The Theoretical And Experimental Investigation Of Phononic Crystals For High Q Acoustic Sensing

In this thesis，due to the low resolution of the acoustic wave sensors, the phononiccrystals have been investigated theoretically and experimentally to improve thequality factor of acoustic wave sensors based on the unique modification of the elasticwaves and the properties of the point defect states. The plane wave expansion methodand the finite element method were used to calculate the complex band structures ofthe phononic crystal plates, the band gaps and the point defect modes of the siliconphononic crystal plates and the slow wave mode of the Lamb wave in the phononiccrystal plates. Then the vibration pattern, the Q factor and the mass sensitivity of thepoint defect modes were investigated by using the heterodyne laser interferometer.1. The complex band structures of the perfect phononic crystal plates wereobtained by applying supercell plane wave expansion method. The calculationresults can provide decay factors of the elastic waves with frequencies in theband gap as propagating in the phononic crystal plates.2. The characters of the point defect modes in silicon PC plate were investigated.Firstly, the micro fabrication processes of the point defect PC plate wereproposed. Secondly, the vibration patterns of point defect modes weretheoretically and experimentally identified. Finally, the Q factor and the masssensitivity of the point defect modes were investigated.3. The anisotropy of mixed modes in phononic crystals with square lattice epoxycylinders embedded in single crystalline silicon were theoretically investigatedby using the PWE method in the long wavelength limit. It is found that: by changing the filling fraction and the crystalline orientation of the material, a) theanisotropy of effective velocities can be tuned in a large range; it can evenapproach to isotropy for the given filling fraction and the crystalline orientation;b) the polarization vector and the energy flow vector (or the deviation angle andthe power flow angle) can be changed; c) the pure vibration mode direction andthe pure propagation mode direction can be tuned to deviate simultaneouslyfrom the symmetric direction of the PC lattice and the material.4. The group velocity of the zero order Lamb modes (A0) of the phononic crystalplate with single layer cylindrical holes parallel to the surface of the plate wereinvestigated theoretically. It is shown that, by increasing the radius or the fillingfractions,(a) the A0mode can be efficiently slowed,(b) the group velocity ofthe A0mode can be changed from the positive to the negative and (c) the zerogroup velocity of the A0mode can be obtained. This zero group velocity of theA0mode can benefit to obtain the high Q factor frequency response.The results can provide valuable methods for the phononic crystals applied in theacoustic sensing for improving the performance of the acoustic wave sensing.