We characterized the diffraction and crystal structure of a crystalline colloidal array (CCA) photonic crystal composed of $270\text{nm}$ diameter polystyrene spheres which have a nearest neighbor spacing of $\sim 540\text{nm}$. This CCA diffracts light in first order at $\approx 1200\text{nm}$ and shows strong diffraction in the visible spectral region from higher order planes. We quantitatively examined the relative diffraction intensities of the putative fcc (111), (200), (220), and (311) planes. Comparing these intensities to those calculated theoretically we find that the crystal structure is fcc with significant stacking faults. Essentially, no light transmits at the Bragg angle for the fcc (111) planes even through thin $\sim 40\mu \mathrm{m}$ thick CCA. However, much of this light is diffusely scattered about the Bragg angle due to crystal imperfections. Significant transmission occurs from thin samples oriented at the Bragg condition for the fcc (200), (220), and (311) planes. We also observe moderately intense two-dimensional diffraction from the first few layers at the crystal surfaces. We also examined the sample thickness dependence of diffraction from CCA photonic crystals prepared from $\sim 120\text{nm}$ polystyrene spheres whose fcc (111) planes diffract in the visible spectral region. These experimental observations, aided by calculations based upon a simple but flexible model of light scattering from an arbitrary collection of colloidal spheres, make clear that fabrication of three-dimensional photonic band gap crystals will be challenged by crystal imperfections.

A recently developed Floquet theory-based formalism for computing electron transport through a molecular bridge coupled to two metalelectrodes in the presence of a monochromatic ac radiation field is applied to an experimentally relevant system, namely a xylyl–dithiol molecule in contact at either end with goldelectrodes. In this treatment, a nondissipative tight-binding model is assumed to describe the conduction of electric current. Net current through the wire is calculated for two configurations of the electrode–wire–electrode system. In one, symmetric, configuration, the electrodes are close (∼2 Å) and equidistant from the bridge molecule. In the other, asymmetric configuration, one electrode is farther away (∼5 Å), representing the tip of a scanning tunneling microscope located at this distance from the bridge molecule (the other end being chemisorbed to a gold substrate). For both configurations, electron current is calculated for a range of experimental inputs, including dc bias and the intensity and frequency of the laser. Via absorption/emission of photons, resonant conditions may be achieved under which electron transport is significantly enhanced compared to the unilluminated analog. Calculations show that this can be accomplished with experimentally accessible laser field strengths.

This paper considers electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic radiation field. Current flow through the wire is calculated within a nondissipative one-electron tight binding model of the quantum dynamics. Using Floquet theory, the field-driven molecular wire is mapped to an effective time-independent quantum system characterized by a tight-binding Hamiltonian with the same essential structure as the nondriven analog. Thus, Green’s Function methods for computing current flow through the wire, which have been profitably applied to the molecular wire problem in the absence of driving, can also be used to analyze the corresponding field-driven system. Illustrative numerical calculations on a simple model system are presented.

Very high diffraction efficiencies (>80%) were observed from two-dimensional (2-D) photonic crystals made of monolayers of ∼ 490 nm diameter dielectric polystyrene spheres arranged in a 2-D hexagonal lattice on top of a liquid mercury surface. These almost close packed 2-D polystyrene particle arrays were prepared by a self-assembly spreading method that utilizes solvent evaporation from the mercury surface. Two-dimensional arrays transferred onto a dielectric glass substrate placed on top of metal mirrors show diffraction efficiencies of over 30%, which is 6- to 8-fold larger than those of the same 2-D monolayers in the absence of mirrors. A simple single particle scattering model with refraction explains the high diffraction efficiencies in terms of reflection of the high intensity forward diffraction.