Big Bad Sound

Half a century since the first working laser kick-started a technological revolution in the field of optics, a new device promises to do the same for acoustics. UK and Ukrainian physicists have built the first "saser", or sound laser, able to generate terahertz-frequency sounds. Imagine getting blasted with that. Ouch.

A laser produces photons that travel in a tight beam instead of dispersing outwards like a regular beam of light. A saser achieves the same for sound waves. Although it's not the first saser ever constructed, it is the first able to produce beams at terahertz frequencies, much higher even than those used for medical ultrasound imaging. As it develops, they could use sasers to manipulate electrons inside semiconductors, resulting in terahertz-frequency computer processors way faster than the gigahertz frequency chips of today.

The new saser is a semiconductor stack, made from thin, alternating layers of gallium arsenide and aluminium arsenide. To fire the device, the upper part of the sandwich is exposed to an intense light beam. That excites electrons in the material, which then release sound waves, or phonons. Those reach the lower part of the sandwich where they bounce off the interfaces between different layers. The spacing of the layers has been carefully chosen so that the weak echoes combine into stronger sounds in which all of the phonons are synchronised. Those strong phonons reflect back into the upper sandwich where they interact with the light-excited electrons – causing them to release further phonons and amplify the signal. The result is the formation of an intense series of synchronised phonons inside the stack, which leaves the device as a narrow saser beam of high-frequency ultrasound.

Saser beams that operate at much lower frequencies, in which the phonons oscillate a billion times per second (gigahertz) rather than a trillion times per second (terahertz), have been made before. However, they have had little impact because there are other methods of generating sound at such frequencies. The team designed and built a similar saser in 2006, but evidence it actually worked was weak. Just recently, they detected the saser by measuring its effect on a surface some distance away and found that it caused phonons to appear in the surface in a confined spot - the equivalent of seeing the dot of a laser pointer on a wall.

Don't forget, like a laser, a powerful beam, could be focused to the point of harm. What, you didn't realize dispersed high frequencies of sound waves can create nausea and confusion when used on people? Oh, you're in for a treat...