PubMed Health⌕ Search

Biomedical subjects

H T Johnson

Publications and source records attributed to H T Johnson.

4 recordsLinked to original sources
PubMed2007

Geometry projection method for optimizing photonic nanostructures.

A geometry projection method for the design of photonic nanostructures is demonstrated and compared with a topology optimization method. By using a higher-dimensional surface to delineate the material interfaces, the projection method restricts the regions of intermediate dielectric properties to a narrow user-defined band and gives some implicit control over feature size. Topology optimization addresses these issues by using a penalization to avoid areas of intermediate dielectric and a filter to obtain implicit control over feature size. The directional emission from a photonic crystal waveguide termination is improved by both methods by generating a series of irregularly shaped dielectric posts. Results are presented, and the relative merits of each method are discussed.

Journal Article↗
PubMed2006

Transformation of hydrogel-based inverse opal photonic sensors from FCC to L1(1) during swelling.

The structural evolution of Bragg diffracting inverse opal hydrogel sensors during swelling is directly observed by two-photon laser scanning fluorescence microscopy and compared to predictions from finite element analysis. A fluorescently labeled pH-sensitive hydrogel is UV-polymerized in a dried polystyrene colloidal crystal template, which is etched to yield an inverse opal. Fluorescence imaging of the hydrogel at different pH values reveals an inhomogeneous deformation of the FCC array of aqueous pores. The pores elongate along the sample normal direction and collapse along the sample parallel directions, consistent with the Bragg response, which indicates a 1-D increase in the interlayer distance. Interconnects between the pores serve as anchor points during hydrogel expansion into the pores. Pinning of the hydrogel to the substrate causes a change of the hydrogel lattice symmetry during deformation, from FCC (ABC stacking) to L1(1) (ABCA'B'C' stacking). Reconstructed cross-sections confirm that a 1-D increase in the interlayer distance along the substrate normal direction is responsible for the diffraction response of an inverse opal hydrogel sensor. Comparison with predictions from finite element analysis shows qualitative agreement, although the experimental mesostructure is significantly more deformed than the calculated data, due to buckling in the experimental system that is not captured by the model.

Biosensing Techniques↗
PubMed2005

Random-walk statistics in moment-based O(N) tight binding and applications in carbon nanotubes.

A computational framework for a moment-based O(N) tight-binding atomistic method is presented, analyzed, and applied to the problem of electronic properties of deformed carbon nanotubes, where N is the number of atoms in the system. The moment-based approach is based on the maximum entropy and kernel polynomial methods for constructing the electronic density of states from local statistical information about the environment around individual atoms. Random-walk statistics are formally presented as the basis for several methods to collect the moments of the density of states in a computationally efficient manner. The computational complexity and accuracy of these methods are systematically analyzed. Using these methods for the problem of deformed carbon nanotubes, it is shown that the computational cost for some cases, per atom, scales as efficiently as O(M log M), where M is the desired number of moments in the expansion of the density of states. These methods are compared to other methods such as direct diagonalization and a Green's function approach.

Journal Article↗