Enhancing Optical Gradient Forces with Metamaterials

We demonstrate how the optical gradient force between two waveguides can be enhanced using transformation optics. A thin layer of double-negative or single-negative metamaterial can shrink the interwaveguide distance perceived by light, resulting in a more than tenfold enhancement of the optical force. This process is remarkably robust to the dissipative loss normally observed in metamaterials. Our results provide an alternative way to boost optical gradient forces in nanophotonic actuation systems and may be combined with existing resonator-based enhancement methods to produce optical forces with an unprecedented amplitude.Comment: 5 pages, 4 figures; supplemental information available from AP

Participant perceptions of physical activity-enhancing interventions for adults with disability: A meta-synthesis of qualitative research

The United Nations Convention on the Rights of Persons with Disabilities enshrines the rights of disabled people to access services in all areas of citizenship including participation in recreational, leisure and sport activities. Despite this protection, people with disabilities face multiple personal, environmental and social barriers to participation in physical activity (PA). As a result, disabled people are more likely to be inactive compared to the able bodied population and are at a greater risk of inactivity-related diseases. Thus, there is an urgent need for behaviour change interventions to increase PA by specifically addressing the situations of people with disabilities and their barriers to participation. This original meta-synthesis of qualitative research was undertaken to explore participants’ perceptions of PA-enhancing interventions for adults with physical disability. To identify published articles relevant to the meta-synthesis, a rigorous systematic search of electronic databases and hand search of relevant journals was undertaken. In total, 76 papers were read in full, and based on the inclusion criteria, 10 papers were included for review. Following a critical appraisal of the papers, methods of thematic synthesis were drawn upon to generate analytical themes through interpretation and conceptual synthesis. Seven interrelated analytical themes were constructed representing both components and outcomes of the interventions. These were: (i) social support; (ii) diversity; (iii) communication; (iv) behavioural strategies; (v) changing thoughts; (vi) knowledge; (vii) health and well-being. The results of this meta-synthesis provide significant new information that will help interventionists design more effective PA-enhancing interventions, and researchers to better identify and measure key mechanisms and outcomes associated with successful PA-enhancing interventions for people with disabilities

Using Network Analysis to Understand Knowledge Mobilization in a Community-based Organization

Background Knowledge mobilization (KM) has been described as putting research in the hands of research users. Network analysis is an empirical approach that has potential for examining the complex process of knowledge mobilization within community-based organizations (CBOs). Yet, conducting a network analysis in a CBO presents challenges. Purpose The purpose of this paper is to demonstrate the value and feasibility of using network analysis as a method for understanding knowledge mobilization within a CBO by (1) presenting challenges and solutions to conducting a network analysis in a CBO, (2) examining the feasibility of our methodology, and (3) demonstrating the utility of this methodology through an example of a network analysis conducted in a CBO engaging in knowledge mobilization activities. Method The final method used by the partnership team to conduct our network analysis of a CBO is described. Results An example of network analysis results of a CBO engaging in knowledge mobilization is presented. In total, 81 participants completed the network survey. All of the feasibility benchmarks set by the CBO were met. Results of the network analysis are highlighted and discussed as a means of identifying (1) prominent and influential individuals in the knowledge mobilization process and (2) areas for improvement in future knowledge mobilization initiatives. Conclusion Findings demonstrate that network analysis can be feasibly used to provide a rich description of a CBO engaging in knowledge mobilization activities

Transforming two-dimensional guided light using nonmagnetic metamaterial waveguides

Almost a decade ago, transformation optics established a geometrical perspective to describe the interaction of light with structured matter, enhancing our understanding and control of light. However, despite their huge technological relevance in applications such as optical circuitry, optical detection, and actuation, guided electromagnetic waves along dielectric waveguides have not yet benefited from the flexibility and conceptual simplicity of transformation optics. Indeed, transformation optics inherently imposes metamaterials not only inside the waveguide's core but also in the surrounding substrate and cladding. Here we restore the two-dimensional nature of guided electromagnetic waves by introducing a thickness variation on an anisotropic dielectric core according to alternative two-dimensional equivalence relations. Our waveguides require metamaterials only inside the core with the additional advantage that the metamaterials need not be magnetic and, hence, our purely dielectric waveguides are low loss. We verify the versatility of our theory with full wave simulations of three crucial functionalities: beam bending, beam splitting, and lensing. Our method opens up the toolbox of transformation optics to a plethora of waveguide-based devices

