A New Kind of Acoustic Laser

Sound amplification by stimulated emission of raciation, or SASER, is the acoustic analog of a laser. Instead of a feedback-built potent wave of electromagnetic radiation, a saser would deliver a potent ultrasound wave.

The concept has been around for years and several labs have implemented models with differing features. In a new version, undertaken by scientists from the University of Nottingham (Anthony Kent, anthony.kent@nottingham.ac.uk) in the U.K. and the Lashkarev Institute of Semiconductor Physics in Ukraine, the gain medium -- that is, the medium where the amplification takes place -- consists of stacks (or a superlattice) of thin layers of semiconductors which together form "quantum wells."

In these wells, really just carefully confined planar regions, electrons can be excited by parcels of ultrasound, which typically possess millielectronvolts of energy, equivalent to a frequency of 0.1-1 terahertz. And just as coherent light can build up in a laser by the concerted, stimulated emission of light from a lot of atoms, so in a saser coherent sound can build up by the concerted emission of phonons from a lot of quantum wells in the superlattice.

In lasers the light buildup is maintained by a reflective optical cavity. In the U.K.-Ukraine saser, the acoustic buildup is maintained by an artful spacing of the lattice layer thicknesses in such a way that the layers act as an acoustic mirror (see figure at Physics News Graphics).

Eventually the sound wave emerges from the device at a narrow angular range, as do laser pulses. The monoenergetic nature of the acoustic emission, however, has not yet been fully probed. The researchers believe their saser is the first to reach the terahertz frequency range while using also modest electrical power input. Terahertz coherent sound is itself a relatively new field of research. Essentially ultrasound with wavelengths measured in nanometers, terahertz acoustical devices might be used in modulating light waves in optoelectronic devices.

Kent et al., Physical Review Letter, 2 June 2006
Contact Anthony Kent, anthony.kent@nottingham.ac.uk
Figure at Physics News Graphics