Phonon sidebands of color centers in hexagonal boron nitride

Low temperature photoluminescence spectra of a color center in hexagonal boron nitride are analyzed. The acoustic phonon sideband can be described by a deformation coupling proportional to strain to a phonon bath that is effectively two dimensional. The optical phonon band is described by Frohlich coupling to the LO-branches, and a deformation coupling proportional to lattice displacement for the TO-branch. The resonances expressed in the optical band vary from defect to defect, in some emitters, coupling to out-of-plane polarized phonons is reported.Comment: 7 pages, 4 fig

Computational simulation of the intervertebral disc

The intervertebral disc is a complex structure unlike any other in the human body. The capability to withstand high loads and deformations in six degrees of freedom is facilitated by the unique soft tissue structures. However, the mechanical behaviour of these tissues is not well understood. The aim of this project was to investigate methods of deriving structural information about the tissues of the intervertebral disc for application in computational simulation, with particular focus on the mechanical function of the annulus fibrosis and how the behaviour of this tissue is governed by its substructures. Magnetic resonance imaging techniques were assessed for potential to inform specimen specific models of the disc. Imaging sequences were developed and validated to image in vitro disc samples in unloaded and compressed states. These images captured the lamellar structure of the annulus in three dimensions to a level of detail not previously published. The image data facilitated the development of a novel method of specimen specific model construction, as well as providing experimental deformation data, against which the models were directly validated. Sensitivity analyses on both generalised and specimen specific models illustrated the influence of interlamellar interaction representation on the gross mechanics of the disc models. The models were adapted to illustrate the effects of tissue degeneration and intervention on disc mechanics Interlamellar interactions and tissue level mechanics were further investigated by developing specimen specific models of disc tissue samples based on microscopy data. Novel methods were developed to implement qualitative histological data into finite element analyses of annulus tissue samples. Interlamellar interactions were shown to provide a strong bond between lamellae. The parameters and variables involved in the mechanical system of the disc pose major challenges for experimental investigation. This study has successfully laid the ground work to negotiate these challenges using a computational approac

Control of spontaneous emission from InP single quantum dots in GaInP photonic crystal nanocavities

We demonstrate semiconductor quantum dots coupled to photonic crystal cavity modes operating in the visible spectrum. We present the design, fabrication, and characterization of two dimensional photonic crystal cavities in GaInP and measure quality factors in excess of 7500 at 680 nm. We demonstrate full control over the spontaneous emission rate of InP quantum dots and by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode we observe a Purcell enhancement of similar to 8. (C) 2010 American Institute of Physics. [doi:10.1063/1.3510469

On kurtosis and extreme waves in crossing directional seas:A laboratory experiment

We examine the statistical properties of extreme and rogue wave activity in crossing directional seas, to constrain the probabilistic distributions of wave heights and wave crests in complex sea states; such crossing seas alter the statistical structure of surface waves and are known to have been involved in several marine accidents. Further, we examine the relationship between the kurtosis as an indicator of nonlinearity in the spectrum and the directionality and crossing angles of the sea-state components. Experimental tests of two-component directionally spread irregular waves with varying frequency, directional spreading and component crossing angles were carried out at the Ocean Basin Laboratory in Trondheim, Norway. The results from the experiments show that wave heights are well described by a first-order (linear) statistical distribution, while for the wave crest heights several cases exceed a second-order distribution. The number of rogue waves is relatively low overall, which agrees with previous findings in directionally spread seas. The kurtosis and wave and crest height exceedance probabilities were more affected by varying the directional spreading of the components than by varying the crossing angles between components; reducing the component directional spreading increases the kurtosis and increases the exceedance probabilities. The kurtosis can be estimated quite well for two-component seas from the directional spreading using an empirical relationship based on the two-dimensional Benjamin–Feir index when the effects of bound modes are included. This result may allow forecasting of the probability of extreme waves from the directional spreading in complex sea states

Gate Tunable Graphene-integrated Metasurface Modulator for Mid-Infrared Beam Steering (article)

This is the final version. Available on open access from Optical Society of America via the DOI in this recordThe data associated with this article is available in ORE at: https://doi.org/10.24378/exe.1304The ability to integrate graphene into metasurface devices has attracted enormous interest as a means of achieving dynamic electrical control of their electromagnetic response. In this manuscript, we experimentally demonstrate a graphene-integrated metasurface modulator that establishes the potential to actively control the amplitude and phase of mid-infrared light with high modulation depth and speed, in good agreement with simulation results. Our simulations also show it is possible to construct a reconfigurable surface with tunable phase profile by incorporating graphene-integrated metasurface modulators with specific geometric parameters. This reconfigurable surface is able to manipulate the orientation of the wave reflected from it, achieving a high-speed, switchable beam steering reflective interface. The results here could inspire research on dynamic reflective display and holograms.Engineering and Physical Sciences Research Council (EPSRC

