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Lasers are most familiar as pinpoint beams of coherent light, but the principle of lasing—amplification by stimulated emission—occurs just fine without light, as two reports of ultrasound lasers demonstrate. One device consists of layers of gallium arsenide and aluminum arsenide that emit and partially trap sound vibrations in the solid (phonons) oscillating in the terahertz range. A voltage creates a burst of phonons, which reverberate and multiply, and the amplified ultrasound exits one end. Physicists describe this so-called saser, for sound laser, in the June 2 Physical Review Letters. In the “uaser” (pronounced “wayzer”), piezoelectric oscillators vibrate an aluminum block, which feeds back into the oscillators and locks the vibrations into a single megahertz frequency. The system generates multidirectional ultrasound and might aid in studying so-called random lasers that likewise produce scattered, coherent light, co-designer Richard Weaver of the University of Illinois informed the Acoustical Society of America on June 8.
