Metal–dielectric photonic crystal superlattice: 1D and 2D models and empty lattice approximation

Periodic nanostructures are one of the main building blocks in modern nanooptics. They are used for constructing photonic crystals and metamaterials and provide optical properties that can be changed by adjusting the geometrical parameters of the structures. In this paper the optical properties of a photonic crystal slab with a 2D superlattice are discussed. The structure consists of a gold layer with a finite periodic pattern of air holes that is itself repeated periodically with a larger superperiod. We propose simplified 1D and 2D models to understand the physical nature of Wood's anomalies in the optical spectra of the investigated structure. The latter are attributed to the Rayleigh anomalies, surface plasmon Bragg resonances and the hole-localized plasmons.     

Correlation of Tc with structure in the density of states in the new high-Tc superconductors

The variation of T_c with hole concentration in the new copper oxide superconductors can readily be understood in terms of a peak in the density of states associated with the CuO₂ planes. The data are consistent with either simple phonon-mediated pairing or an indirect excitonic pairing. Effects due to interlayer coupling are considered. The sharp decrease of T_c near half-filling is probable due to strong spin fluctuations.     

The thermoelectric power of the metallic glass Ca0.8Al0.2

We report the first measurement of the thermoelectric power Q, of a metallic glass that contains only normal metals. The thermoelectric power of amorphous Ca_0.8Al_0.2 has been measured as a function of temperature from 10 K to 420 K. It is found that Q is positive, varies linearly with temperature and has a small slope. This is similar to the thermoelectric power found for other metallic glasses containing large concentrations of transition metal atoms.     

Interstitial site of Cu in Ca

Fluorescence EXAFS was used to study the siting of a dilute solution of Cu in Ca. The Cu is interstitial in Ca, and no more than .4Å away from the center of the octahedral site. No lattice dilation was seen about the Cu atom, even though Cu is “too big” to fit into the interstitial site.     

Manipulation of supercontinuum generation by stimulated cascaded four-wave mixing in tapered fibers

Tailoring supercontinuum generation is important for a number of applications. Stimulated four-wave mixing in tapered optical fibers can lead to modified initial conditions for the evolution of supercontinua. We experimentally show that synchronizing a pump pulse with an additional signal pulse induces cascaded four-wave mixing components. The spectral evolution and the interaction with the pump wavelength are investigated in detail upon increasing pump power. Furthermore it will be shown that four-wave mixing components provide the initiation for an altered shape of the spectral fiber output when soliton dynamics are involved. We finally demonstrate the altered evolution in a synchronized feedback configuration. This work can lead to a control over specific spectral components of supercontinua. PACS  42.81.-i; 42.25.Kb     

Stable MHz-repetition-rate passively Q-switched microchip laser frequency doubled by MgO:PPLN

We present an Nd³⁺:YVO₄ microchip laser that is passively Q-switched by a semiconductor saturable absorber mirror. The system generates 520 ps pulses at 1064 nm with 340 mW average output power at up to 2.3 MHz repetition rate. Single longitudinal and transverse mode operation with a peak-to-peak timing jitter less than 1 % is achieved. We discuss the influence of different setup parameters by using numerical simulations of the coupled rate equations and FEM simulations of the thermal management. The infrared light was frequency doubled in an MgO:PPLN crystal with up to 75 % conversion efficiency, which to our knowledge is the highest conversion efficiency that was ever achieved with passively Q-switched microchip lasers.     

Models for stiffening in cross-linked biopolymer networks: A comparative study

In a recent publication, we studied the mechanical stiffening behavior in two-dimensional (2D) cross-linked networks of semiflexible biopolymer filaments under simple shear [Onck, P.R., Koeman, T., Van Dillen, T., Van der Giessen, E., 2005. Alternative explanation of stiffening in cross-linked semiflexible networks. Phys. Rev. Lett. 95, 178102]. These simulations make use of a geometrically nonlinear finite-element technique, taking into account the discreteness of the biopolymer network. As an alternative to the prevalent view, these computations relate the stiffening to nonaffine network reorientations. However, this discrete-network model neglects any interaction of the filaments with the surrounding fluid, which is the origin of entropic stiffening in single filaments and in biopolymer networks, according to MacKintosh et al. For this reason, this article is devoted to a thorough study of the difference between both approaches on the 2D single-filament level. In addition, we investigate the deviation from affine deformation behavior, by comparing the discrete calculations with an affine-network model.     

Coupling effects in optical metamaterials

Metamaterials have become one of the hottest fields of photonics since the pioneering work of John Pendry on negative refractive index, invisibility cloaking, and perfect lensing. Three-dimensional metamaterials are required for practical applications. In these materials, coupling effects between individual constituents play a dominant role for the optical and electronic properties. Metamaterials can show both electric and magnetic responses at optical frequencies. Thus, electric as well as magnetic dipolar and higher-order multipolar coupling is the essential mechanism. Depending on the structural composition, both longitudinal and transverse coupling occur. The intricate interplay between different coupling effects in a plasmon hybridization picture provides a useful tool to intuitively understand the evolution from molecule-like states to solid-state-like bands.     

Fabrication of a fiber-based microcavity with spherical concave fiber tips

A straightforward method for manufacturing concave fiber ends with optical quality surfaces is presented. This method allows for the indention of bare cleaved fiber ends of single-mode Bi₂O₃ fibers by way of conventional heating and subsequent contact with ultra-smooth sinter metal spheres. The resulting high quality surfaces show promise for microcavity experiments. Results for a simple plano-concave cavity composed of a single indented fiber and a plane mirror are presented.