Artificial Life in an Exciton-Polariton Lattice

We show theoretically that a lattice of exciton-polaritons can behave as a life-like cellular automaton when simultaneously excited by a continuous wave coherent field and a time-periodic sequence of non-resonant pulses. This provides a mechanism of realizing a range of highly sought spatiotemporal structures under the same conditions, including: discrete solitons, oscillating solitons, rotating solitons, breathers, soliton trains, guns, and choatic behaviour. These structures can survive in the system indefinitely, despite the presence of dissipation, and allow universal computation.Comment: 14 pages, 14 figure

Anti-chiral edge states in an exciton polariton strip

We present a scheme to obtain anti-chiral edge states in an exciton-polariton honeycomb lattice with strip geometry, where the modes corresponding to both edges propagate in the same direction. Under resonant pumping the effect of a polariton condensate with nonzero velocity in one linear polarization is predicted to tilt the dispersion of polaritons in the other, which results in an energy shift between two Dirac cones and the otherwise flat edge states become tilted. Our simulations show that due to the spatial separation from the bulk modes the edge modes are robust against disorder.Comment: 6 pages, 5 figure

Planning for Development using Social Impact

Economic development activities change the physical and social environments in which individuals live. For planners, it is important to anticipate the types of changes that might occur, and to put measures in place that mitigate negative impacts and promote positive impacts on people and communities. Social Impact Assessment (SIA) was introduced as a tool for understanding the social impacts of development. There are three factors, however, that limit the use of SIA in developing countries. First, the original SIA tool was designed in a developed country, and as such the list of indicators developed may not suitable for local conditions. Second, there is no specific theoretical underpinning of the SIA tool, and thus no link between the SIA tool and particular theories of social behaviour. Third, there is no particular link between what SIA measures, and what should be done to mitigate the effects of development activities. The purpose of this paper is to address these three issues and in doing so, provide a SIA tool that can be applied usefully and practically in a developing country. The theoretical basis of SIA used in the paper is Actor-Network Theory (ANT). The tool, which was developed using ANT, principles consists of five stages of analysis: identification of principal actors (human and non-human) and the changes due to development; exploration of the ownership of resources (capital) that enables principle actors to change; identification of change agents attached to the capital of principal actors; tracing which interests of actors are aligned to deal with the development; and an analysis of the social change platform (mobilization of actors) based on connections of all principal actors with other actors. Each of these stages provides the basis for determining what should be assessed in SIA, how to structure the assessment, and how to interpret the results of a SIA.Social Impact Assessment (SIA), Actor-Network Theory (ANT), development impact, Community/Rural/Urban Development, Public Economics,

Patient experiences of an ankle fracture and the most important factors in their recovery : a qualitative interview study

Objective The objective of this qualitative research study is to explore patient experiences of ankle fracture and the factors most important to them in recovery. Design Semistructured interviews exploring patient experiences of ankle fracture recovery at 16–23 weeks following injury. Interviews followed a topic guide and were recorded with an encrypted audio recorder and then transcribed verbatim. Thematic content analysis was used to identify themes in the data. Setting Individuals were recruited from a sample of participants of a UK-based clinical trial of immobilisation methods for ankle fracture (ISRCTN15537280 at the pre-results stage at time of writing). Interviews were conducted at the participants’ own homes or on a university campus setting. Participants A purposive sample was used to account for key variables of age, gender and fracture management. Participants recruited from the clinical trial sample were adults aged 18 years or over with a closed ankle fracture. Results Ten participants were interviewed, five of whom were female and six of whom needed an operation to fix their ankle fracture. The age range of participants was 21–75 years with a mean of 51.6 years. Eight themes emerged from the data during analysis; mobility, loss of independence, healthcare, psychological effects, social and family life, ankle symptoms, sleep disturbance and fatigue, and activities of daily living. Factors of importance to participants included regaining their independence, sleep quality and quantity, ability to drive, ability to walk without walking aids or weight-bearing restrictions, and radiological union. Conclusions The results of this research demonstrates the extensive impact of ankle fracture on individuals’ lives, including social and family life, sleep, their sense of independence and psychological well-being. The results of this study will enable an increased understanding of the factors of relevance to individuals with ankle fracture, allowing collection of appropriate outcomes in clinical studies for this condition. Ultimately these results will help formulate appropriate patient-centred rehabilitation plans for these patients

The Design for a Nanoscale Single-Photon Spin Splitter

We propose using the effective spin-orbit interaction of light in Bragg-modulated cylindrical waveguides for the effcient separation of spin-up and spin-down photons emitted by a single photon emitter. Due to the spin and directional dependence of photonic stopbands in the waveguides, spin-up (down) photon propagation in the negative (positive) direction along the waveguide axis is blocked while the same photon freely propagates in the opposite direction.Comment: 5 pages, 3 figure

Spontaneous Pattern Formation in a Polariton Condensate

Polariton condensation can be regarded as a self-organization phenomenon, where phase ordering is established among particles in the system. In such condensed systems, further ordering can possibly occur in the particle density distribution, under particular experimental conditions. In this work we report on spontaneous pattern formation in a polariton condensate under non-resonant optical pumping. The slightly elliptical ring-shaped excitation laser we employ is such to force condensation to occur in a single-energy state with periodic boundary conditions, giving rise to a multi-lobe standing wave patterned state

Ballistic spin transport in exciton gases

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed; Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They can form highly coherent quantum liquids and carry spins over macroscopic distances. The price to pay is a finite life-time of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems where bosonic spin transport has been reported, in particular, to indirect excitons in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit interaction of electrons and holes on the exciton spin, account for the Zeeman effect induced by external magnetic fields, long range and short range exchange splittings of the exciton resonances. We also consider exciton transport in the non-linear regime and discuss the definitions of exciton spin current, polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.

Spin rings in bi-stable planar semiconductor microcavities

A unique feature of exciton-polaritons, inherited from their mixed light-matter origin, is the strongly spin-dependent polariton-polariton interaction, which has been predicted to result in the formation of spin rings in real space [Shelykh et al., Phys. Rev. Lett. 100, 116401 (2008)]. Here we experimentally demonstrate the spin bi-stability of exciton-polaritons in an InGaAs-based semiconductor microcavity under resonant optical pumping. We observe the formation of spin rings whose size can be finely controlled in a spatial scale down to the micrometer range, much smaller than the spot size. We additionally evaluate the sign and magnitude of the antiparallel polariton spin interaction constant.Comment: 5 pages, 4 figure