Franco Laeri

Biomimetic Morphogenesis of Fluorapatite-Gelatin Composites: Fractal Growth, the Question of Intrinsic Electric Fields, Core/Shell Assemblies, Hollow Spheres and Reorganization of Denatured Collagen

The biomimetic growth of fluorapatite in gelatin matrices at ambient temperature (double-diffusion technique) starts with elongated hexagonal-prismatic seeds followed by self-similar branching (fractal growth) and ends up with anisotropic spherical aggregates. The chemical system fluorapatite/gelatin is closely related to in vivo conditions for bone or tooth formation and is well suited to a detailed investigation of the formation of an inorganic solid with complex morphology (morphogenesis). The fractal stage of the morphogenesis leads to the formation of closed spheres with diameters of up to 150 μm. The self-assembled hierarchical growth thereby shows immediate parallels to the topological branching criteria of the macromolecular starburst dendrimers. A second growth stage around the closed spheres of the first stage is characterized by the formation of concentric shells consisting of elongated prismatic fluorapatite units with nearly parallel orientation (maximum diameter of the complete core/shell spheres of 1 mm). The specific structure of the core/shell assembly is similar to the dentin/enamel structure in teeth. Together with the idea of the biological significance of electric fields (pyro-, piezoelectricity) during apatite formation under in vivo or biomimetic conditions the present paper considers the composite character of the material and the mechanisms of fractal growth (branching criteria and architecture, the influence of intrinsic electric fields etc.).

Hexagonal microlasers based on organic dyes in nanoporous crystals

Abstract.Molecular sieves, such as nanoporous AlPO4-5, can host a wide variety of laser active dyes. We embeded pyridine-2 molecules as a representative of a commercially available dye which fits into the channel pores of the host matrix. Many efficient dye molecules, such as rhodamines, do not fit into the pores. But modifying the structure of the dyes to appear like the used templates allows us to increase the amount of encapsulated dyes. The properties of resulting microlasers depend on size and shape of the microresonators, and we discuss a model for microscopic hexagonal ring resonators. In terms of pump needed to reach lasing threshold molecular sieve microlasers are comparable to VCSELs. For dyes that fit into the pores we observed a partial regeneration of photo-induced damage.

Coherent CW image amplifier and oscillator using two-wave interaction in a BaTiO3-crystal

Abstract A BaTiO 3 crystal was used to amplify an image illuminated by an Ar-laser. The amplification mechanism is two-wave mixing in an electrooptic crystal, usually called dynamic holography. We achieved gain factors of 4000 with pump powers of 150 mW and holographic resolution. As the interaction between signal and pump wave is not effected through excited states of the crystal, the amplification process is virtually noise free. This has to be traded in with slow response, 0.1-1 s. To demonstrate the high optical quality of the amplifier, it was incorporated in a ring resonator as gain medium. It was possible to excite pure discrete transversal modes of higher order. That reveals the high optical homogeneity of the crystal over the mode cross-section of > 1 mm. The coupling process of the pump with the two counter-propagating waves forming the standing resonator wave is not symmetric. Consequently a travelling wave develops in the resonator.

PHOTOREFRACTION IN THE ULTRAVIOLET : MATERIALS AND EFFECTS

Doped as well as nominally pure crystals of Lithium Niobate (LiNbO3), ι-Arginine Phosphate (LAP), Lithium Iodate (LiIO3), Potassium Dihydrogen Phosphate (KDP), Lithium Formate (LFM), Beta-Barium Borate (BBO), and lithium tetra borate were grown and investigated for photorefractive effects at ultraviolet wavelengths down to 333 nm. In nominally undoped LiNbO3 crystals strong beam coupling effects were observed. In contrast to the visible we revealed a diffusion-dominated charge transport mechanism based on holes, and a low photovoltaic field in the order of 550 V/cm. With such a crystal we investigated the modulation transfer function of a lensless image projection system based on a phase conjugation scheme. A spatial frequency response beyond 2800 line pairs per millimeter was observed. Photorefractive beam coupling was also obtained in LiIO3. Light-induced scattering was detected in iron-doped LiIO3 whereas as-grown LAP material did not exhibit any observable photorefractive effects. However, 100 kV X-ray irradiation seems to induce material defects which can lead to weak light-induced scattering at 351 nm. In all other above-mentioned materials, doped as well as undoped, light-induced scattering could not be observed. On the other hand, this is appreciated in all the applications where the crystals are used as nonlinear material for optical frequency conversion.

