Investigation Of Point Defect Modes In Phononic Crystals For High-Resolution Sensing

Point defect modes in phononic crystals could form high qulity factor(Q factor) acoustic resonances because of the energy localization, indicating a promsing approach to complete high-resolution acoustic mass sensor. This thesis focuses on the domainant loss of point defect modes, integration of mass sensing system and the design of phononic crystals for measuring. The main contents and conclusions are as follows:1. On the basis of simulation analysis of point defect modes, a method based on heterodyne interferometer laser vibrometer for mode displacement profile testing and accurate identification is given, laying the foundation for analysis of the energy loss and mass sensitivity of point defect modes;2. Asymmetric frequency response is analysised based on coherent resonance model. An accurate approach is proposed to fit modes response curves for obtaining accurate Q factors and to replace the 3dB method which results in large errors in calculating Q factors, laying the foundation for analysis of the energy loss of point defect modes;3. The relationship between the modes displacement profiles and their Q factors is analysised. The domainant loss of modes is material loss of piezoelectric thin film AlN;4. Based on DDS(Direct Digital Synthesis) chip with the amplitude and phase detection chip, a miniaturized, digitized and real-time mass sensing systems is designed and fabricated. Dual-mode measurement method is used to achieve temperature compensation of the sensor. Such metod decouples temperature and mass by simultaneously measuring the resonant frequency shift and by using the property that point modes differ in mass sensitivity. The frequency resolution of sensor is about 0.25 Hz, corresponding the mass resolution of 46 pg.5. To solve problem that it is difficult to load mass and to deal with liquid in sensing experiment with holed phononic crystal plate, a tubular pillared phononic plate is proposed. Band gap generation mechanism and its evolution with respect to structure parameters are analysised based on numerical simulation. The results show that such structure enhances the acoustic multiple scattering and band gap opens when the pillar height is 50% of that in conventional cylindrical pillared phononic plate, thereby enhancing the stability of the structure and reducing processing costs. Moreover, the double-pillar structure is conducive to broaden the band gap. Its width is two times of that in single-pillar with same filling factor.The thesis provides theoretical and experimental basises for application of the phononic crystal point defect modes in high-resolution acoustic mass sensing.