L. Palmieri

Incidence angle and polarization dependence of light diffracted by acoustic surface waves

A theoretical analysis of the diffracting effects produced by ASW on light beams contained on the sagittal plane of the wave is presented. The separate contributions have been explicitly studied, produced by periodical changes of the refractive index and by corrugation of the surface in cases of x‐ and y ‐cut crystalline quartz and y ‐cut LiNbO3 for both states of polarization of the incident light. In all cases analyzed, measurements of light‐deflection efficiency have been performed as a function of the angle of incidence in light‐transmission experiments.

Acoustic Lamb wave‐electric field nonlinear interaction in YZ LiNbO3 plates

The nonlinear electroacoustic effect has been investigated for acoustic Lamb wave propagation in a YZ LiNbO3 plate under a uniform bias electric field. A theoretical model is briefly outlined, which shows how the effect can be interpreted in terms of second and third order material constants. The strength of the nonlinear interaction has been experimentally evaluated through measurements of the change in phase velocity of Lamb modes produced by the bias field. The experiments were performed in the frequency range 8–33 MHz on most of the Lamb modes excited in a 1.25‐mm‐thick plate by an interdigital transducer with a periodicity λ=0.396 mm. The results evidence a strong dependence of the nonlinear effect on the order and symmetry of the analyzed modes.

Phase and amplitude relations between fundamental and second harmonic acoustic surface waves on SiO2 and LiNbO3

The spread of results obtained by different theoretical approaches in studying acoustic surface wave nonlinear interaction and difficulties in treating strongly piezoelectric materials indicate a need for more experimental data. In the present work the spectral asymmetries of the light diffracted by surface waves are analyzed in order to evaluate the complex coupling coefficient in second harmonic surface‐wave generation. The experimental conditions are such as to keep the acoustic energy mostly confined to within fundamental and second harmonic waves. Amplitude and phase of nonlinear coupling coefficients were separately determined for LiNbO3 and crystalline SiO2 substrates through different geometries of the acousto‐optical interaction, crystal cuts, and propagation directions of the waves.

Electroelastic effect in layer acoustic mode propagation along ZnO films on Si substrates

The influence of a bias electric field on the phase velocity of acoustic layer modes propagating along a ZnO overlay on a Si substrate is analyzed. Measurements of the fractional change in phase velocity as a function of the applied electric field were performed on both Rayleigh and Sezawa modes. The strength of the electroelastic effect has been experimentally determined for two samples obtained by different sputtering runs. An evident dependence of this parameter on the acoustic mode and on the quality of the sputtered film was observed. The maximum value of 15.8×10−6 μm/V was attained by the Sezawa wave in one of the specimens, while a weaker effect was observed in the other cases.

Image scanning through the acousto−optical effect produced by acoustic surface waves

The scanning of a transparency is done by letting a collimated light wave front illuminate it and be diffracted by very short pulses of ultrasonic surface waves. Detection of diffracted light follows a time dependence that equals the transparency brightness along each line that is successively scanned. 94−MHz wave trains, about 10 wavelengths long, give a potential number of resolved spots per second almost equal to 107. Interaction geometry by back−surface reflection has been used, which greatly enhances the diffraction efficiency and yet retains the large acousto−optical bandwidth proper to surface waves.

Lamb-wave electroacoustic voltage sensor

The effect of a bias electric field on the phase velocity of Lamb waves propagating in a piezoelectric plate has been investigated in order to develop an electroacoustic voltage sensor. Experiments were performed on YX and YZ LiNbO3 Lamb‐wave delay lines, used as the stabilizing element in the feedback loop of an oscillator, by measuring the fractional frequency change produced by a bias electric field normal to the plate surfaces. The results obtained have shown that Lamb‐wave sensors can exhibit a higher sensitivity and a better linearity than the corresponding Rayleigh‐wave devices.

Determination of coupling coefficient in second harmonic generation of acoustic surface waves

In a slightly dispersive y‐cut sample of LiNbO3 the complex coupling coefficient between fundamental and second harmonic acoustic surface waves has been measured through the intensities of the light reflected from the corrugated surface into second diffraction orders. The values have been separately obtained for both +z and −z propagation directions.

INTERNAL AND SURFACE CONTRIBUTION TO LIGHT DIFFRACTION BY SURFACE‐ACOUSTIC WAVES

An analysis is done of depth interaction between polarized light and acoustic‐surface waves, which gives account for the actual dependence of the scattered‐light intensity vs the angle of incidence, in transmission experiments. Measurements were performed at 11.8 and 18.8 MHz for surface waves propagating in Y direction on X‐cut, α‐quartz, and Z‐polarized light.

Real time acousto-optical spectrum analyzer through unguided light-surface acoustic waves interaction

Abstract An acousto-optical spectrum analyzer is described, based on the linear relationship between frequency and angular deviation of an acousto-optical cell. A side-entry internal reflection geometry of interaction is used with surface acoustic waves propagating on a Y-Z LiNbO 3 sample. A CCD photodiode array is the detecting device, allowing 300 kHz resolution and 50 MHz bandwidth.

Interdigital transducer method of acoustic beam steering for bulk wave acousto‐optic Bragg cells

A LiNbO3 acousto‐optic Bragg cell is described, which uses an interdigital transducer deposited on the x surface of the crystal so as to excite longitudinal bulk acoustic waves along a direction almost normal to the surface itself. By a proper choice of the periodicity of the transducer electrodes, a frequency‐dependent steering of the acoustic beam can be obtained, which allows the Bragg conditions to be satisfied over a wide range of frequencies. Results are reported relative to a cell operating in the frequency range 1.4–2.1 GHz, with a diffraction efficiency of approximately 2% per watt of acoustic power.