Hypersonic Phononic CrystalsEdwin L. Thomas, Massachusetts Institute of Technology, DMR 0308133

Fig. 2. (a) Single Crystal Hypersonic Reflector(b) Theoretically Calculated Dispersion relation of the epoxy-air phononic crystals along[10] (c) Experimental Phononic dispersion relation along the [10] direction showing a partial band gap between 1.21 and 1.57 GHz (in grey)

Fig. 1. A sound wave is incident on the surface of a two dimensional phononic crystal consisting of air cylinders on a triangular lattice in a solid film. As the sound wave has a frequency within the bandgap, propagation is not allowed and the wave is reflected backwards.

J.-H. Jang et al., Applied Physics Letters (submitted)

Phononic crystals are structures possessing periodic variations in density and/or elastic constants, which result in band gaps for sound and other mechanical waves. Sonic crystals can be used for sound and vibration isolation, ultrasonic crystals for acoustic imaging. Our interest is in hypersonic crystals with 100nm feature sizes for the enhancement of acousto-optical interactions: engineering of phonon –photon as well as electron-phonon interactions.

E. Thomas et al., Nature Materials, 5(10), 773, 2006

We fabricated hypersonic crystals with band gaps in GHz frequency range using interference lithography and measured their phonon dispersion relation with Brillouin light scattering (BLS). We reported the first experimentally measured band gap at hypersonic frequencies in a single crystalline hypersonic phononic crystal.

Hypersonic phononic crystals

Edwin L. Thomas, Massachusetts Institute of Technology, DMR 0308133

Education

This grant was used to provide support for T. Gorishnyy, a graduate student who just completed his Ph.D. in Materials Science and Engineering (July, 2007) and provides partial support for Henry Koh, a 3rd year graduate student in Materials Science and Engineering and Boris Rasin a sophomore in Materials Science and Engineering at MIT who is working on fabricating the hypersonic crystals using interference lithography during spring term 2007 and over the summer of 2007.

References:

•T. Gorishnyy et al., PRL, 94 115501, 2005T. Gorishnyy et al., Physics World, 18(12), 24, 2005J.-H. Jang et al., Nano Letters, 6(4): 740, 2006•E. Thomas et al, Nature Mat.5,10,773 (2006)•W. Cheng, Macromolecules,39,9614(2006)

Outreach

As a part of this investigation we have established a very productive collaboration with Prof. G. Fytas from Max Plank Institute (MPI) for Polymer Research in Mainz, Germany. T. Gorishnyy has visited MPI twice to perform Brillouin light scattering measurements on phononic crystals. This work has resulted in several publications and was presented at number of international conferences. The Interference Lithography Lab (ILL) has offered tours and demos to Freshman Explorations as well as Parents Weekend activities.