Transforming Cherenkov radiation in metamaterials

In this contribution, we explore the generation of light in transformation-optical media. When charged particles move through a transformation-optical material with a speed larger than the phase velocity of light in the medium, Cherenkov light is emitted. We show that the emitted Cherenkov cone can be modified with longitudinal and transverse stretching of the coordinates. Transverse coordinates stretching alters only the dimensions of the cone, whereas longitudinal stretching also changes the apparent velocity of the charged particle. These results demonstrate that the geometric formalism of transformation optics can be used not only for the manipulation of light beam trajectories, but also for controlling the emission of light, here for describing the Cherenkov cone in an arbitrary anisotropic medium. Subsequently, we illustrate this point by designing a radiator for a ring imaging Cherenkov radiator. Cherenkov radiators are used to identify unknown elementary particles by determining their mass from the Cherenkov radiation cone that is emitted as they pass through the detector apparatus. However, at higher particle momentum, the angle of the Cherenkov cone saturates to a value independent of the mass of the generating particle, making it difficult to effectively distinguish between different particles. Using our transformation optics description, we show how the Cherenkov cone and the cut-off can be controlled to yield a radiator medium with enhanced sensitivity for particle identification at higher momentum [Phys. Rey. Lett. 113, 167402 (2014)]

Intraocular scattering compensation in retinal imaging

Intraocular scattering affects fundus imaging in a similar way that affects vision; it causes a decrease in contrast which depends on both the intrinsic scattering of the eye but also on the dynamic range of the image. Consequently, in cases where the absolute intensity in the fundus image is important, scattering can lead to a wrong estimation. In this paper, a setup capable of acquiring fundus images and estimating objectively intraocular scattering was built, and the acquired images were then used for scattering compensation in fundus imaging. The method consists of two parts: first, reconstruct the individual’s wide-angle Point Spread Function (PSF) at a specific wavelength to be used within an enhancement algorithm on an acquired fundus image to compensate for scattering. As a proof of concept, a single pass measurement with a scatter filter was carried out first and the complete algorithm of the PSF reconstruction and the scattering compensation was applied. The advantage of the single pass test is that one can compare the reconstructed image with the original one and see the validity, thus testing the efficiency of the method. Following the test, the algorithm was applied in actual fundus images in human eyes and the effect on the contrast of the image before and after the compensation was compared. The comparison showed that depending on the wavelength, contrast can be reduced by 8.6% under certain conditions

Broadband metasurfaces enabling arbitrarily large delay-bandwidth products

Metasurfaces allow for advanced manipulation of optical signals by imposing phase discontinuities across flat interfaces. Unfortunately, these phase shifts remain restricted to values between 0 and 2 pi, limiting the delay-bandwidth product of such sheets. Here, we develop an analytical tool to design metasurfaces that mimic three-dimensional materials of arbitrary thickness. In this way, we demonstrate how large phase discontinuities can be realized by combining several subwavelength Lorentzian resonances in the unit cell of the surface. Our methods open up the temporal response of metasurfaces and may lead to the construction of metasurfaces with a plethora of new optical functions

The Linear Zero-Forcing Crosstalk Canceller is Near-optimal in DSL Channels

The design and optimization of orthogonal frequency division multiplex (OFDM) systems typically take the following form: The design objective is usually to maximize the total sum rate which is the sum of individual rates in each frequency tone. The design constraints are usually linear constraints imposed across all tones. This paper explains why dual methods are ideally suited for this class of problems. The main result is the following: Regardless of whether the objective or the constraints are convex, the duality gap for this class of problems is always zero in the limit as the number of frequency tones goes to infinity. As the dual problem typically decouples into many smaller per-tone problems, solving the dual is much more efficient. This gives an efficient method to find the global optimum of non-convex optimization problems for the OFDM system. Multiuser optimal power allocation, optimal frequency planning, and optimal low-complexity crosstalk cancellation for vectored DSL are used to illustrate this point

A Near-Optimal Linear Crosstalk Canceler for VDSL

Crosstalk is the major source of performance degradation in VDSL. Several crosstalk cancelers have been proposed to address this. Unfortunately they suffer from error propagation, high complexity and long latency. In this paper we present a simple, linear zero forcing (ZF) crosstalk canceler. This design has a low complexity, no latency and does not suffer from error propagation. Furthermore, due to the well conditioned structure of the VDSL channel matrix, the ZF design causes negligible noise enhancement. A lower bound on the performance of the linear ZF canceler is derived. This allows performance to be predicted without explicit knowledge of the crosstalk channels, which simplies service provisioning considerably. This bound shows that the linear ZF canceler operates close to the single user bound. So the linear ZF canceler is a low complexity, low latency design with predictable, near-optimal performance. The combination of spectral optimization and crosstalk cancellation is also considered. Spectra optimization in a multi-access channel generally involves a highly complex optimization problem. Since the linear ZF canceler decouples transmission on each line, the spectrum on each modem can be optimized independently, leading to a signicant reduction in complexity.