Quantum leap for phonon lasers

February 22, 2010
These are schemes for phonon amplification and lasing: (Top) Two coupled microcavities are excited by an optical pulse traveling through an optical fiber (blue). (Center) Pump photons entering through the fiber are converted to lower energy photons and coherent phonons. At a threshold pump power, phonon gain exceeds phonon loss resulting in phonon lasing. (Bottom) Phonon amplification in a superlattice. Tunneling of electrons from one quantum well to the next is accompanied by phonon emission. When a strong phonon wave is applied, it leads to phonon amplification. Credit: Alan Stonebraker

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 . The papers are highlighted with a Viewpoint by Jacob Khurgin of Johns Hopkins University in the February 22 issue of Physics.

Light and sound are similar in various ways: they both can be thought of in terms of waves, and they both come in quantum mechanical units (photons in the case of light, and phonons in the case of sound). In addition, both light and sound can be produced as random collections of quanta (consider the light emitted by a light bulb) or orderly waves that travel in coordinated fashion (as is the case for laser light). Many physicists believed that the parallels imply that lasers should be as feasible with sound as they are with light. While low sound in the range that humans can hear (up to 20 kilohertz) is easy to produce in either a random or orderly fashion, things get more difficult at the terahertz (trillions of hertz) frequencies that are the regime of potential phonon laser applications. The problem stems from the fact that sound travels much slower than light, which in turn means that the wavelength of sound is much shorter than light at a given frequency. Instead of resulting in orderly, coherent phonon lasers, miniscule structures that can produce terahertz sound tend to emit phonons randomly.

Researchers at Caltech have overcome the problem by assembling a pair of microscopic cavities that only permit specific frequencies of phonons to be emitted. They can also tune the system to emit phonons of different frequencies by changing the relative separation of the microcavities.

The group from the UK's University of Nottingham took a different approach. They built their device out of electrons moving through a series of structures known as quantum wells. As an electron hops from one quantum well to the next, it produces a phonon. So far, the Nottingham group has not demonstrated a true phonon lasing, but their system amplifies high-frequency sound in a way that suggests it could be it a key component in future phonon laser designs.

Regardless of the approach, the recent developments are landmark breakthroughs on the route to practical phonon lasers. Phonon lasers would have to go a long way to match the utility of their optical cousins, but the many applications that physicists have in mind already, including medical imaging, high precision measurement devices, and high-energy focused , suggest that sound-based lasers may have a future nearly as bright as light lasers.

Explore further: Terahertz imaging goes the distance

More information:
-- Phonon Laser Action in a Tunable Two-Level System, Ivan S. Grudinin, Hansuek Lee, O. Painter, and Kerry J. Vahala
Phys. Rev. Lett. 104, 083901 (2010) - Published February 22, 2010, Download PDF (free)
-- Coherent Terahertz Sound Amplification and Spectral Line Narrowing in a Stark Ladder Superlattice, R. P. Beardsley, A. V. Akimov, M. Henini, and A. J. Kent, Phys. Rev. Lett. 104, 085501 (2010) - Published February 22, 2010, Download PDF (free)

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3.3 / 5 (7) Feb 22, 2010
If it emits sound it is not a L.A.S.E.R.
Feb 22, 2010
This comment has been removed by a moderator.
5 / 5 (2) Feb 22, 2010
Aww, if it's a phonon laser I wanna call it a phaser. Although, I suppose to be a proper acronym it would a SASEPH, sound amplification through the stimulated emission of phonons.
5 / 5 (1) Feb 22, 2010
anyone want to speculate on what kind of scifi goodies that could emerge from this?
5 / 5 (1) Feb 22, 2010
With this new technology we could send Bob Saget back in time to meet Charlemagne!
Feb 22, 2010
This comment has been removed by a moderator.
1 / 5 (1) Feb 22, 2010
Good for rupturing your hated enemies' eardrums from miles away!
2 / 5 (3) Feb 22, 2010
What sort of sounds in Nature are there at these teraherz frequencies? Quasar emissions? Anyone got an answer?
not rated yet Feb 22, 2010

Forget about long-range applications. Sound propagation through air is problematic: air suffers from turbulence, adiabatic losses, winds, and gradients in temperature, pressure, and humidity. IOW even a tight sound pulse will quickly lose shape and dissipate. It could work as a short-range weapon, though: firing a "needle" of air through an opponent. The cool thing is that there's no slug left over, so no way to trace the weapon. Criminals of the world, rejoice!


Quasars don't make sound, because in space nobody can hear you scream. And there's probably nothing in nature that elicits terahertz sound from any structure larger than a single molecule (it's pretty energy-intensive to make anything with noticeable mass oscillate so rapidly.)
1 / 5 (2) Feb 22, 2010
Thanks. That's what I was trying to visualze-what kind of macro-object could produce such a frequency.
1 / 5 (2) Feb 23, 2010
"The problem stems from the fact that sound travels much slower than light, which in turn means that the wavelength of sound is much shorter" Dont they mean longer? Also i'd like to see this applied to cerenkov radiation counters.
not rated yet Feb 23, 2010
It's use will probably be to study molecules and crystals in vacuum since the coherence probably evaporates after a few collisions. Mechanically accelerating atoms with compression waves at those speeds might give new insights in how complex reactions take place. It took years to figure out what lasers were good for.
not rated yet Feb 23, 2010
"The problem stems from the fact that sound travels much slower than light, which in turn means that the wavelength of sound is much shorter" Dont they mean longer?
No, they really mean shorter. At terahertz frequencies, you get 1 trillion oscillations per second regardless of how quickly the wave propagates. Now, light happens to be MUCH faster than sound, so in 1-trillionth of a second (which corresponds to 1 wavelength), light travels a lot farther. Hence, at any given fixed frequency light has a much longer wavelength than sound of the same frequency.
Also i'd like to see this applied to cerenkov radiation counters.
What do you mean? Cerenkov radiation is photonic; this article talks about sound waves...
4 / 5 (1) Feb 23, 2010
Think of the terahertz waves as being the carrier signal of the sound wave. Once that initial wave is produced, it can be oscillated to produce specific, high-resolution sounds.
not rated yet Feb 26, 2010
I presume, as an equivalent of laser could serve Rijke tube resonator

..forget about long-range applications. Sound propagation through air is problematic..
Check this...


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