No abstract

We study the dynamics of a three-dimensional laser bullet propagating inside a nonlinear saturable medium. We show that an increase of the pump parameter destabilizes the bullet and leads to its destruction through oscillations with increasing amplitude. We propose an inhomogeneous and anisotropic external excitation mechanism leading to a stable oscillating bullet. By varying the frequency of the external excitation, a stable quasi-in-phase or quasi-antiphase internal state can be reached.

Previous studies have shown that interference between fast waves and slow waves can lead to observed negative dispersion in cancellous bone. In this study, the effects of overlapping fast and slow waves on measurements of the apparent attenuation as a function of propagation distance are investigated along with methods of analysis used to determine the attenuation properties. Two methods are applied to simulated data that were generated based on experimentally acquired signals taken from a bovine specimen. The first method uses a time-domain approach that was dictated by constraints imposed by the partial overlap of fast and slow waves. The second method uses a frequency-domain log-spectral subtraction technique on the separated fast and slow waves. Applying the time-domain analysis to the broadband data yields apparent attenuation behavior that is larger in the early stages of propagation and decreases as the wave travels deeper. In contrast, performing frequency-domain analysis on the separated fast waves and slow waves results in attenuation coefficients that are independent of propagation distance. Results suggest that features arising from the analysis of overlapping two-mode data may represent an alternate explanation for the previously reported apparent dependence on propagation distance of the attenuation coefficient of cancellous bone.

The question of whether sound vibration of a medium can bring about any kind of molecular or macromolecular events is a long-standing scientific controversy. Although it is known that ultrasonic vibrations with frequencies of more than 1 MHz are able to align certain macromolecules in solution, no effect has yet been reported with audible sound, the frequency of which is much lower (20-20,000 Hz). Here, we report on the design of a supramolecular nanofibre that in solution becomes preferentially aligned parallel to the propagation direction of audible sound. This phenomenon can be used to spectroscopically visualize sound-induced vibrations in liquids and may find application in a wide range of vibration sensing technologies.

The received signal in through-transmission ultrasound measurements of cancellous bone consists of two longitudinal waves, called the fast and slow waves. Analysis of these fast and slow waves may reveal characteristics of the cancellous bone that would be good indicators of osteoporosis. Because the two waves often overlap, decomposition of the received signal is an important problem in the characterization of bone quality. This study proposes a fast and accurate decomposition method based on the frequency domain interferometry imaging method with a modified wave transfer function that uses a phase rotation parameter. The proposed method accurately characterized the fast and slow waves in the experimental study, and the residual intensity, which was normalized with respect to the received signal intensity, was less than -20 dB over the bone specimen thickness range from 6 to 15 mm. In the simulation study, the residual intensity was less than -20 dB over the specimen thickness range from 3 to 8 mm. Decomposition of a single received signal takes only 5 s using a laptop personal computer with a single central processing unit. The proposed method has great potential to provide accurate and rapid measurements of indicators of osteoporosis in cancellous bone.