Acoustic phase conjugation is a set of techniques meant to perform

phase conjugation Phase conjugation is a physical transformation of a wave field where the resulting field has a reversed propagation direction but keeps its amplitudes and phases. Description It is distinguished from Time Reversal Signal Processing by the fact t ...

acoustic wave Acoustic waves are a type of energy propagation through a medium by means of adiabatic loading and unloading. Important quantities for describing acoustic waves are acoustic pressure, particle velocity, particle displacement and acoustic intensit ...

s.A. P. Brysev et al., ''Wave phase conjugation of ultrasonic beams'', Physics-Uspekhi (1998)
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Techniques

Acoustic phase conjugation can appear in a solid when the sound velocity is modulated by an electromagnetic field. The generation of the conjugate wave can be seen as the decay of a photon into two phonons, as seen on the diagram. The two phonons have opposite wave vectors k and -k (they will propagate in opposite directions) and a frequency two time smaller than that of the photon. Parametric pumping techniques can be performed in several media: * In piezoelectric crystals, a nonlinear piezoelectric effect will produce a modulation of fractions of a percent. * In magnetic crystals, a modulation of dozens of percent can be reached through the magneto-acoustic coupling, which can be improved by combining the magnetostriction and spin reorientation transition effects or using the magnetoacoustic resonance. A "supercritical" or "giant" amplification, up to 80 dB, can be obtained beyond the threshold of instability of phonons in magnetoacoustic media. * In semiconductors, parametric interaction between phonons and plasmon can be generated by an alternative electric field or a modulated optical pump.

Applications

The auto-compensation of phase distortion and auto-focusing properties of the conjugate wave are used in non-destructive testing techniques. In medical therapy, they can be combined with giant amplification for tumor destruction, like

lithotripsy Lithotripsy is a non-invasive procedure involving the physical destruction of hardened masses like kidney stones, bezoars or gallstones. The term is derived from the Greek words meaning "breaking (or pulverizing) stones" ( litho- + τρίψω rip ...

and

hyperthermia therapy Hyperthermia therapy ''(or hyperthermia, or thermotherapy)'' is a type of medical treatment in which body tissue is exposed to temperatures above body temperature, in the region of 40–45 °C (104–113 °F). Hyperthermia is usually ...

. Acoustic imaging can be improved by applying selective phase conjugation on some harmonics of the incident wave. This narrows the focal distribution of those harmonics and reduces the sidelobes and reverberation noise, thus increasing the image resolution.V. L. Preobrazhensky, ''Parametrically phase-conjugate waves: application in nonlinear acoustic imaging and diagnostics'', Physics-Uspekhi (2006)
IEMN Selective acoustic phase conjugation can be used to detect isoechogenic objects whose nonlinear parameters differ from that of the medium. The linear acoustic properties of such objects are close to that of the medium which make them invisible with traditional echography techniques.V. L. Preobrazhensky et al., ''Nonlinear Acoustic Imaging of Isoechogenic Objects and Flows Using Ultrasound Wave Phase Conjugation'', Acta Acustica (2009)
IEMN Another field of application is nonlinear ultrasonic

velocimetry Velocimetry is the measurement of the velocity of fluids. This is a task often taken for granted, and involves far more complex processes than one might expect. It is often used to solve fluid dynamics problems, study fluid networks, in industria ...

, one order of magnitude more precise than with the usual

Doppler effect The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...

. Phase conjugate velocimeters have proved to correctly measure the flow velocity in the case of laminar flows in tubes, vortex flows under rotating disks and immersed jets in water.

Techniques

Applications

References

See also