial and error. Here we take a
completely different approach: we consider the design of photonic topological
insulators as an inverse design problem and use topology optimization to
maximize the transmission through an edge mode with a sharp bend. Two design
domains composed of two different, but initially identical,
C-symmetric unit cells define the geometrical design problem.
Remarkably, the optimization results in a photonic topological insulator
reminiscent of the shrink-and-grow approach to quantum-spin-Hall photonic
topological insulators but with notable differences in the topology of the
crystal as well as qualitatively different band structures and with
significantly improved performance as gauged by the band-gap sizes, which are
at least 50 \% larger than previous designs. Furthermore, we find a directional
beta factor exceeding 99 \%, and very low losses for sharp bends. Our approach
allows for the introduction of fabrication limitations by design and opens an
avenue towards designing PTIs with hitherto unexplored symmetry constraints.Comment: 7 pages, 5 figure
3D architected isotropic materials with tunable stiffness and buckling strength
This paper presents a class of 3D single-scale isotropic materials with
tunable stiffness and buckling strength obtained via topology optimization and
subsequent shape optimization. Compared to stiffness-optimal closed-cell plate
material, the material class reduces the Young's modulus to a range from 79% to
58%, but improves the uniaxial buckling strength to a range from 180% to 767%.
Based on small deformation theory, material stiffness is evaluated using the
homogenization method. Buckling strength under a given macroscopic stress state
is estimated using linear buckling analysis with Block-Floquet boundary
conditions to capture both short and long wavelength buckling modes. The 3D
isotropic single-scale materials with tunable properties are designed using
topology optimization, and are then further simplified using shape
optimization. Both topology and shape optimized results demonstrate that
material buckling strength can be significantly enhanced by hybrids between
truss and variable thickness plate structures
Maximizing the quality factor to mode volume ratio for ultra-small photonic crystal cavities
Small manufacturing-tolerant photonic crystal cavities are systematically
designed using topology optimization to enhance the ratio between quality
factor and mode volume, Q/V. For relaxed manufacturing tolerance, a cavity with
bow-tie shape is obtained which confines light beyond the diffraction limit
into a deep-subwavelength volume. Imposition of a small manufacturing tolerance
still results in efficient designs, however, with diffraction-limited
confinement. Inspired by numerical results, an elliptic ring grating cavity
concept is extracted via geometric fitting. Numerical evaluations demonstrate
that for small sizes, topology-optimized cavities enhance the Q/V-ratio by up
to two orders of magnitude relative to standard L1 cavities and more than one
order of magnitude relative to shape-optimized L1 cavities. An increase in
cavity size can enhance the Q/V-ratio by an increase of the Q-factor without
significant increase of V. Comparison between optimized and reference cavities
illustrates that significant reduction of V requires big topological changes in
the cavity
On the competition for ultimately stiff and strong architected materials
Advances in manufacturing techniques may now realize virtually any imaginable
microstructures, paving the way for architected materials with properties
beyond those found in nature. This has lead to a quest for closing gaps in
property-space by carefully designed metamaterials. Development of mechanical
metamaterials has gone from open truss lattice structures to closed plate
lattice structures with stiffness close to theoretical bounds. However, the
quest for optimally stiff and strong materials is complex. Plate lattice
structures have higher stiffness and (yield) strength but are prone to buckling
at low volume fractions. Hence here, truss lattice structures may still be
optimal. To make things more complicated, hollow trusses or structural
hierarchy bring closed-walled microstructures back in the competition. Based on
analytical and numerical studies of common microstructures from the literature,
we provide higher order interpolation schemes for their effective stiffness and
(buckling) strength. Furthermore, we provide a case study based on
multi-property Ashby charts for weight-optimal porous beams under bending, that
demonstrates the intricate interplay between structure and microarchitecture
that plays the key role in the design of ultimate load carrying structures. The
provided interpolation schemes may also be used to account for microstructural
yield and buckling in multiscale design optimization schemes.Comment: 18 pages main manuscript with 6 figures and 10 pages appendix with 2
figure
Fundamental limitations to gain enhancement in periodic media and waveguides
A common strategy to compensate for losses in optical nanostructures is to
add gain material in the system. By exploiting slow-light effects it is
expected that the gain may be enhanced beyond its bulk value. Here we show that
this route cannot be followed uncritically: inclusion of gain inevitably
modifies the underlying dispersion law, and thereby may degrade the slow-light
properties underlying the device operation and the anticipated gain enhancement
itself. This degradation is generic; we demonstrate it for three different
systems of current interest (coupled resonator optical waveguides, Bragg
stacks, and photonic crystal waveguides). Nevertheless, a small amount of added
gain may be beneficial
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