Biomedical subjects

PA Robinson

Publications and source records attributed to PA Robinson.

9 recordsLinked to original sources

PubMed2000

Effects of geometric and refractive index disorder on wave propagation in two-dimensional photonic crystals

The effects of disorder in the geometry and refractive index on the transmittance of two-dimensional photonic crystals composed of dielectric circular cylinders are considered, including randomness of radii, positions of the cylinder centers, and thickness of each layer of the photonic crystal. The effects of combinations of different types of strong disorder are also considered. The localization and homogenization properties of disordered photonic crystals are investigated. Analytical expressions for the two-dimensional localization length in the form of integrals are presented for both polarizations. It is shown numerically that the slope of the exponential divergence of the localization length in two dimensions is proportional to the inverse of the square of randomness for strong disorder and proportional to the inverse of the randomness for weak disorder. The effective dielectric constants for both polarizations in the case of strong disorder are also found. The transition from localization to homogenization is discussed and the terms responsible for this transition are identified and investigated.

Journal Article↗

PubMed1999

Self-organized criticality in termite architecture: a role for crowding in ensuring ordered nest expansion

A cellular automaton model of the vertical and lateral growth that occurs in a termite nest expansion is formulated. The model consists of a lattice of building sites permeated by a trail network of termites. Termites are recruited from the network to build at these sites by an attractive pheromone which emanates from building pellets. The experimentally observed behaviour in which termites halt and then retreat from overcrowded sites is also incorporated along with the assumption that termites which retreat then build on nearby sites. Simulations without this response to crowding result in disordered, unrealistic nest structures, while its inclusion qualitatively reproduces two previously unexplained features of nest expansion. First, the active surface formed by joining the tips of growing pillars maintains a smooth appearance through time. Second, the model explains how building activity spreads laterally over the building domain. The competition between the attractive pheromone and inhibitory crowding stimuli drives the system to a critical state where the correlation length (distance over which building sites interact) diverges, i.e. all building sites interact with each other. An explicit mapping of a particular case of the model onto the sandpile model of self-organized criticality is exhibited. The criticality of the model determines power-law scalings of the system's dependence on the density of termites on the trail network and the size of the building domain. The model makes several experimentally testable predictions. Copyright 1999 Academic Press.

Journal Article↗

PubMed1996