Hammouche Khales CDTA/MEMS & Sensors Group, Centre de De ´veloppement, des Technologies Avance ´es, Algiers 16303, Algeria e-mail: hkhales@cdta.dz Abdelkader Hassein-Bey FUNDAPL, Faculty of Sciences, Saad Dahlab University, Blida 09000, Algeria e-mail: a_hassein@univ-blida.dz Abdelkrim Khelif FEMTO-ST Institut, CNRS UMR 6174, Besanc ¸on 25044, France e-mail: abdelkrim.khelif@femto-st.fr Evidence of Ultrasonic Band Gap in Aluminum Phononic Crystal Beam In this paper, we prove theoretically and experimentally the existence of complete ultra- sonic band gap in phononic crystal beam. The phononic beam structure studied is com- posed of a linear lattice array of square pillars on a beam, made with aluminum-fortal easily machinable at centimetric scale. Ultrasonic characterization of phononic beam guides shows the existence of a frequency range where the transmitted signals are strongly attenuated, due to the presence of ultrasonic band gap, in agreement with theoretical results predicted by finite element simulation. These structures present a potential for the use as energy loss reduction in micromechanical resonators at high frequency regime. [DOI: 10.1115/1.4023827] 1 Introduction During the last two decades, the propagation of acoustic and elastic waves in inhomogeneous media has attracted a lot of inter- est. Specifically, periodic structures with spatially modulated elas- tic moduli and mass density possess a number of important properties, such as the occurrence of frequency band gaps [1–3]. These artificial phononic crystals (PCs) are periodic arrangements of materials in one, two, or three-dimensions attenuating fre- quency ranges. Actually, PCs have received high interest due to their ability to create powerful functionalities like confining, guid- ing, or filtering acoustic energy [4–6]. Such properties are useful in a wide variety of applications including acoustic isolation, radio frequency communication, and sensing [7]. The research on peri- odic beams has received significant attention for use in many domains, from geotechnical structures in general [8], vibration isolation [9], and aerospace load-bearing, to MEMS components [10]. For instance, it has been shown that a PCs band gap enhan- ces the quality factor (Q), which is greatly desired for use in radio frequency devices in the case of MEMS resonators [11,12]. This is achieved by structuring the resonator support with phononic band-gaps structures to block the propagation of acoustic waves toward the surrounding area [12]. The theoretical work of Hsu et al. [13,14] showed that designing anchor support with phononic structures to have a band gap range around the resonator operating frequency strongly reduces the energy transmission to the anchors and thus gives rise to energy confinement in the resonator, improving the Q factor of the resonators. Recently, Sorenson et al. reported [15] a linear acoustic bandgap structure realized with coupled ring-structures integrated on tethers of a high-frequency AlN-on-Si resonator, which significantly improves the resonator properties. These basic (1D PC) structures are common for many applica- tions; they are considered as structures realized by a joined unit cell periodically repeated to form a phononic waveguide beam. Several works made for the comprehension of propagation proper- ties in such a medium are well established [16–19]. In general, the finite element method (FEM) is mostly used to determine the acoustic modes propagating in the band gap structures. Two types of band gaps, Bragg and resonance gaps, can exist in this medium [2,7,20]. The first type is due to the spatial modulation of the elas- ticity, and the second gap type is due to the local resonance in the case of the inclusions hosted in the matrix beam. In this paper, we show experimentally the existence of com- plete phononic band gap in a phononic beam structure made with pillars. For that purpose, numerical results based on finite element simulations of one dimensional phononic crystal beam are, at first, presented. The structure considered in this study is made with aluminum-fortal material, and composed of periodic array of square pillars on beam. After that, an ultrasonic characterization based on the transmission measurements of the aluminum-fortal phononic beam is shown. Finally, a comparison between the measured data with simulation results is given. 2 Method and Numerical Results As illustrated in Fig. 1, the considered PC beam structure is composed of periodic cells along the x direction, where each unit cell consists of square pillar on the beam. The lattice parameter is Fig. 1 Square pillar-based phononic beam. The unit cell do- main is meshed in three dimensions to be used for band struc- ture calculations. The beam thickness and the lattice parameter is a and the square cross-section of pillars have height h and length d. Bloch–Floquet periodic boundary conditions are applied in the x direction. Contributed by the Design Engineering Division of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received April 30, 2012; final manuscript received January 28, 2013; published online June 6, 2013. Assoc. Editor: Michael Leamy. Journal of Vibration and Acoustics AUGUST 2013, Vol. 135 / 041007-1 Copyright V C 2013 by ASME Downloaded From: http://asmedigitalcollection.asme.org/ on 06/19/2013 Terms of Use: http://asme.org/terms