Optical control of electric-field poling in LiTaO₃

We present a room temperature technique for optically inducing periodic domain-inverted structures in bulk (0.2mm thick) LiTaO3. By simultaneous application of an electric field and patterned illumination using UV wavelengths (351nm and 364nm) we demonstrate modulation of the resulting domain profile. We discuss the origins of the observed optical effect and describe our results from repeated domain switching, by cycling the electric field

The Risk of Gambling Problems in the General Population: A Reconsideration

We examine the manner in which the population prevalence of disordered gambling has usually been estimated, on the basis of surveys that suffer from a potential sample selection bias. General population surveys screen respondents using seemingly innocuous “trigger,” “gateway” or “diagnostic stem” questions, applied before they ask the actual questions about gambling behavior and attitudes. Modeling the latent sample selection behavior generated by these trigger questions using up-to-date econometrics for sample selection bias correction leads to dramatically different inferences about population prevalence and comorbidities with other psychiatric disorders. The population prevalence of problem or pathological gambling in the United States is inferred to be 7.7%, rather than 1.3% when this behavioral response is ignored. Comorbidities are inferred to be much smaller than the received wisdom, particularly when considering the marginal association with other mental health problems rather than the total association. The issues identified here apply, in principle, to every psychiatric disorder covered by standard mental health surveys, and not just gambling disorder. We discuss ways in which these behavioral biases can be mitigated in future surveys

Competition and coexistence of multiple mutually pumped oscillations in the visible and infra-red

A photorefractive oscillator. mutually pumped by three wavelengths is presented in various configurations and competition effects demonstrated. The theoretical model used to simulate the behaviour of the oscillation beams is in good agreement with experimental data

Topology, Entropy and Witten Index of Dilaton Black Holes

We have found that for extreme dilaton black holes an inner boundary must be introduced in addition to the outer boundary to give an integer value to the Euler number. The resulting manifolds have (if one identifies imaginary time) topology $S^1 \times R \times S^2$ and Euler number $\chi = 0$ in contrast to the non-extreme case with $\chi=2$. The entropy of extreme $U(1)$ dilaton black holes is already known to be zero. We include a review of some recent ideas due to Hawking on the Reissner-Nordstr\"om case. By regarding all extreme black holes as having an inner boundary, we conclude that the entropy of {\sl all} extreme black holes, including $[U(1)]^2$ black holes, vanishes. We discuss the relevance of this to the vanishing of quantum corrections and the idea that the functional integral for extreme holes gives a Witten Index. We have studied also the topology of ``moduli space'' of multi black holes. The quantum mechanics on black hole moduli spaces is expected to be supersymmetric despite the fact that they are not HyperK\"ahler since the corresponding geometry has torsion unlike the BPS monopole case. Finally, we describe the possibility of extreme black hole fission for states with an energy gap. The energy released, as a proportion of the initial rest mass, during the decay of an electro-magnetic black hole is 300 times greater than that released by the fission of an ${}^{235} U$ nucleus.Comment: 51 pages, 4 figures, LaTeX. Considerably extended version. New sections include discussion of the Witten index, topology of the moduli space, black hole sigma model, and black hole fission with huge energy releas

Superconducting p-branes and Extremal Black Holes

In Einstein-Maxwell theory, magnetic flux lines are `expelled' from a black hole as extremality is approached, in the sense that the component of the field strength normal to the horizon goes to zero. Thus, extremal black holes are found to exhibit the sort of `Meissner effect' which is characteristic of superconducting media. We review some of the evidence for this effect, and do present new evidence for it using recently found black hole solutions in string theory and Kaluza-Klein theory. We also present some new solutions, which arise naturally in string theory, which are non-superconducting extremal black holes. We present a nice geometrical interpretation of these effects derived by looking carefully at the higher dimensional configurations from which the lower dimensional black hole solutions are obtained. We show that other extremal solitonic objects in string theory (such as p-branes) can also display superconducting properties. In particular, we argue that the relativistic London equation will hold on the worldvolume of `light' superconducting p-branes (which are embedded in flat space), and that minimally coupled zero modes will propagate in the adS factor of the near-horizon geometries of `heavy', or gravitating, superconducting p-branes.Comment: 22 pages, 2 figure

