Acoustoelectric Coupling Research Articles

Surface acoustic wave gas sensor based on film conductivity changes

The first surface acoustic wave (SAW) sensor that functions via changes in conductivity of a thin surface film is reported. A lead phthalocyanine (PbPc) thin film is deposited on the acoustic propagation path of a LiNbO 3 SAW delay line, which serves as the feedback element of an oscillator circuit. Reaction with strongly oxidizing gases, in particular NO 2, increases the conductivity of the PbPc film. Acousto-electric coupling of the traveling electric potential wave associated with the SAW to charge carriers in the PbPc film slows the acoustic wave velocity, altering the oscillation frequency of the circuit. This sensor is about 1000 times more sensitive, in terms of the number of NO 2 molecules that can be detected (10 16 molecules/cm 3 of PbPc film), than an identical SAW sensor functioning via mass loading would be. Sensitivity to a few ppm of NO 2 in N 2 has been demonstrated.

Quantum anomaly in acoustic parametric interaction

A quantum theory of acoustic parametric interaction in piezoelectric semiconductors is developed. The rate of acoustic sum frequency generation is expressed in terms of the nonlinear conductivity coefficient gamma , which is obtained for arbitrarily degenerate electron statistics by solving the equation of motion for the one-electron density operator to second order in the wave amplitudes. An anomalous variation of gamma is found to occur when the acoustic wavenumber becomes comparable with the characteristic electron wavenumber. This anomaly, which is associated with the cut-off of the linear acoustoelectric coupling, gives rise to an enhancement of the acoustic parametric interaction. In particular it becomes possible for acoustic sum frequency signals to be efficiently generated on the high-frequency side of the cut-off of the linear acoustoelectric coupling. Using X-ray diffraction techniques, this effect was observed in the piezoelectrically amplified sound flux of a highly doped GaAs single crystal.

Photoenhancement of Surface Wave Convolution on GaAs

Convolution signal produced by a nonlinear interaction between acoustic surface waves on GaAs has been found to be markedly enhanced by light illumination. Spectral response of the convolution showed, besides a gradual change due to ordinary acoustoelectric coupling, a sharp "resonant" peak which can supposedly be attributed to the enhancement of elasto-optical coupling by near-band-gap illumination.

Quantum theory of acoustoelectric interaction

Within the self-consistent-field approximation, a quantum-mechanical derivation is given for the dielectric response function of an arbitrarily degenerate free-electron gas which is subjected to a drift field. Neglecting in the equation of motion for the one-electron density operator a convection term, significant in the classical-collision-dominated regime only, the dielectric response function and the acoustic gain factor for a piezoelectrically active sound wave are obtained for the quantum and semiclassical-microscopic regimes. The manner in which the theory can be extended to the collision-dominated regime is discussed. For a collision-free electron gas, the requirements of energy and momentum conservation in individual electron-phonon interactions lead to a cutoff in the acoustoelectric coupling when the acoustic wave number exceeds the characteristic electron wave number. The broadening of this cutoff due to collisions is investigated and compared with thermal broadening. A number of useful approximations for the acoustic gain factor are derived.

Photoenhancement of Surface Wave Convolution on GaAs

Convolution signal produced by a nonlinear interaction between acoustic surface waves on GaAs has been found to be markedly enhanced by light illumination. Spectral response of the convolution showed, besides a gradual change due to ordinary acoustoelectric coupling, a sharp “resonant” peak which can supposedly be attributed to the enhancement of elasto-optical coupling by near-band-gap illumination.

Acoustoelectric interaction in degenerately doped piezoelectric semiconductors

The acoustoelectric gain constant is calculated quantum mechanically for arbitrary degeneracy of the electron gas. The requirements of energy and momentum conservation in individual electron-phonon interactions result in a rather sharp reduction of the gain factor when the acoustic wave number exceeds the characteristic linear extension of the electron distribution in reciprocal space. A simple two-band model is developed to describe effects of impurity banding upon the acoustoelectric interaction. According to this model, impurity banding enhances the acoustoelectric coupling and widens the active acoustic spectrum. These effects are associated with the increased density of states near the conduction-band edge and with the presence of electrons of higher momentum effective mass than the conduction-band electrons. At low temperatures it becomes possible for the linear gain factor as a function of acoustic frequency to have two maxima.

An Approximate Theory of Skin-Effect Acoustic Generation in Conductors

Abeles has shown that a marked acoustoelectric coupling can occur due to the penetration of a high-frequency electric field into a conductor. Calculations are presented here of the coupling due to the spatial phase shift between the Coulomb force on the lattice ions and the reaction force of the electrons, and due to the localized electron reaction on the surface. The latter is largest for diffuse electron reflection, and is the most important coupling mechanism at frequencies above 30 GHz. Efficiencies are fairly low, being in the 10−2 to 10−3 range under ideal conditions; however, they remain of this magnitude up to the lattice vibration frequency. The model used is approximate: a classical free-electron model, idealized cases of fixed electron free paths with either uniaxial or isotropic electron motion being considered. For specular surface reflection of electrons, the exact model of Quinn is available for comparison, and it is shown that the results are very similar. Single and multilayered film structures are considered in addition to the semi-infinite conductor.

PARAMETRIC AMPLIFICATION OF ULTRASONIC WAVES IN CdS

Traveling-wave parametric amplification of 0.5-GHz acoustic shear waves has been observed in CdS at room temperature. Acoustoelectric coupling provides the nonlinear interaction. Observed gain was about 10 dB/mm, which is comparable to that of the conventional acoustic amplifier.

On sound amplification in crystals with strong current-striction and elasto-conductivity effects

It is pointed out that our previous equations describing acousto-electric coupling in non-polar crystals should be modified by adding a current-dependent term which gives rise to additional attenuation. Comparison with sound transmission experiments seems to indicate that the effect in Ge is much smaller than suggested by independent results.