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The Phaser
March 25, 2013
The regulation
side arm for members of
Star Trek reconnaissance teams (away-teams) is the
phaser, which seems to act much like a
laser. Instead of
vaporizing things, however, the objects just seem to disappear, which makes me think the operating principle involves some sort of
decoherence of matter.
Quantum decoherence is the loss of the order of the
phases of the components of a system in
quantum superposition; so, we can get the word, "phase," from quantum decoherence. The only problem with this sort of phaser operation is that coherent
quantum states are artificial states used in
quantum computing. They aren't really a
glue that holds things together, but who knows what tricks twenty-third century
physicists might concoct.
Although the Star Trek phaser is
fiction, the laser has been with us for more than fifty years. I wrote about the fiftieth anniversary of the
invention of the laser in a
previous article (Fifty Years of Lasers, June 8, 2010). Nearly every home has devices with
semiconductor lasers, such as
CD and
DVD players, and people with a
fiber cable connection use a semiconductor laser for communication.
What scientist doesn't have a
laser pointer, although we should be careful using them.[1]. Some
do-it-yourself enthusiasts might have a
laser line level. The ubiquity of the laser can be seen in the long
list of laser articles on
Wikipedia.
The laser was first demonstrated by
Theodore Maiman of
Hughes Research Labs. He achieved laser action in a
chromium-doped piece of
aluminum oxide crystal (a.k.a.,
ruby) on May 16, 1960. The fundamental process behind lasing in ruby is shown in the following figure.
Energy level diagram of a ruby laser.
(Author's rendition with Inkscape.)
In a short summary of how the
ruby laser works, an intense burst of light from a
flash lamp excites
electrons from a
ground state (
4A
2 in a
notation developed by
spectroscopists) to two
excited states (
4T
1 and
4T
2). The electrons then transit to another energy level (
2E), and from there they revert back to the ground state, emitting light at two deep red colors (about 692.7 and 694.3
nm).
I've remarked in previous articles how many of the same processing techniques applied to
light waves, such as
focusing,
reflection and
diffraction, will work for
sound waves as well. Sound waves in materials are
quantized, and these
phonons behave quantum mechanically the same as
photons. Physicists have taken the optical-acoustic
analogy to heart, finally developing some acoustic lasers in 2010.[2-3]
It took fifty years from the time of the optical laser to these first acoustic "phasers" (for "phonon amplification by stimulated emission of radiation") because of the differences between photon and phonon systems. Laser
wavelengths are about a
micrometer, whereas phonon wavelengths can be as short as a few
atomic spacing in crystals. The
speed of sound c in a typical crystal is a few thousand
meters per second, so phonon frequencies
ν = c/λ, where λ is the wavelength, can be as high as the 10
12 Hz range (
terahertz). furthermore, most materials are not that
transparent at phonon frequencies.
The early phasers were optically-excited, so they weren't a pure analog of the laser. Scientists at the
NTT Basic Research Laboratories (
Kanagawa, Japan) have recently published a description of an acoustically-excited phaser.[4-6] The basic energy levels of this phaser are shown in the figure.
Phaser energy level diagram.
The NTT phaser has three levels, with the stimulated phonon emission at the frequency ωL = ωH - ωM
(Author's rendition with Inkscape.)
The NTT phaser uses a
mechanical oscillator as an excitation source. This emits phonons into the solid, where they subsequently produce a narrow frequency acoustic signal at about 1.7
MHz.[5] The device, as shown in the
photomicrograph, is contained in a 1 cm x 0.5
cm chip.
A false-color scanning electron micrograph of the NTT phaser.
(Image courtesy of Imran Mahboob, used with permission.)
As can be seen in the photomicrograph, the excitation resonator is a beam, formed from a
gallium arsenide piezoelectric heterostructure, clamped on two sides. All the phaser action happens in this beam, the three-levels being three coupled acoustic modes of vibrations of the beam.[5]
A
bias voltage is applied to the piezoelectric
electrodes to tune the beam to attain the
resonance condition
ωL = ωH - ωM. Upon resonance, excitation of the upper frequency mode
ωH results in amplified emission of acoustic radiation.[5] As in a laser, phaser operation occurs only if the
gain exceeds the losses.
The NTT scientists proved that their device is an acoustic analog of a laser by showing that there is a required
threshold of pump intensity for the device to "phase"; and, by measuring the
spectral purity of the emitted signal. The acoustic
bandwidth was as narrow as 175 millihertz, which is about 10
-7 of the carrier frequency.[5]
Just as in the early years of the laser, the ultimate utility of phasers is yet to be imagined. One limitation is that the phaser signals are now confined to the chip.[6] The smaller wavelength of phaser emissions would allow better resolution in many applications, including precision measurement,
tomography and
ultrasound imaging.[5]
References:
- Michael Cooney, "Laser pointers produce too much energy, pose risks for the careless," Network World, March 20, 2013.
- Jacob B. Khurgin, "Viewpoint: Phonon lasers gain a sound foundation," APS Physics, vol 3, no. 16 (February 22, 2010), DOI: 10.1103/Physics.3.16 .
- Ivan S. Grudinin, Hansuek Lee, O. Painter and Kerry J. Vahala, "Phonon Laser Action in a Tunable Two-Level System," Phys. Rev. Lett., vol. 104, no. 8 (February 26, 2010), Document No. 085501.
- I. Mahboob, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi, "Phonon Lasing in an Electromechanical Resonator," Phys. Rev. Lett., vol. 110, no. 12 (March 22, 2013), Document No. 127202.
- José Tito Mendonca, "Viewpoint: Lasers of Pure Sound," APS Physics, vol. 6, no. 32 (March 18, 2013), DOI: 10.1103/Physics.6.32.
- Adam Mann, "Pew Pew! Scientists Build Lasers Out of Sound, Call Them Phasers," Wired, March 18, 2013.
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