Highly efficient self-pumped phase conjugation at near-infrared wavelengths by using nominally undoped BaTiO3

Using a nominally undoped crystal of photorefractive BaTiO3, we have examined self-pumped phase conjugation at near-infrared wavelengths. We report reflectivities as high as 74% between 720 and 800 nm. As expected, the crystal response time increases significantly at longer wavelengths. We believe that this value of reflectivity in the self-pumped geometry is the highest reported to date for this range of wavelengths with the use of a nominally undoped crystal

Double phase conjugate mirror with x6 gain in photorefractive BaTiO₃ at near infrared wavelengths

We describe incoherent beam coupling using the double phase-conjugate mirror arrangement between a laser diode and a Ti:sapphire laser at the near-infrared wavelengths of ~800nm using a nominally undoped sample of BaTiO3. We report phase-conjugate reflectivities of greater than 6 times, which we believe to be the highest reported to date at these wavelengths. We also examine the fidelity of the phase-conjugate beam and the wavelength response of the double-color-pumped oscillator

Strong photorefractive response of Rh:doped BaTiO₃ at red and infrared wavelengths

BaTiO3 crystal is one of the most efficient photorefractive materials, showing strong beam coupling and phase conjugation effects. Its attractive properties are, however, normally limited to visible wavelengths. In order to improve its infrared response, a new type of BaTiO3 has been grown [1] with additional impurities, mainly rhodium. We have investigated this new Rh:BaTiO3 to characterise its response and optical parameters. Our first results show an enhanced absorption at red and infrared wavelengths [2] and high reflectivities in self-pumped phase conjugate configurations. Additionally, strong laser-induced effects have been observed [3]. The aim of the studies presented in this contribution was to provide information about photorefractive centres present in Rh:BaTiO3. Using dual-wavelength illumination we were able to investigate more thoroughly the laser-induced change in absorption. Laser-induced absorption and transparency was measured at both visible (514.5, 633 and 750 nm) and infrared (800 and 1060 nm) wavelengths. Further, we have successfully modelled the observed changes, using numerical simulation of photorefractive centres, and achieved a good agreement between theory and experiment. The strong changes of absorption influence the beam-coupling processes and phase conjugation. We will present the results of extremely high two-beam coupling gain, namely bigger than 20,000, obtained in only a 3 mm thick crystal. We believe that this is the highest photorefractive gain ever reported. We will discuss this experimental data together with theoretical modelling

Erbium fibre laser pumped nanosecond optical parametric oscillator

We report a periodically-poled lithium niobate optical parametric oscillator pumped by a frequency doubled Q-switched erbium-fibre laser. Pump tuning of the OPO signal with thresholds below 10µJ was demonstrated. Signal tuning range was 0.99-1.45µm

Virtual Black Holes

One would expect spacetime to have a foam-like structure on the Planck scale with a very high topology. If spacetime is simply connected (which is assumed in this paper), the non-trivial homology occurs in dimension two, and spacetime can be regarded as being essentially the topological sum of $S^2\times S^2$ and $K3$ bubbles. Comparison with the instantons for pair creation of black holes shows that the $S^2\times S^2$ bubbles can be interpreted as closed loops of virtual black holes. It is shown that scattering in such topological fluctuations leads to loss of quantum coherence, or in other words, to a superscattering matrix $\$$ that does not factorise into an $S$ matrix and its adjoint. This loss of quantum coherence is very small at low energies for everything except scalar fields, leading to the prediction that we may never observe the Higgs particle. Another possible observational consequence may be that the $\theta$ angle of QCD is zero without having to invoke the problematical existence of a light axion. The picture of virtual black holes given here also suggests that macroscopic black holes will evaporate down to the Planck size and then disappear in the sea of virtual black holes.Comment: 24p, LaTeX, 3 postscript figures included with epsf sent in a seperate uuencoded fil