139 research outputs found
Group Theory of Chiral Photonic Crystals with 4-fold Symmetry: Band Structure and S-Parameters of Eight-Fold Intergrown Gyroid Nets
The Single Gyroid, or srs, nanostructure has attracted interest as a
circular-polarisation sensitive photonic material. We develop a group
theoretical and scattering matrix method, applicable to any photonic crystal
with symmetry I432, to demonstrate the remarkable chiral-optical properties of
a generalised structure called 8-srs, obtained by intergrowth of eight
equal-handed srs nets. Exploiting the presence of four-fold rotations, Bloch
modes corresponding to the irreducible representations E- and E+ are identified
as the sole and non-interacting transmission channels for right- and
left-circularly polarised light, respectively. For plane waves incident on a
finite slab of the 8-srs, the reflection rates for both circular polarisations
are identical for all frequencies and transmission rates are identical up to a
critical frequency below which scattering in the far field is restricted to
zero grating order. Simulations show the optical activity of the lossless
dielectric 8-srs to be large, comparable to metallic metamaterials,
demonstrating its potential as a nanofabricated photonic material
Group Theory of Circular-Polarization Effects in Chiral Photonic Crystals with Four-Fold Rotation Axes, Applied to the Eight-Fold Intergrowth of Gyroid Nets
We use group or representation theory and scattering matrix calculations to
derive analytical results for the band structure topology and the scattering
parameters, applicable to any chiral photonic crystal with body-centered cubic
symmetry I432 for circularly-polarised incident light. We demonstrate in
particular that all bands along the cubic [100] direction can be identified
with the irreducible representations E+/-,A and B of the C4 point group. E+ and
E- modes represent the only transmission channels for plane waves with wave
vector along the ? line, and can be identified as non-interacting transmission
channels for right- (E-) and left-circularly polarised light (E+),
respectively. Scattering matrix calculations provide explicit relationships for
the transmission and reflectance amplitudes through a finite slab which
guarantee equal transmission rates for both polarisations and vanishing
ellipticity below a critical frequency, yet allowing for finite rotation of the
polarisation plane. All results are verified numerically for the so-called
8-srs geometry, consisting of eight interwoven equal-handed dielectric Gyroid
networks embedded in air. The combination of vanishing losses, vanishing
ellipticity, near-perfect transmission and optical activity comparable to that
of metallic meta-materials makes this geometry an attractive design for
nanofabricated photonic materials
Nature of topological protection in photonic spin and valley Hall insulators
Recent interest in optical analogs to the quantum spin Hall and quantum valley Hall effects is driven by the
promise to establish topologically protected photonic edge modes at telecommunication and optical wavelengths
on a simple platform suitable for industrial applications. While first theoretical and experimental efforts have
been made, these approaches so far both lack a rigorous understanding of the nature of topological protection and
the limits of backscattering immunity. We here use a generic group theoretical methodology to fill this gap and
obtain general design principles for purely dielectric two-dimensional topological photonic systems. The method
comprehensively characterizes possible two-dimensional hexagonal designs and reveals their topological nature,
potential, and limits
Designing Refractive Index Fluids using the Kramers-Kronig Relations
For a number of optical applications, it is advantageous to precisely tune
the refractive index of a liquid. Here, we harness a well-established concept
in optics for this purpose. The Kramers-Kronig relation provides physical
connection between the spectral variation of the (real) refractive index and
the absorption coefficient. In particular a sharp spectral variation of the
absorption coefficient gives rise to either an enhancement or reduction of the
refractive index in the spectral vicinity of this variation. By using bright
commodity dyes that fulfil this absorption requirement, we demonstrate the use
of the Kramers-Kronig relation to predictively dial-in refractive index values
in water solutions that are otherwise only attained by toxic specialised
liquids
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A functional genetic toolbox for human tissue-derived organoids.
