The SASER (sonic laser) is not a new idea and several have been built in recent years. But these always operated in gigahertz frequency ranges and below. Now a tetrahertz frequency SASER has been built by scientists from Nottingham University in the UK and the Lashkarev Institute of Semiconductor Physics in the Ukraine.

While a LASER operates by building up similar quantum steps by electrons that release photons of the same frequency (coherent light), the SASER builds up sonic vibrations (phonons) that form coherent sound waves in a tightly focused beam. Light stimulation is used to fire semiconductors which produce sound that is synchronized into an amplifying cascade which eventually escapes the device as high frequency ultrasound.

A sonic boom in the world of lasers – []

The Saser mimics this technology but using sound, to produce a sonic beam of ‘phonons’ which travels, not through an optical cavity like a laser, but through a tiny manmade structure called a ‘superlattice’. This is made out of around 50 super-thin sheets of two alternating semiconductor materials, Gallium Arsenide and Aluminium Arsenide, each layer just a few atoms thick. When stimulated by a power source (a light beam), the phonons multiply, bouncing back and forth between the layers of the lattice, until they escape out of the structure in the form of an ultra-high frequency phonon beam.

A key factor in this new science is that the Saser is the first device to emit sound waves in the terahertz frequency range… the beam of coherent acoustic waves it produces has nanometre wavelengths (billionths of a metre). Crucially the ‘superlattice’ device can be used to generate, manipulate and detect these soundwaves making the Saser capable of widespread scientific and technological applications. One example of its potential is as a sonogram, to look for defects in nanometre scale objects like micro-electric circuits. Another idea is to convert the Saser beam to THz electromagnetic waves, which may be used for medical imaging and security screening. High intensity sound waves can also change the electronic properties of nanostructures so a Saser could be used as a high-speed terahertz clock to make the computers of the future a thousand times faster.

Most powerful ‘sound laser’ to shake up acoustics
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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, says Tony Kent at the University of Nottingham, UK.

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. Terahertz sound may be largely a curiosity today, says Kent, but being able to produce it in controllable beams could unleash new ideas and applications.

“Fifty years ago many eminent scientists said that light amplification by the stimulated emission of radiation [lasers] was no more than a scientific curiosity,” says Kent, but lasers are now used for everything from digital storage and cancer treatment to weaponry.

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