X-waves Generated at Second Harmonic

The process of optical frequency doubling can lead, in the undepleted regime, to the generation of a X-wave envelope with group velocity locked to the pump beam. Its parameters and its angular spectrum, are directly related to the zero- and first-order dispersive features of the nonlinear process. This constitutes a novel mechanism for spatio-temporal localization of light.Comment: 11 pages, 1 figure, revised version submitted to Optics Letter

The University of Hong Kong's New PCLL

A recent review of legal education criticised Hong Kong's professional legal education programme (the PCLL). The review said that the PCLL was not doing a good enough job in preparing students for legal practice. This article responds to the review by outlining proposals for a new PCLL. The proposals are based on the premise that professional legal education needs to develop students' ability to learn new skills after graduation. This is a complicated, but crucial, educational goal that is more easily understood with the help of concepts such as problem solving and transfer of learning. The paper argues that if the PCLL is designed to enable students to transfer problem solving skills from one legal context to another then students will more easily be able to transfer what they have learned in the programme to new problems in legal practice. To achieve this goal, however, requires painstaking effort in using several interdependent, curriculum-design features that other programmes around the world have used successfully.published_or_final_versio

Defect Sizing Using Distance-Gain-Size Diagrams for Flat-Bottomed Holes in a Solid: Theoretical Analysis and Experimental Verification

Although there are a number of potential pitfalls, the classical method of relating defect area to echo amplitude is still the most widely used method to size defects using ultrasonic pulse-echo techniques. In 1959 Krautkramer [1] was the first to introduce a set of curves (DGS diagrams) showing the variation of echo amplitude with range and target size. As Krautkramer made clear, such curves are dependent on transducer pulse shape. For the very far field he gave theoretical results assuming a fluid-like medium of propagation, but he had to resort to a large number of experimental measurements to construct the near field portion of the curves. Well known problems in using DGS diagrams include the sensitivity of echo amplitudes to target angular and lateral alignment and the need to construct a new set of curves for each transducer pulse shape. Furthermore, when sizing targets in solids there are likely to be errors if curves constructed assuming a fluid medium are used. In 1987, McLaren and Weight [2] gave an impulse-response method to predict echo amplitudes for arbitrary target position in the field and for any transducer pulse shape. Normally-aligned, flat-ended cylindrical targets and a fluid medium were assumed. More recently, Schmerr and Sedov [3,4] have calculated single frequency DGS diagrams for flat-bottomed holes (FBH’s), for both direct and water coupling, but the holes are assumed to be in a fluid-like material. Their method takes account of diffraction and refraction effects but not mode conversion. A more exact treatment of the effect of a solid medium of propagation on DGS diagrams has been given by Sumbatyan and Buyove [5] who developed DGS diagrams for disc-like targets using a boundary element method to solve the elastodynamic equations, but again, only for the case of continuous sinusoidal waves. One disadvantage of such an approach is that the calculations can be rather time consuming

Response of a Focused Transducer Facing a Rigid Reflector

Focused ultrasonic transducers can be useful in material characterization and tissue property measurement because of the improved signal to noise ratio as compared to flat transducers. To fully understand the transducer output, it is useful to know the system transfer function or impulse response of a focused transducer first acting as a transmitter and then receiving the signal reflected off a rigid plane. The resulting received pressure of a pulse-echo system can also be characterized in terms of a diffraction correction factor. The determination of the impulse response or diffraction correction factor is well understood for flat disk transducers, but is more complex for focused transducers

X-wave mediated instability of plane waves in Kerr media

Plane waves in Kerr media spontaneously generate paraxial X-waves (i.e. non-dispersive and non-diffractive pulsed beams) that get amplified along propagation. This effect can be considered a form of conical emission (i.e. spatio-temporal modulational instability), and can be used as a key for the interpretation of the out of axis energy emission in the splitting process of focused pulses in normally dispersive materials. A new class of spatio-temporal localized wave patterns is identified. X-waves instability, and nonlinear X-waves, are also expected in periodical Bose condensed gases.Comment: 4 pages, 6 figure

Visualization of Broadband Sound Sources

In this paper the method of imaging of wideband audio sources based on the 2D microphone array measurements of the sound field at the same time in all the microphones is proposed. Designed microphone array consists of 160 microphones allowing to digitize signals with a frequency of 7200 Hz. Measured signals are processed using the special algorithm that makes it possible to obtain a flat image of wideband sound sources. It is shown experimentally that the visualization is not dependent on the waveform, but determined by the bandwidth. Developed system allows to visualize sources with a resolution of up to 10 cm

A hybrid boundary element method for shallow water acoustic propagation over an irregular bottom

A hybrid numerical model combining a boundary element method (BEM) and eigenfunction expansions is developed to solve acoustic wave propagation in shallow water. Waves are assumed harmonic and, therefore, the governing equation reduces to a Helmholtz equation. Accurate numerical integration techniques are implemented in the BEM, for calculating singular and quasi-singular integrals. For the latter, an adaptive integration technique is developed and tested for computational domains with very small aspect ratios, representative of shallow water environments. The model is validated by comparing the numerical solution to analytic solutions for problems with simple boundary geometries (e.g. rectangular, step, and sloped domains). Results indicate good agreement between the two solutions. Effects of node resolution, adaptive integrations, and number of modes in radiated fields, on the accuracy of the solution, are assessed. Finally, the model is used to study acoustic transmission over a rectangular bump in the bottom, as a function of frequency and bump geometry. Expected results are obtained below the first cut-off frequency over the bump, i.e. the transmission of energy beyond the bump through evanescent modes. A similar effect is commonly noted in layered media and is known as \u27tunneling\u27. © 1998 Elsevier Science Ltd. All rights reserved