Ion-implanted lanthanum fluoride waveguides

The effects of He+ and H+ implantation into LaF3 substrates have been investigated at both room temperature and 77 K. At room temperature the surface of the samples becomes black, possibly due to chemical reduction effects, but at low temperature the crystals remain clear and a refractive index change is observed which produces a surface waveguide. The dark mode spectra of these guides have been recorded using the prism coupling technique, and their refractive index profiles have been deduced from the spacing of these modes. The ion implantation produces a thin optical barrier layer of low refractive index at the end of the ion range. This paper discusses the use of multiple-energy implantation to produce a broad optical barrier in order to reduce the tunnelling losses of the guides, and subsequent thermal annealing to reduce absorption and scattering losses caused by colour centre formation in the electronic stopping surface region of the guides. It is concluded that either single- or multiple-energy He+ implants can be ideally used to form waveguides, for applications in the blue/UV or red/IR wavelength regions, respectively, with losses of a few dB/cm

A plasticity model for powder compaction processes incorporating particle deformation and rearrangement

This article is available open access through the publisher’s website at the link below. Copyright @ 2008 Elsevier Ltd.This paper develops a mechanistic model of granular materials that can be used with a commercial finite element package (ABAQUS). The model draws on the ideas of critical state soil mechanics and combines them with the theory of envelopes to develop an elasto-plastic model with a non-associated flow rule. The model incorporates both local deformation at the granule contacts, and rearrangement of the granules so that jointly they account for any bulk deformation. The mechanics of the model closely reflect the physicality of the material behaviour and the model parameters are closely linked (although not simplistically identical) to the characteristics of the granules. This not only gives an insight into the material behaviour, but also enables the model to be used to facilitate design of the material, its processing properties and, hence, component development. The model is used to simulate drained triaxial tests, settlement of a powder in a bin, and some examples of die pressing. Simulations are compared with experimental data and with predictions obtained using other models

Pulsed laser deposition of KNbO₃ thin films

The laser ablation of stationary KNbO3 single crystal targets induces a Nb enrichment of the target surface. In rotated targets this effect is observed only in those areas irradiated with low laser fluence. The composition of the plasma formed close to the target surface is congruent with the target composition; however, at further distances K-deficient films are formed due to the preferential backscattering of K in the plasma. This loss may be compensated for by using K-rich ceramic targets. Best results so far have been obtained with [K]/[Nb] = 2.85 target composition, and crystalline KNbO3 films are formed when heating the substrates to 650 °C. Films formed on (100)MgO single crystals are usually single phase and oriented with the (110) film plane parallel to the (100) substrate surface. (100)NbO may coexist with KNbO3 on (100)MgO. At substrate temperatures higher than 650 °C, niobium diffuses into MgO forming Mg4Nb2O9 and NbO, leading to K evaporation from the film. Films formed on (001) alpha-Al2O3 (sapphire) show the coexistence of (111), (110), and (001) orientations of KNbO3, and the presence of NbO2 is also observed. KNbO3 films deposited on (001)LiNbO3 crystallize with the (111) plane of the film parallel to the substrate surface. For the latter two substrates the Nb diffusion into the substrate is lower than in MgO and consequently the K concentration retained in the film is comparatively larger

Ion beam manipulation of the photorefractive properties of SBN planar waveguides

Photo refractive planar waveguides have been fabricated in cerium doped Strontium Barium Niobate (SrxBa(1-x)Nb2O6 : SBN) single crystals by ion beam implantation. The losses measured were as low as 0.1 dB /cm and 7.0 dB /cm for the TM and TE modes respectively. Subsequent two beam coupling experiments performed on the waveguides showed that, unlike BaTiO3 and KNbO3 waveguides formed by ion beam implantation, the two-beam coupling gain direction did not reverse. The response time had also been reduced by two orders of magnitude

Gain reversal studies in photorefractive waveguides

We report on low-loss photorefractive BaTiO3 H+ implanted waveguides exhibiting reversal of two-beam-coupling gain direction, caused by induced colour centres. The anomalous two-beam-coupling gain has been investigated as a function of the input beam ratio

Two-wave mixing in photorefractive SBN planar waveguides

Planar waveguides have been fabricated in SBN:61 and SBN:75 by ion-beam implantation. The implantation was carried out using 2.0MeV H+ ions with doses ranging from 2x10 for the TE mode. These losses are significantly lower than those observed for planar waveguides fabricated previously using sulphur diffusion [1]. Two beam coupling experiments were then carried out on the SBN waveguide where it was observed that the gain direction was the same as that of the bulk crystal. Previously, in both BaTiO3 [2] and KNbO3 [3] planar waveguides fabricated in the same manner, the gain direction for two beam coupling had been shown to reverse. This phenomenon may be due to the ion beam implantation process reducing the waveguide layer and subsequently changing the ratio of impurity atom oxidation states - in the case of BaTiO3, the ratio of Fe2+:Fe3+. It is thought that the predominant photoexcited charge carrier in both BaTiO3 and KNbO3 is changed from the hole to the electron whereas in SBN, the electron remains the predominant carrier even after the implantation process. Modelling of these effects has been undertaken, giving an explanation of why gain reversal should occur in certain photorefractive planar waveguides fabricated by ion-beam implantation. This paper will discuss the gain and response time of the SBN waveguide in comparison to the bulk and the theoretical predictions of the effects of ion-beam implantation on photorefractive materials

Photorefractive planar waveguides in BaTiO₃ fabricated by ion-beam implantation

For the first time to our knowledge, photorefractive properties have been observed in planar waveguides fabricated by the technique of ion-beam implantation in BaTiO3 single crystals. The implantation was carried out by using 1.5 MeV H+ ions at a dose of 10-16 ions/cm2. For a given input power, a decrease in the effective photo-refractive two-beam coupling response time of ≥102 has been observed, owing to a combination of optical confinement within the waveguide and possible modification of charge-transport properties induced through implantation. Experiments carried out on the two-beam coupling gain show that the gain direction has been reversed in the waveguide compared with that of the bulk crystal

Mean Field Fluid Behavior of the Gaussian Core Model

We show that the Gaussian core model of particles interacting via a penetrable repulsive Gaussian potential, first considered by Stillinger (J. Chem. Phys. 65, 3968 (1976)), behaves like a weakly correlated ``mean field fluid'' over a surprisingly wide density and temperature range. In the bulk the structure of the fluid phase is accurately described by the random phase approximation for the direct correlation function, and by the more sophisticated HNC integral equation. The resulting pressure deviates very little from a simple, mean-field like, quadratic form in the density, while the low density virial expansion turns out to have an extremely small radius of convergence. Density profiles near a hard wall are also very accurately described by the corresponding mean-field free-energy functional. The binary version of the model exhibits a spinodal instability against de-mixing at high densities. Possible implications for semi-dilute polymer solutions are discussed.Comment: 13 pages, 2 columns, ReVTeX epsfig,multicol,amssym, 15 figures; submitted to Phys. Rev. E (change: important reference added

BaTiO₃ waveguide self-pumped phase conjugator

For the first time to our knowledge, self-pumped phase conjugation is reported in a planar waveguide structure in a BaTiO3 single crystal. The waveguide was fabricated by the technique of ion implantation, with 1.5-MeV H+ ions at a dose of 1016 ions/cm2. Phase-conjugate reflectivities >20% have been measured for waveguide self-pumped phase conjugation, and, for a given input power, an order-of-magnitude reduction in the response time is observed in the waveguide compared with the bulk. The fidelity of phase conjugation in the guide is also discussed