1 Designing a Ultrasound Coupler on a Phononic Crystal Platform Ali Abdollahi a , Mohammad Kazem Moravvej-Farshi b* a Faculty of Electrical and Computer Engineering, Tarbiat Modares University, PO Box 14115-194,Tehran 1411713116, Iran. ali.abdollahi@modares.ac.ir b Faculty of Electrical and Computer Engineering, Tarbiat Modares University, PO Box 14115-194,Tehran 1411713116, Iran. * Corresponding author e-mail: farshi_k@modares.ac.ir Abstract We are reporting the design of a new ultrasound coupler with the operating frequency of ~28.69 MHz, by means of coupling a W1 waveguide to a ring resonator based on a 2-D phononic crystal (PnC) platform. The 2-D PnC is composed of a periodic array of infinitely long cylindrical rods of steel, arranged in a square lattice of constant a 0 =6 µm, in air background. The radii of the steel rods are r 0 =0.42a 0 =2.52 µm. Mechanical waves propagating through this particular PnC are of acoustic types, experiencing a 10-MHz wide phononic bandgap within the frequency range of 25 MHz≤ f ≤ 35 MHz. The PnC based W1 waveguide that can be made by removing a row of rods in a straight line guides an acoustic mode within the phononic bandgap. The PnC ring resonator that is assumed to be made by removal of the rods from the periphery of a square of side lengths L=3a 0 =18 µm. Simulations are done by employing plane wave expansion (PWE) and finite element (FE) methods. Keywords: PnC; Resonator; Coupler; Waveguide 1. Introduction Ultrasound devices are used to detect objects and measure distances [1]. Heterogeneous mate- rials with periodic structures known as phononic crystals (PnCs) have attracted a great deal of re- searchers’ attentions, in recent years [2-5]. A PnC is a man-made crystal composed of two different elastic materials arranged in a periodic manner. Depending on its structural and physical parame- ters, for a PnC there exists a range of frequencies within which the mechanical waves are not al- lowed to propagate. This forbidden range is alternatively called phononic band gap (PnBG) [6]. An appropriate adjustment of the constituents’ elastic properties can facilitate a complete PnBG ex- tended along all directions in the first Brillouin zone (BZ). Moreover, by manipulating PnCs’ topol- ogy, the constituents’ fill-fraction, and the contrasts of the scatterers and host material parameters,