|M.Sc Student||Uri Bar-Ziv|
|Subject||Accelerating Non Diffracting Acoustic Beams in Liquids:|
Theory and Experiment
|Department||Department of Physics||Supervisor||? 18? Segev Mordechai|
|Full Thesis text|
Accelerating beams are recently attracting considerable research interest. They were first proposed in the context of quantum mechanics, where shape-preserving temporally-accelerating solutions of the Schrödinger equation were found. However, they have become an important research topic only three decades later when they were introduced into optics, in the form of Airy beams that "bend" in space. The field has attracted significant attention for several reasons: (i) the mathematical aesthetics of these beams; (ii) the search after similar phenomena in different fields of physics; and (iii) numerous promising applications. Indeed, soon after their first experimental demonstration, many applications were proposed and demonstrated, ranging from micromanipulation of particles, generation of curved plasma channels in the air, micromachining of curved surfaces, light sheet microscopy and 3D fluorescent imaging. In a different important domain, non-diffracting acoustic Bessel beams were demonstrated more than two decades ago, but until recently it remained a challenge to generate acoustic accelerating beams, that is, acoustic beams that bend in space. Recently, a pioneering work has demonstrated ultrasonic accelerating beams in gas (air), where an array of speakers with a specifically designed phase was used to launch acoustic bending beams in two and three dimensions. However, many applications of acoustics are in liquids. Examples range from ultrasound imaging and ultrasonic therapy in the medical arena, SONAR, hydrography and underwater communication, to ultrasonic detection of cracks in metallic components where the specimen is immersed in liquid to guarantee good coupling of the acoustic wave. Importantly, it has recently been suggested that acoustic half-Bessel beams could be used for micro-particle transport in liquids, but until now acoustic accelerating beams have never been demonstrated in liquids. Liquids are typically characterized by acoustic impedance which is orders of magnitude higher than in gas. As such, new methods, completely different than those used in gas, are required for generating such beams in liquids. Here, we demonstrate the first acoustic accelerating beam in a liquid: an underwater ultrasonic shape-preserving accelerating beam. The beam was generated by phase modulating a single projector using a tailored acoustic phase mask. The beam is propagating for a range in excess of 800 wavelengths, which are about 6 Rayleigh lengths, while preserving its shape and transversely accelerating. On top of many promising applications, such beams could provide new means to study non-linear interaction of acoustic beams.