Negative refraction and the spectral filtering of terahertz radiation by a photonic crystal prism.

We demonstrate how micromachined photonic crystals can be used to negatively refract terahertz frequency light. The photonic crystals, which are constructed from conventional dielectric materials, manipulate the incident beam via interaction with their photonic bands. Consequently, we show that different components of a broadband beam incident on the structure may be positively or negatively refracted, depending upon its frequency and that the structure can be used as an effective spectral filter of THz radiation.

Negative refraction can make non-diffracting beams.

We report the results of simulations relating to the illumination of a structure consisting of a slab constructed from a 2-D hexagonal array of metal rods with a terahertz frequency source. As a consequence of negative refraction an essentially non-divergent beam pattern is observed. Although the results presented relate to the terahertz regime they should also be applicable at other frequencies.

Terahertz filter based on refractive properties of metallic photonic crystal.

We propose a new type of pass-band filter, in this case designed to operate in the terahertz frequency regime, possessing two separate passbands utilizing the distinction between positive and negative refraction in a photonic crystal prism. The prism is formed from a two-dimensional hexagonal arrangement of metallic rods. In order to understand the operation of the filter we both consider the photonic bandstructure of the associated infinite photonic structure and carry out simulations of the refraction properties of the prism using finite-difference time-domain software.

Statistics of the eigenmodes and optical properties of one-dimensional disordered photonic crystals.

The eigenmode spectrum and transmission properties of a certain class of one-dimensional disordered photonic crystals have been studied statistically. It is shown that the relative fluctuation of the optical width of the period of the photonic crystal is a universal parameter allowing a quantitative description of the disordered photonic crystal for various models of disorder. It is shown that the tail of the density of states is characterized by a certain penetration depth and a quantitative relation between the penetration depth, the relative band gap width, and the disorder parameter is obtained. It is found that a threshold level of disorder exists, below which the density of states in the center of the photonic band gap vanishes, and the ensemble-averaged transmission coefficient does not change significantly with increasing disorder. Also, the standard deviation of the transmission coefficient is less than its mean value. Above threshold, the ensemble averaged transmission coefficient and density of states increase with the level of disorder rapidly, and the standard deviation of the transmission coefficient exceeds its mean value. A scaling formula is presented, which relates the logarithm of the transmission to the periodic refractive index modulation and the disorder.

Ab initio dynamics study of poly-para-phenylene vinylene.

We present an ab initio dynamics investigation within a density-functional perturbation theory framework of the properties of the conjugated polymer poly-para-phenylene vinylene (PPV) in both the isolated chain and crystalline states. The calculated results show that for an isolated chain, most of the vibrational modes correspond to experimentally observed modes in crystalline PPV. However, additional hitherto unidentified modes have been observed in experiment and our calculations on crystalline material have allowed us to assign these. We also present the results of calculations of the polarizability and permittivity tensors of the material, which are in reasonable agreement with the typical values for conjugated polymers. Dynamical Born effective charges [S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73, 515 (2001)] are calculated and compared with atomic charges obtained from Mulliken population analysis [M. D. Segall, C. J. Pickard, R. Shah, and M. C. Payne, Mol. Phys. 89, 571 (1996)] and we conclude that effective charges are more appropriate for use in the study of the dynamics of the system. Notable differences are found in the infrared-absorption spectra obtained for the isolated chain and crystalline states, which can be attributed to the differences in the crystalline packing effects, which clearly play a key role in influencing the lattice dynamics of PPV.

Theoretical adlayer surface morphology of wurtzite 2 × 2 reconstructions of the GaN(0001) surface.

In the first-principles calculations presented here we employ a density functional formalism using a pseudopotential plane-wave basis set in order to obtain the minimum energy configurations of various GaN(0001) 2 × 2 surfaces involving N atoms. The calculated formation energies of the 2 × 2 ideal model are compared with a previously proposed laterally contracted Ga bilayer model. We show how the order and stability of the different reconstructions are influenced by the choice of the standard 2 × 2 ideal or contracted bilayer model. On the basis of these results, we have characterized the effect on the adlayer surface of N segregation on the top of the surface, and the stability dependence on the number of substitutions for the different models employed. Our results predict that not all the adlayer structures containing nitrogen are unstable relative to the commonly considered N (H3) adatom configuration.

Analysis of the dynamics of a blue-violet Inx Ga1-x N laser with a saturable absorber.

We report the results of numerical investigations of the dynamical behavior of a blue-violet InGaN laser with a specially incorporated saturable absorber. We have identified the nature of the bifurcation that occurs in the device dynamics and also the conditions that are necessary for self-pulsating and excitable operations. We also demonstrate the influence of the relevant device parameters on the laser dynamics and show how the properties of the saturable absorber and its position in the device have a considerable influence on the laser behavior. Finally theoretical investigations of the excitable behavior and a confirmation of the excitability properties of an InGaN laser are presented and discussed.

Excitability of excitons and biexcitons in a ring cavity.

We discuss the excitable behavior of excitons and biexcitons in a nonlinear optical ring cavity. The nonlinearity is due to the process of the creation of biexcitons by photon-assisted conversion of excitons. In the bifurcation analysis a region where a saddle point is close to an equilibrium has been found. In this region the system shows excitability. It is shown that the mechanism of the excitable behavior of excitons and biexcitons in a ring cavity is different from that of two-level atoms in the same system. The possible applications of an excitable ring cavity are discussed.