Metamaterials for acoustic sensing

Abstract

Acoustic sensing has played an important role in engineering and daily life, especially in biomedical imaging and in the recently developed field of the Internet of Things. Acoustic metamaterials are man-made materials composed of subwavelength unit cells, and can be viewed as macroscopically homogenized media with effective material properties non-existing in nature. Acoustic metamaterials provide new opportunities for solving the challenges in acoustic sensing. This thesis is about exploring new acoustic metamaterials and their applications in acoustic sensing. The first part of the thesis is about new acoustic metamaterials. In this part, two types of phase shifters are designed. When the frequency of the incident wave changes, one type of them has constant time delay and the other has constant phase shift.

New acoustic metasurfaces are designed based on the phase shifters, including acoustic binary phase grating that can realize highly efficient wave steering with much less complex structure compared to previous metasurfaces, and acoustic flat lenses with different steering angles and focusing locations for waves having different frequencies. Besides the phase shifters for phase modulation, tunable amplitude modulation is also proposed and experimentally demonstrated by creating iD channels in a hydrogel sheet. When the channels are filled with different materials and with different filling ratios, the acoustic properties can be tuned by orders of magnitudes over broad frequency ranges. The second part of the thesis is about new acoustic sensing methods and systems. First, an acoustic imaging system that can expand the evanescent wave travel distance and improve imaging resolution in far-field is proposed based on the binary phase gratings and additional filter layers.

Subwavelength imaging and edge detection for 1D slit objects is demonstrated experimentally with 3D printed prototypes. Second, a system that can select the direction of acoustic transmission is designed and experimentally demonstrated based on the combination of two acoustic binary phase gratings. The double-grating structure is shown to select the direction and the frequency at the same time. By stacking multiple double-grating structures that are configured for different frequencies, the system for broadband direction selection is proposed.