Fundamental Acoustics

Fundamental Acoustics graphic showing the terms linear momentum, spin, orbital

This research area focuses on the theoretical and experimental study of acoustic wave propagation and wave-mater interaction. In previous research, acoustic metamaterials with properties unprecedented in nature such as negative mass density and bulk modulus were developed to realize desired refractive index distribution and control acoustic wave propagation based on resonance. We have designed parity-time symmetric acoustic device using passive loss and active gain elements, which has been used to demonstrate unidirectional reflection. Based on resonating elements, we have also developed an acoustic metamaterial with simultaneous zero mass density and compressibility, whose refractive index is zero. With this zero refractive index metamaterial, we have collimated cylindrical acoustic wave emitted by a point source into directional plane wave. All of these designs focus on the control of linear momentum of acoustic waves.

In other works, we have generated acoustic vortex beams carrying orbital angular momentum (OAM) using active phase array, which was applied to realize high-speed acoustic communication with multiplexing OAM. An ongoing project focuses on the study of optimal spatial information encoding using the active array. Different acoustic communication techniques are applied to further enhance information capacity and overcome the volume scattering  problem in actual ocean environments. We have also observed acoustic spin angular momentum in interfering and evanescent fields. This acoustic spin provides a new control degree of freedom for  acoustic wave propagation. An interesting topic focuses on the coupling between the acoustic OAM, spin, and linear momentum as well as the application of these relations on realizing unconventional control of acoustic wave propagation.

Ultrasound Imaging

Black and white image of the brain

Ultrasonic imaging is a major biomedical imaging technique used in clinics. In recent developments, high intensity focused ultrasound (HIFU) was also used in lithotripsy and histotripsy for treatments of kidney stone, tumor, and thrombosis. More recently, low intensity focused ultrasound (LIFU) was used to stimulate brain, whereas HIFU was used for the attempt of non-invasive tremor treatment. Even with the advanced technology, the presence of skull still creates many challenges. The porosity of skull induces a strong acoustic attenuation, resulting in severe heating in the skull when ultrasound propagates through it. Thus, the non-invasive HIFU treatment requires a cooling system around the skull and cannot be used for a long period. The attenuation of high frequency ultrasound is even worse, which limits the high frequency ultrasound to penetrate through skull, and non-invasive ultrasonic brain imaging becomes impossible. The strong impedance mismatch between skull and brain results in a large reflection, further reducing the penetration efficiency of ultrasound through skull. The non-uniformity of skull varies the acoustic phase of transmitted ultrasound, which makes the control of focal point in the brain to be difficult. Indeed, ultrasound can only be focused in a small region of brain, making the non-invasive treatment for diseases located outside the area impossible. This research direction focuses on developing complementary metamaterials with acoustic properties opposite to skull such that the acoustic effect of the skull is suppressed and overcome the aforementioned problems.

Another fundamental limitation of ultrasonic imaging is due to the diffraction limit where the resolution of ultrasonic waves cannot be smaller than their wavelength, typically at millimeter scalar. While the neurons in the brain are normally around 10 microns, the ultrasonic waves cannot visualize the neurons. Current techniques of single neuron imaging requires the histology of the brain so that the neurons can be imaged under optical microscope. These techniques prevent the observation of neuron behaviors in living animals. We are currently developing a super-resolution technique that will allow us to achieve single neuron ultrasonic imaging for both in vitro and in vivo experiments. With this new imaging method, we will be able to observe the neuron behaviors in animals when they are alive under different circumstances.


A Model of High-Speed Endovascular Sonothrombolysis with Vortex Ultrasound-Induced Shear Stress to Treat Cerebral Venous Sinus Thrombosis

Sonothrombolysis is a treatment of blood clot formation using ultrasound. Traditional sonothrombolysis focuses on applying ultrasound induced cavitation to enhance anti-coagulant drugs into blood clots to assist the lysis. We are interested in designing sonothrombolysis catheters to generate different types of ultrasound to enhance cavitation effect and radiation stress in the blood clots to significantly increase the lytic rate and thrombolysis efficiency that will dramatically reduce the treatment time and improve the treatment effectiveness of patients.

Single Phonon Detector

Phonon as a fundamental boson particle characterize a quantum of vibrational energy in a lattice of atoms. Like photons, phonons have a great application potential in quantum computing and quantum communication. The detection of phonons require highly sensitive devices because of their low energy nature. This research direction focuses on developing highly sensitive devices based on 2D electron gases for single phonon detection and using these devices to study the quantum properties of phonons.


Portrait of Chengzhi Shi in a button up collared shirt and dark suit jacket

Chengzhi Shi

Mechanical Engineering

Recent News

  • Professor Shi gave an invited seminar at the University of Toledo. (April 2024)
  • Our lab has moved to the Department of Mechanical Engineering at the University of Michigan. (January 2024)
  • Professor Shi gave an invited seminar at the School of Earth and Atmospheric Sciences at Georgia Tech. (September 2023)
  • Professor Shi organized and chaired the “Vibration and Stability of Mechanical Systems” session at the ASME 2023 International Design Engineering Technical Conference. (August 2023)
  • Professor Shi gave an invited talk at the 13th International Conference on Metamaterials, Photonic Crystals and Plasmonics in Paris, France. (July 2023)
  • Mark Kelly won the Best Student Paper Award First Prize from the 184th Acoustical Society of America Meeting in the Underwater Acoustics session. (May 2023)
  • Professor Shi organized and chaired the “Brian and Super-Resolution Ultrasound Imaging” session at the 184th Acoustical Society of America Meeting. (May 2023)
  • Professor Shi gave an invited seminar at the University of Michigan. (April 2023)
  • Professor Shi gave an invited seminar at the University of California, Berkeley. (February 2023)
  • Our work on vortex ultrasound thrombolysis was highlighted by Physics WorldASMENSFMedical DialoguesScience DailyThe Engineer, etc. (January 2023)
  • Professor Shi gave an invited seminar at the Rutgers University. (November 2022)
  • Professor Shi gave an invited seminar at Shanghai Jiao Tong University. (October 2022)
  • Professor Shi gave an invited seminar at the Institute for Electronics and Nanotechnology of Georgia Tech. (October 2022)
  • Professor Shi gave an invited seminar at the META Series of the Imperial College London. (October 2022)
  • Phoebe Welch received the Christopher Ruffin Graduate Leadership Award (May 2022)
  • Professor Shi won the prestigious Office of Naval Research Young Investigator Program (ONR YIP) Award. (January 2022)
  • Professor Shi won the prestigious National Science Foundation Faculty Early Career Development Program (NSF CAREER) Award. (January 2022)