Funder: Alzheimers Research UK Stem Cell Research CentreHuman organoid systems recapitulate key features of organs offering platforms for modelling developmental biology and disease. Tissue-derived organoids have been widely used to study the impact of extrinsic niche factors on stem cells. However, they are rarely used to study endogenous gene function due to the lack of efficient gene manipulation tools. Previously, we established a human foetal lung organoid system (Nikolić et al., 2017). Here, using this organoid system as an example we have systematically developed and optimised a complete genetic toolbox for use in tissue-derived organoids. This includes 'Organoid Easytag' our efficient workflow for targeting all types of gene loci through CRISPR-mediated homologous recombination followed by flow cytometry for enriching correctly-targeted cells. Our toolbox also incorporates conditional gene knock-down or overexpression using tightly-inducible CRISPR interference and CRISPR activation which is the first efficient application of these techniques to tissue-derived organoids. These tools will facilitate gene perturbation studies in tissue-derived organoids facilitating human disease modelling and providing a functional counterpart to many on-going descriptive studies, such as the Human Cell Atlas Project
Laser-induced forced evaporative cooling of molecular anions below 4 Kelvin
The study of cold and controlled molecular ions is pivotal for fundamental
research in modern physics and chemistry. Investigations into cooling molecular
anions, in particular, have proven to be of key consequence for the production
of cold antihydrogen, the creation, and study of anionic Coulomb crystals as
well as in atmospheric research and astrochemistry. The commonly used anion
cooling technique via collisions with a buffer gas is limited by the
temperature of the used cryogenic cooling medium. Here, we demonstrate forced
evaporative cooling of anions via a laser beam with photon energies far above
the photodetachment threshold of the anion. We reach runaway evaporative
cooling of an anionic OH ensemble from an initial temperature of 370(12)
K down to 2.2(8) K. This corresponds to three orders of magnitude increase in
the ions' phase space density approaching the near-strong Coulomb coupling
regime. A quantitative analysis of the experimental results, via a full
thermodynamic model including the role of intrinsic collisional heating,
represents the anion cooling dynamics without any fitting parameters. This
technique can be used to cool, in principle, any anionic species below liquid
helium temperature, providing a novel tool to push the frontiers of anion
cooling to much lower than the state-of-the-art temperature regimes
Biopolymer photonics: from nature to nanotechnology
Biopolymers offer vast potential for renewable and sustainable devices. While nature mastered the use of biopolymers to create highly complex 3D structures and optimized their photonic response, artificially created structures still lack nature's diversity. To bridge this gap between natural and engineered biophotonic structures, fundamental questions such as the natural formation process and the interplay of structural order and disorder must be answered. Herein, biological photonic structures and their characterization techniques are reviewed, focusing on those structures not yet artificially manufactured. Then, employed and potential nanofabrication strategies for biomimetic, bio-templated, and artificially created biopolymeric photonic structures are discussed. The discussion is extended to responsive biopolymer photonic structures and hybrid structures. Last, future fundamental physics, chemistry, and nanotechnology challenges related to biopolymer photonics are foreseen.Peer ReviewedPostprint (published version
Gapless unidirectional photonic transport using all-dielectric kagome lattices
Photonic topological insulators are a promising photonic platform due to the possibility of unidirectional edge states with insensitivity to bending, fabrication imperfections or environmental fluctuation. Here we demonstrate highly efficient unidirectional photonic edge mode propagation facilitated by an optical analogue of the quantum valley Hall effect. With an all-dielectric kagome lattice design, we demonstrate broadband suppressed reflection in the presence of sharp corners and further show negligible vertical losses in a semiconductor-based device at telecommunication wavelengths
Group Theoretical Route to Deterministic Weyl Points in Chiral Photonic Lattices.
Topological phases derived from point degeneracies in photonic band structures show intriguing and unique behavior. Previously identified band degeneracies are based on accidental degeneracies and subject to engineering on a case-by-case basis. Here we show that deterministic pseudo Weyl points with nontrivial topology and hyperconic dispersion exist at the Brillouin zone center of chiral cubic symmetries. This conceivably allows realization of topologically protected frequency isolated surface bands in 3D and n=0 properties as demonstrated for a nanoplasmonic system and a photonic crystal
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