Phononic crystals (PnCs), also called phononic band gap (PnBG) materials, are the acoustic wave equivalent of the well-known photonic crystals (PhCs), which consist of periodically arranged scattering centres embedded in a homogeneous background matrix, prohibiting elastic waves within a certain frequency range from travelling in any direction. From the aspect of elastic properties, PnCs are inhomogeneous materials with periodic variations. Thus, the dispersion characteristics of the PnCs lead to the existence of phononic band gaps, in which the propagation of elastic waves within a certain frequency range is prohibited in any direction. PnCs with properly engineered band gaps can be the basis of realizing a variety of functionalities such as acoustic waveguides, cavities and filters. Various types of acoustic and elastic wave propagation have been studied in different classes of PnC structures. Surface acoustic waves in semi-infinite PnCs with cylindrical air holes etched in a single material or a solid/solid composition whereby the air holes are filled with other kinds of materials, have been studied. Currently, our group is focusing on a pure PnC band gap structure based on two-dimensional (2-D) silicon phononic crystal (PnC) slab of a square array of cylindrical air holes in a 10μm thick free-standing silicon plate.

Selected Publications:

1. Nan Wang, Julius Ming-Lin Tsai, Fu-Li Hsiao, Bo Woon Soon, Dim-Lee Kwong, Moorthi Palaniapan and Chengkuo Lee, Experimental investigation of a cavity-mode resonator using a micromachined two-dimensional silicon phononic crystal in a square lattice, IEEE Electron Device Lett., vol. 32, no. 6, pp. 821-823, 2011. [PDF] [DOI]

By adding defects to PnC structure, devices of various functionalities like waveguides and resonators can be realised. Various types of resonating structures have been studied in our group. The first type of PnC resonating structure is a PnC cavity-mode resonator formed by removing three rows of air holes (cavity width, w=3a) at the center from the 2-D PnC structure. This structure is essentially a Fabry-Perot resonating structure with a resonant cavity at the centre of the structure. We characterized the resonator in terms of its resonant frequency, Q factor and insertion loss (IL). The achieved f-Q product is as high as 1.5 e11. The second type of resonating structure is a PnC Bloch-mode resonators which are formed by removing two rows of air holes (cavity width, w=2a) and replacing them with one row of air holes at the center. The third type of PnC Bloch-mode resonators are formed by removing three rows of air holes (w=3a) and replacing them with one row of air holes at the center. Structures with various radii of the central holes (r’) are also designed and characterized. The fourth type of Bloch mode PnC resonator is PnC resonators with alternate defects. The resonators are made by alternately removing the central four rows of air holes while the radii of the remaining central air holes (r’) are varied and its effect on the performance of the resonators is studied. We also show that by creating alternate defects, the mode mismatch could be decreased, resulting in both the enhancement in the Q factor and the reduction in IL, as compared to the Fabry-Perot resonator.

Selected Publications:

1. Nan Wang, Fu-Li Hsiao, Julius Ming-Lin Tsai, Moorthi Palaniapan, Dim-Lee Kwong and Chengkuo Lee, Investigation on the optimized design of alternate-hole-defect for 2-D phononic crystal based silicon microresonators, J. Appl. Phys., vol. 112, no. 2, pp. 024910, 2012. [PDF] [DOI]
2. Nan Wang, Julius Ming-Lin Tsai, Fu-Li Hsiao, Bo Woon Soon, Dim-Lee Kwong, Moorthi Palaniapan and Chengkuo Lee, Micromechanical resonators based on silicon two-dimensional phononic crystals of square lattice, IEEE/ASME J. Microelectromech. Syst., vol. 21, no. 4, pp. 801-810, 2012. [PDF] [DOI]
3. Nan Wang, Fu-Li Hsiao, Moorthi Palaniapan and Chengkuo Lee, Silicon two-dimensional phononic crystal resonators using alternate defects, Appl. Phys. Lett., vol. 99, no. 23, 234102, 2011. [PDF] [DOI]

Special Mention:

1. Our team member cum Ph.D. student, Mr Wang Nan, along with Prof. Fu-Li Hsiao, Dr. Julius Tsai, Prof. Moorthi Palaniapan, Prof. Dim-Lee Kwong and Prof. Chengkuo Lee, received the Best Paper Award for the article "Experimental Demonstration of Fano Resonance in Microfabricated Phononic Crystal Resonators Based on Two-Dimensional Silicon Slab."[LINK]

 

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