PL EU: emoji, language games and political polarisation

Are emoji political? In an increasing body of research, emoji have variably been viewed as emotional data or personality identifiers. However, little attention has been paid to the social and political import of emoji. Using a dataset of politically active Twitter users in Poland, including 334 members of parliament and their 1,288,950 followers, we ask whether emoji are used for political self-representation, and discuss the implications for political identity formation and mobilisation online. Adapting a new method of ideal point estimation, we identify patterns in the employment of emoji in user Twitter bios across a latent political space computed from a Twitter following network. We find that emoji are used as stand-ins for o昀ffiine political symbols such as, and . Additionally, we find that the use of emoji without recognisable political meaning, such as,, and is contingent on a users estimated political ideal point. Users on the left are likelier to employ and, while those on the right are likelier to employ and . Using Ludwig Wittgenstein’s theory of language games, we argue that this points to the use of emoji for communication of both political values and affect, and to the development of a new political language game of emoji

Modulation characteristics of graphene-based thermal emitters

© 2016 The Japan Society of Applied Physics. We have investigated the modulation characteristics of the emission from a graphene-based thermal emitter both experimentally and through simulations using finite element method modelling. Measurements were performed on devices containing square multilayer graphene emitting areas, with the devices driven by a pulsed DC drive current over a range of frequencies. Simulations show that the dominant heat path is from the emitter to the underlying substrate, and that the thermal resistance between the graphene and the substrate determines the modulation characteristics. This is confirmed by measurements made on devices in which the emitting area is encapsulated by hexagonal boron nitride.This work has been undertaken as part of a UK EPSRC Fellowship in Frontier Manufacturing (GRN) grant no. EP=J018651=1, and of the project “GOSFEL”, which has received funding from the European Union for this research. The authors would also like to thank Choon How Gan for useful discussions

Highly tunable hybrid metamaterials employing split-ring resonators strongly coupled to graphene surface plasmons

Metamaterials and plasmonics are powerful tools for unconventional manipulation and harnessing of light. Metamaterials can be engineered to possess intriguing properties lacking in natural materials, such as negative refractive index. Plasmonics offers capabilities to confine light in subwavelength dimensions and to enhance light-matter interactions. Recently,graphene-based plasmonics has revealed emerging technological potential as it features large tunability, higher field-confinement and lower loss compared to metal-based plasmonics. Here,we introduce hybrid structures comprising graphene plasmonic resonators efficiently coupled to conventional split-ring resonators, thus demonstrating a type of highly tunable metamaterial, where the interaction between the two resonances reaches the strong-coupling regime. Such hybrid metamaterials are employed as high-speed THz modulators, exhibiting over 60% transmission modulation and operating speed in excess of 40 MHz. This device concept also provides a platform for exploring cavity-enhanced light-matter interactions and optical processes in graphene plasmonic structures for applications including sensing, photo-detection and nonlinear frequency generation

Metamaterial-based graphene thermal emitter

This is the final version of the article. Available from Tsinghua University Press / Springer Verlag via the DOI in this record.The publisher's erratum to this article is in ORE: http://hdl.handle.net/10871/34353A thermal emitter composed of a frequency-selective surface metamaterial layer and a hexagonal boron nitride-encapsulated graphene filament is demonstrated. The broadband thermal emission of the metamaterial (consisting of ring resonators) was tailored into two discrete bands, and the measured reflection and emission spectra agreed well with the simulation results. The high modulation frequencies that can be obtained in these devices, coupled with their operation in air, confirm their feasibility for use in applications such as gas sensing.C.S., I.J.L. and G.R.N. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom via the Centre for Doctoral Training in Electromagnetic Metamaterials (No. EP/L015331/1). G.R.N. also acknowledges the support of EPSRC via a Fellowship in Frontier Manufacturing (No. EP/J018651/1)

Analytical performance assessment of a novel active mooring system for load reduction in marine energy converters

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Reliability and storm survival of Marine Energy Converters are critical to their commercial development and deployment. The Intelligent Active Mooring System (IAMS) is a novel device intended to minimise extreme and fatigue loading in mooring lines through a non-linear load–extension curve that is variable in operation to adjust to the prevailing metocean conditions. An analytical model of IAMS, validated by physical model tests at the Dynamic Marine Component test facility at the University of Exeter, is used in a numerical simulation of the performance of IAMS as part of the mooring system of the South West Mooring Test Facility buoy. A 10 m length of IAMS can reduce the rms line tension in normal operating conditions by up to 21% and the peak line tension in storm conditions by up to 21% when compared to braided nylon mooring lines. Peak line tension reductions of over 50% can be achieved if a longer IAMS unit is used. The resulting mooring system can be optimised to give load reductions in a wide range of metocean conditions; while variable pre-tension could be used for tidal range compensation or to ease access for installation and maintenance.The work reported here is part of a joint project between AWS Ocean Ltd., Teqniqa Systems Ltd. and the University of Exeter. The project was funded in part by the Technology Strategy Board (now Innovate UK) grant number 101970