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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

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 (4A2 in a notation developed by spectroscopists) to two excited states (4T1 and 4T2). 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 1012 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.

Figure caption

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.

Figure caption

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:

  1. Michael Cooney, "Laser pointers produce too much energy, pose risks for the careless," Network World, March 20, 2013.
  2. 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 .
  3. 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.
  4. 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.
  5. José Tito Mendonca, "Viewpoint: Lasers of Pure Sound," APS Physics, vol. 6, no. 32 (March 18, 2013), DOI: 10.1103/Physics.6.32.
  6. Adam Mann, "Pew Pew! Scientists Build Lasers Out of Sound, Call Them Phasers," Wired, March 18, 2013.

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