Giancarlo Cappellini

Third-order three-wave mixing in single-mode fibers: exact solutions and spatial instability effects

Exact solutions are presented to the steady-state coupled-mode equations that govern the nonlinear parametric interaction of a central-frequency wave with a pair of upshifted and downshifted sidebands in isotropic single-mode optical fibers. This solution accounts for pump depletion as well for as a possible phase mismatch among the waves. The existence is predicted of eigensolutions propagating unchanged along the fiber, which may be either spatially stable or spatially unstable, depending on the total power and the propagation-constant mismatch. The presence of spatially unstable eigensolutions dramatically affects the power exchange among the three waves. The physical implications of this instability for the frequency-conversion process, as well as its potential application to all-optical switching, are discussed.

Electronic and optical properties of families of polycyclic aromatic hydrocarbons: A systematic (time-dependent) density functional theory study

Abstract Homologous classes of polycyclic aromatic hydrocarbons (PAHs) in their crystalline state are among the most promising materials for organic opto-electronics. Following previous works on oligoacenes we present a systematic comparative study of the electronic, optical, and transport properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using density functional theory (DFT) and time-dependent DFT we computed: (i) electron affinities and first ionization energies; (ii) quasiparticle correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap; (iii) molecular reorganization energies and (iv) electronic absorption spectra of neutral and ±1 charged systems. The excitonic effects are estimated by comparing the optical gap and the quasiparticle corrected HOMO–LUMO energy gap. For each molecular property computed, general trends as a function of molecular size and charge state are discussed. Overall, we find that circumacenes have the best transport properties, displaying a steeper decrease of the molecular reorganization energy at increasing sizes, while oligorylenes are much more efficient in absorbing low-energy photons in comparison to the other classes.

An efficient method for calculating quasiparticle energies in semiconductors

We present a method for the efficient calculation of the electronic structure of semiconductors within the GW approach. It approximately includes dynamical-screening and local-field effects, previously disregarded in simplified GW approaches, without increasing the computational effort. Such effects substantially affect the gap corrections. We find quasiparticle shifts in good agreement with the complete GW calculations or experiment for Si, AlAs, GaAs and ZnSe.

Theoretical electron affinities of PAHs and electronic absorption spectra of their mono-anions

We present theoretical electron affinities, calculated as total energy differences, for a large sample of polycyclic aromatic hydrocarbons (PAHs), ranging in size from azulene (C10H8) to dicoronylene (C48H20). For 20 out of 22 molecules under study we obtained electron affinity values in the range 0.4-2.0 eV, showing them to be able to accept an additional electron in their LUMO π � orbital. For the mono-anions we computed the absolute photo-absorption cross-sections up to the vacuum ultraviolet (VUV) using an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD-DFT), an approach which has already been proven to yield accurate results for neutral and cationic PAHs. Comparison with available experimental data hints that this is the case for mono-anions as well. We find that PAH anions, like their parent molecules and the corresponding cations, display strong π ∗ ← π electronic transitions in the UV. The present results provide a quantitative foundation to estimate the fraction of specific PAHs which can be singly negatively charged in various interstellar environments, to simulate their photophysics in detail and to evaluate their contribution to the interstellar extinction curve.

Optical properties of BN in cubic and layered hexagonal phases

INFM Sezione di Roma-2 and Dipartimento di Fisica, Universita`di Tor Vergata, Via della Ricerca Scientifica 1, I-00133, Rome, Italy~Received 24 January 2001; revised manuscript received 6 April 2001; published 26 June 2001!Linear optical functions of cubic and hexagonal BN have been studied within first principles density func-tional theory in the local density approximation. Calculated energy-loss functions show reasonable agreementwith experiments and previous theoretical results both for h-BN and forc-BN. Discrepancies arise betweentheoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible expla-nations of this mismatch are proposed and evaluated: lattice constant variations, h-BN contamination inc-BNsamples, and self-energy effects.DOI: 10.1103/PhysRevB.64.035104 PACS number~s!: 78.20.Ci, 68.35.Ja, 71.45.GmI. INTRODUCTION

Anomalous relaxations and chemical trends at III-V semiconductor nitride nonpolar surfaces

Relaxations at nonpolar surfaces of semiconductor III-V compounds result from a competition between dehybridization and charge transfer. First-principles calculations for the (110) and (10

Structural properties and quasiparticle energies of cubic SrO, MgO and SrTiO3

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Two-wave mixing in a quadratic nonlinear medium: bifurcations, spatial instabilities, and chaos

0) faces of zinc-blende and wurtzite AlN, GaN, and InN reveal an anomalous behavior as compared with ordinary III-V semiconductors. Additional calculations for GaAs and ZnO suggest close analogies with the latter. We interpret our results in terms of the larger ionicity (charge asymmetry) and bonding strength (cohesive energy) in the nitrides with respect to other III-V compounds, both essentially due to the strong valence potential and absence of p core states in the lighter anion. The same interpretation applies to Zn II-VI compounds.

Effects of TIPS-Functionalization and Perhalogenation on the Electronic, Optical, and Transport Properties of Angular and Compact Dibenzochrysene

The structural properties and the band structures of the charge-transfer insulating oxides SrO, MgO and SrTiO3 are computed both within density functional theory in the local density approximation (LDA) and in Hedins GW -scheme for self-energy corrections, by using a model dielectric function, which approximately includes local field and dynamical effects. The deep valence states are shifted by the GW -method to higher binding energies, in very good agreement with photoemission spectra. Since in all of these oxides the direct gaps at high-symmetry points of the Brillouin zone may be very sensitive to the actual value of the lattice parameter a already at the LDA level, self-energy corrections are computed both at the theoretical and the experimental a . For MgO and SrO, the values of the energies of transition between the valence and the conduction bands are improved by GW -corrections, while for SrTiO3 they are overestimated. The results are discussed in relation to the importance of local field effects and to the nature of the electronic states in these insulating oxides.

Ground state properties and excitation energies of cubic SrO and MgO.

We present the qualitative dynamics of parametric mixing of two waves in a quadratic nonlinear medium by introducing a reduced phase-plane description. We find the nonlinear frequency-conversion eigenmodes and their bifurcations and instabilities. We discuss applications of this description to the phase dependence of seeded second-harmonic generation, frequency division, and spatial chaos in a medium with periodic mismatch.