Efficient ultraviolet photorefraction in LiNbO3

A nominally undoped LiNbO3 crystal with a slightly broadened absorption edge is used to study beam coupling effects in the UV at 351 nm. At this wavelength the crystal exhibits a diffusion-dominated charge transport mechanism, which allows steady state beam amplification of up to 700 times, comparable to BaTiO3 in the visible. The used crystal material was characterized by an absorption coefficient α=2.68 cm−1 at 351 nm and a maximal gain coefficient Г=13.94 cm−1. This high gain value in the UV can be attributed to a hole diffusion-dominated charge transport mechanism together with a low bulk photovoltaic effect. We measured photovoltaic fields of the order of 550 V/cm.

Designing resonators with large mode volume and high mode discrimination.

We present a new approach for the calculation of resonator mirror shapes based on a simulated annealing optimization algorithm. Compared with the possibilities of conventional resonators with spherical mirrors, the method presented permits a large mode volume of the stationary field distribution of the oscillating lowestloss mode and an increase in its loss discrimination against the remaining ensemble of other modes at the same time. We present results for circular symmetric graded-phase mirrors that yield a difference of 30% between the diffraction losses of the fundamental mode and of other evaluated higher modes.

Nearly degenerate four-wave mixing in photorefractive crystals, an analytical treatment

Abstract We extend the standard model of four-wave mixing in photorefractive crystals to the case where the Bragg-condition is broken by a slight departure from degeneracy. Our analytical treatment shows that for this conditions an increased phase conjugate reflectivity can be obtained.

Active coherent optical feedback system with phase-conjugating image amplifier

We present an image-transmitting optical feedback system containing a phase-conjugating mirror (PCM) as an amplifying element. The PCM is based on four-wave mixing in a photorefractive BaTiO3 crystal. The ring structure constitutes a new phase-conjugate-resonator configuration with the use of the aberration-compensating properties of the phase-conjugating element. Injection of a spatially modulated beam into this active feedback system causes image-bearing oscillation. Experimental results concerning image transmission and self-oscillation are reported.

Isotropic diffraction induced by concentration gratings in α-LiIO3:Fe

Abstract Iron doped lithium iodate (α-LiIO 3 :Fe) of hexagonal modification is investigated by holographic experiments in a geometry where an electrooptic contribution to the diagonal elements of the change of the dielectric permittivity tensor is forbidden by symmetry. Anisotropic diffraction (electrooptically allowed) which we expected to observe in that geometry did not occur. Instead, isotropic diffraction (electrooptically forbidden) from light-induced gratings was found. To clarify its origin the properties of these gratings are studied as well as photo-induced transparency occurring in the ultraviolet. The experiments show that two types of gratings exist, one of them being an absorption grating. The second grating is attributed to originate from concentration changes which arise from the transport of ions triggered by non-uniform illumination.

Some comments on maximum entropy spectral analysis of time series

For spectrum estimation of stationary time series the method of maximum entropy (ME) is compared to the method using fast Fourier transforms (FFT). As the maximum entropy method is highly nonlinear such a comparison is done on the basis of a simulation. The ME method is introduced and it is demonstrated that especially for short data sequences the ME method is capable of delivering superior spectral estimates. How the ME method works and why the ME estimation produces such unexpectedly good results is discussed. Further, the influence of a digitizing signal acquisition apparatus, of additive white noise, and of the initial phase on the variability of the estimated spectra are investigated. Fitting an autoregressive process to the time signal is equivalent to the ME procedure. An autoregressive fit code is presented.