Ever heard of a ‘phonon torpedo’? How about a ‘phonon laser’? Not that either? No wonder, they don’t exist. Although a phonon is to sound as the photon is to light, we do not know much about working with phonons. However, here is news concerning research that – one day – may bring about devices based on phonons.
One of the reasons there are no phonon devices is fundamental difference between light and sound. While they both can be thought of as waves and both have units defined by quantum mechanics (the photon and phonon), the problem is that sound travels much more slowly than light – meaning that at any given frequency the wavelength of sound is much shorter than light. To work in a laser, sound would have to be in the range of terahertz (trillions of hertz) frequencies; but because of the tiny wavelengths high-frequency sounds tend to result not in orderly laser-like focus but a more random emission like a light bulb.
Two different approaches to this problem have recently been announced. One by researchers at the California Institute of Technology (USA) overcame the problem by using a pair of microscopic cavities that permit only specific frequencies of phonons to be emitted. This approach allows for precise tuning of frequencies.
Another approach, coming from the University of Nottingham (UK), constructed a device using quantum wells (typically a semiconductor that forces electrons into a two-dimensional plane) so that electrons hopping from one well to another emit phonons. They have not built a true laser, but can demonstrate a system that amplifies high-frequency sounds to a level that could be used in sonic lasers.
Physicists have taken major step forward in the development of practical phonon lasers, which emit sound in much the same way that optical lasers emit light. The development should lead to new, high-resolution imaging devices and medical applications. Just as optical lasers have been incorporated into countless, ubiquitous devices, a phonon laser is likely to be critical to a host of as yet unimaginable applications.
Two separate research groups, one located in the US and the other in the UK, are reporting dramatic advances in the development of phonon lasers in the current issue of Physical Review Letters. The papers are highlighted with a Viewpoint by Jacob Khurgin of Johns Hopkins University in the February 22 issue of Physics.