Acoustic nanocavities

Phonons pulsed at around 100 GHz, with wavelengths of a few nanometers, have been confined in the same kind of resonant cavity used in photonics. A collaboration of physicists in France and Argentina used a superlattice made of carefully grown alternating layers of gallium arsenide and aluminum arsenide, materials with different acoustic impedances—the acoustic analog of refractive index for light. Two sets of multilayers in the superlattice act as Bragg mirrors for phonons, while a single nanometer-thick layer of GaAs in the center serves as the cavity. A femtosecond laser focused on the bottom of the stack generates the high-frequency sound, which is reflected multiple times through the nanocavity. After some delay, narrow phonon wavepackets at certain allowable sharp frequencies are detected by a laser probing the top of the device. Bernard Jusserand (CNRS and University of Paris VI and VII) says that he and his colleagues hope to reach the terahertz acoustic range. The researchers think that a new field of nanophononics has been inaugurated and that the acoustical properties of semiconductor nanodevices will play important roles. One envisaged use is high-frequency modulation of the flow of charges or light in small spaces. Another is for novel forms of tomography that could image the interior of opaque solids. (A. Huynh et al., Phys. Rev. Lett. 97 , 115502, 2006 http://dx.doi.org/10.1103/PhysRevLett.97.115502 .)