G.F. Calvo

Dipole-mode vector solitons in anisotropic nonlocal self-focusing media.

We demonstrate, theoretically and experimentally, that dipole-mode vector solitons created in biased photorefractive media possess a number of anisotropy-driven properties, such as stability of a selected orientation, wobbling, and incomplete rotation, owing to the anisotropic nonlocal response of the photorefractive non-linearity. Such features are found for higher-order (multipole) vector solitons, and they are carefully verified in an experiment.

Grating translation technique for vectorial beam coupling and its applications to linear signal detection

We develop the theory of vectorial beam coupling in cubic photorefractive crystals to describe the effect of fast phase modulation on output intensities and polarizations. Special emphasis is given to new features of the grating translation technique compared with its scalar variant. We show, in particular, that a strong nonlocal ac response in crystals of the sillenite family can be effectively used for the linear detection of fast signals. Theoretical results are supported by polarization ac experiments with Bi12TiO20 crystals.

Linear phase demodulation in photorefractive crystals with nonlocal response

On the basis of the vectorial theory of light diffraction in cubic photorefractive crystals, we derive analytical expressions to describe the output intensities and polarization states of two coherent beams coupled via a dynamic index grating and subjected at the input to a fast phase modulation. It is shown that the linear transformation of fast phase excursions into intensity modulation can be achieved in the case of a nonlocal ac response by means of proper polarization filtering. Theoretical results are supported by the data obtained in experiments with Bi12TiO20 crystals of different orientations. The polarization technique is used for the evaluation of the space-charge fields created in different crystals.

Parametric scattering processes in photorefractive periodically poled lithium niobate

We present an analysis of photorefractive properties of bulk periodically poled lithium niobate. The results obtained are applied to description and interpretation of phase-matched four-wave processes found recently in this novel nonlinear material. These processes manifest themselves in rings, lines, and dots of light-induced scattering that are essentially different from those known for single-domain crystals. We conclude that periodically poled lithium niobate is a new nonlinear material promising for various photorefractive applications.

Models for polymers and biopolymers based on quantum mechanics

A study has been made of several types of 3-dimensional macromolecular chains: open and closed-ring linear chains, star polymers and double-stranded chains. Chains in thermal equilibrium at room temperature are assumed, and typical harmonic-oscillator-like vibrational potentials with frequencies ωi for the interactions of nearest-neighbour atoms along the same chain. For the double-stranded chain, the interactions between atoms in different strands behave like the Morse potential. In physically interesting cases ωi is large, so that all rapidly varying vibrational degrees of freedom have to be treated quantum mechanically, and these are represented approximately by ground state wavefunctions; they decouple consistently, yielding constant bond lengths. This generates effective quantum Hamiltonians and partition functions for the macromolecules, depending only on the remaining and relevant slowly varying degrees of freedom (angles determining internal rotations). At room temperature, the classical limit of the slowly varying quantum description yields simpler effective classical Hamiltonians and partition functions. Previous results for the open linear chain are discussed together with former work for the closed-ring linear chain, and new models are presented for star polymers and double-stranded macromolecules. The model appears to describe consistently a double-stranded chain at temperatures below thermal denaturation.

Quantum statistical mechanics of closed-ring molecular chains

A model for a closed-ring unhindered three-dimensional macromolecular chain, based on Quantum Mechanics, is presented. Upon starting from an exact non-relativistic Hamiltonian operator, we integrate out all electronic degrees of freedom, in the Born-Oppenheimer framework, giving rise to an effective vibro-rotational Hamiltonian for the chain. Then, assuming a harmonic oscillator-like vibrational potential between nearest-neighbour atoms, the integration of the atomic radial degrees of freedom is carried in the limit of high frequencies. Thus, all bond lengths become fixed, including the one which makes the chain to become fixed, including the one which makes the chain to become a closed ring. This formulation leads to a specific Hamiltonian for the remaining angular variables of the closed-ring chain, and constitutes an alternative in comparison with standard Gaussian models, which do not. Use is made of a variational inequality by Peierls to find an approximate quantum partition function for the angular variables of the system. We then proceed to obtain approximately another representation for the angular partition function in the classical limit. Several features of the classical partition function are disscussed.

Self-trapping along light-induced singularity of space charge in fast photorefractive materials

We investigate light beam propagation in a fast photorefractive medium placed in an alternating electric ac-field to enhance the nonlinear response. It is shown that the joint action of the optical and material nonlinearities leads to formation of a narrow singularity of the light-induced space charge at the intensity maximum and to self-trapping of the light energy near the corresponding discontinuity of the index profile. Owing to the strong saturation of the material nonlinearity, the trapped beam propagates over long distances with only a weak loss of its power.

Photorefractive ac response beyond the low-contrast limit

We show that a photorefractive crystal, placed in an ac field and exposed to nonuniform light, exhibits singularities of the induced space charge and discontinuities of the corresponding space-charge field. For a symmetric beam, the singularity appears at the intensity maximum when the light contrast exceeds a certain (rather low) threshold value. Also, we analyze a diffraction model to understand the effect of the singular ac response on the propagation of a Gaussian light beam.

Bipolar two-dimensional analysis of grating dynamics in photorefractive thin films

A two-dimensional analytical model is developed to describe transient grating formation for bipolar transport in photorefractive devices. This approach is suitable for structures in which the film thickness is comparable with or smaller than the grating vector. A crucial feature of the model is that it takes into account the edge effects by including both parallel and perpendicular components of the fields and the currents. The treatment is entirely general and valid for any geometry, in particular, for parallel (image amplifiers, etc) and for perpendicular (Pockels readout optical modulators, etc) photorefractive devices. The role of bulk and surface charges in each configuration is discussed. The results are illustrated in a few examples showing the effect the material and geometrical parameters have on the device resolution thus imposing more stringent limits upon the resolution than those found in previous studies.