2018-2019 Final Year Project Titles

Dr. Guancong Ma

SCTY906, phgcma@hkbu.edu.hk, ext. 7095

Measurement of wavefunctions near an exceptional point in non-Hermitian acoustic system (3/6 unit)


Non-Hermitian ¬Hamiltonians can be used to describe a class of systems that is associated with loss and/or gain. The concept of non-Hermitian system has recently been used to demonstrate long-distance wireless energy transfer (link). In this project, we study a fundamental problem in such systems. A particularly interesting characteristics of a non-Hermitian system, is that the variation of parameters (usually two or more) can drive the systems across a phase-transition point, known as an “exceptional point”. At this point, two (or more) eigenstates coalesce. In this project, we instigate the system’s behavior near an exceptional point by measuring the spatial pattern of wavefunctions. We use coupled acoustic cavities for such a task. The experiment is a simple AC signal measurement, in which we need to acquire both amplitude and phase information of the sound wave in the cavities at sufficient locations. The information provided by the wavefunctions is important for further studies, including the topological characteristics of the exceptional point.

Design, fabrication and testing of multiple-phase actively controllable membrane acoustics metamaterial units (3/6 unit)


In this project, we will try to design a metamaterial unit that can be switched between at least three states by electricity. A multi-phase metamaterial unit is the foundation of many intriguing functionalities, such as active beam forming, wavefield shaping, and even energy harvesting. Ideally, the three states shall have similar amplitude transmission coefficient, by different transmission phase. The transmission phases in the three states ideally shall cover 360-deg. The main idea is to use electromagnets to shift the resonant frequency of a thin piece of elastic membrane. We will use numerical simulations to guide our design. Prototyping will be done with 3D printing. Testing will be carried out using the impedance tube method.

Simulation of acoustic metasurface (3/6 unit)


Metasurface has been a recent focus in the field of acoustics field research. It can generate a designed phase profile for the transmitted or reflected sound. Many exotic functionalities, such as flat-lens focusing, beam forming, negative refraction, have been demonstrated. A factor that limits the application of metasurfaces is that they are it is hard to change once fabricated. We believe with some efforts, we will be able to find a solution to overcome this problem. The first step in this endeavor is to understand the relation between the target phenomenon and the required phase profile. We will investigate by using finite-element simulations.