74 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
Inverse design in photonics by topology optimization: tutorial
Topology optimization methods for inverse design of nano-photonic systems
have recently become extremely popular and are presented in various forms and
under various names. Approaches comprise gradient and non-gradient based
algorithms combined with more or less systematic ways to improve convergence,
discreteness of solutions and satisfaction of manufacturing constraints. We
here provide a tutorial for the systematic and efficient design of
nano-photonic structures by Topology Optimization (TopOpt). The implementation
is based on the advanced and systematic approaches developed in TopOpt for
structural optimization during the last three decades. The tutorial presents a
step-by-step guide for deriving the continuous constrained optimization problem
forming the foundation of the Topology Optimization method, using a cylindrical
metalens design problem as an example. It demonstrates the effect and necessity
of applying a number of auxiliary tools in the design process in order to
ensure good numerical modelling practice and to achieve physically realisable
designs. Application examples also include an optical demultiplexer.Comment: 8 figures, 19 page
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
Topology Optimization of Surface-enhanced Raman Scattering Substrates
Surface-enhanced Raman spectroscopy is a powerful and versatile sensing
method with a detection limit down to the single molecule level. In this
article, we demonstrate how topology optimization (TopOpt) can be used for
designing surface enhanced Raman scattering (SERS) substrates adhering to
realistic fabrication constraints. As an example, we experimentally
demonstrated a SERS enhancement factor of 5*10e4 for the 604 cm-1 Raman line of
rhodamine 6G using metal nanostructures with a critical dimension of 20 nm. We
then show that, by relaxing the fabrication constraints, TopOpt may be used to
design SERS substrates with orders of magnitude larger enhancement factor. The
results validate topology optimization as an effective method for engineering
nanostructures with optimal performance and fabrication tolerance.Comment: 12 page
Experimental validation of systematically designed acoustic hyperbolic meta material slab exhibiting negative refraction
Designing Structures that Maximize Spatially Averaged Surface-Enhanced Raman Spectra
We present a general framework for inverse design of nanopatterned surfaces
that maximize spatially averaged surface-enhanced Raman (SERS) spectra from
molecules distributed randomly throughout a material or fluid, building upon a
recently proposed trace formulation for optimizing incoherent emission. This
leads to radically different designs than optimizing SERS emission at a single
known location, as we illustrate using several 2D design problems addressing
effects of hot-spot density, angular selectivity, and nonlinear damage. We
obtain optimized structures that perform about 4 times better than coating with
optimized spheres or bowtie structures and about 20 times better when the
nonlinear damage effects are included
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