Research On Point Defect Modes Of Phononic Crystal And Its Sensing Characteristics

Phononic crystals(PC) are novel artificial acoustic materials and their defect states feature high Q, multiple resonant modes and tiny resonating mass, which open up new approaches for high Q, self-compensated sensing applications. In this paper, theoretical and experimental investigations on the selectively mode excitation and decoupling measurement using multiple modes of PC point defects on silicon slabs are carried out. The main contents are as follows:(1) The parameters of the point defects on silicon based PC slabs are chosen by using finite element method(FEM) calculations and the resonating characteristics of the cavity modes are discussed, which lay foundations for the next studies.(2) Point defects on silicon based PC slabs are fabricated using micro fabrication processes. The mode frequencies and vibration shapes of the 380 μm are measured with optical heterodyne interferometry and good agreement is achieved with calculations. Q factors as high as 24000 and 150000 in air and vacuum, respectively, are obtained, which are the highest reported yet. The factors dominating the Q performance of the 380 μm device are proved to be air damping in air and material loss in AlN film in vacuum.(3) Due to the geometry complexity, PC point defects support multiple, distinct resonant modes with close frequencies. An electrode design method is proposed to selectively excite certain defect modes and the method is verified by one-port devices with electrodes for intended mode excitations. Performances of devices with different port configurarions are discussed.(4) The mass sensitivities of the 380 μm and 80 μm devices are measured to be ~5 Hz/ng and ~2000 Hz/ng. The temperature sensitivities of all defect modes is proved to be ~-31 ppm/℃. A dual-mode passive temperature method is implemented to compensate temperature changes and after compensation, a mass sensitivity of 5.4 Hz/ng and mass resolution of 85 pg are revealed.The results and conclusions can promote the application of phononic crystals in sensing and are very significant in both theory and engineering.