Phonon amplification

By stimulated emission of radiation. The concept of the saser, the acoustic analogue of the laser, has been around for years. New work by scientists from the University of Nottingham in the UK and the Lashkarev Institute of Semiconductor Physics in Ukraine has brought the realization of a saser one step closer. In the new experiments, the amplification medium consists of a semiconductor superlattice—stacks of thin layers of alternating semiconductors that confine electrons in so-called quantum wells. When a voltage is applied across the superlattice, electrons can hop from one well to the next and emit a phonon along the way. Because the superlattice also forms an acoustic cavity, phonons at certain frequencies will be resonant and remain in the superlattice for long periods of time. Measurements indicate that such resonant phonons stimulate the emission of additional phonons of the same frequency: When the applied voltage is tuned so that the hopping electrons emit resonant phonons, both the number of emitted phonons and the electric current through the superlattice increase. The researchers estimate that the generated phonon power is on the order of milliwatts per square centimeter of cross section. With improved efficiency, a semiconductor superlattice may form the basis of a saser that would emit terahertz coherent acoustic phonons. Sasers have potential applications in phonon optics, spectroscopy, and acoustical imaging of nanostructures. (A. J. Kent et al., Phys. Rev. Lett. 96 , 215504, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.215504 .)