Supriyo Datta

First‐order reflection coefficient of surface acoustic waves from thin‐strip overlays

The problem of surface‐wave reflection at thin‐strip overlays occurs frequently in signal‐processing devices. Grooved array reflectors employ strip overlays of the same material as the substrate, while metallic transducers incorporate strips of a material different from the substrate. A simple model for calculating the reflection coefficient of thin overlays is described, based on the normal‐mode analysis developed by Auld. Calculated values are in close agreement with reported experimental values for grooves in ST‐X quartz and Y‐Z lithium niobate, and for aluminum on ST‐X quartz.

An analytical theory for the scattering of surface acoustic waves by a single electrode in a periodic array on a piezoelectric substrate

An analytical theory is presented that leads to the scatter matrix of a sngle electrode for an acoustic surface wave propagating through a periodic array of electrodes on an anisotropic piezoelectric substrate. The scatter matrix elements are obtained analytically as functions of frequency and metallization ratio, considering only the electrical loading by the electrodes; mechanical loading effects are neglected. The results may be used directly in the broad band analysis of surface acoustic wave reflectors and multistrip couplers. Previous works have provided these parameters numerically for electrically shorted and open arrays at stop band frequencies (frequencies where the array period is an integer number of half wavelengths). This work extends the analysis in three ways: (a) the analysis is valid for all frequencies, (b) the external electrical load connected to the electrodes is included as a parameter, and (c) the scatter matrix elements are determined analytically instead of numerically.

Room temperature Coulomb blockade and Coulomb staircase from self‐assembled nanostructures

The self‐assembly of well‐characterized, nanometer‐size Au clusters into ordered monolayer arrays spanning several microns has been achieved. Techniques to insert molecular wires to link adjacent clusters in the self‐assembled array have also been developed. ‘‘Unit cell’’ nanostructures formed from individual Au clusters supported on a self‐assembled monolayer film of the double‐ended thiol molecule p‐xylene‐α,α′‐ dithiol show evidence for reproducible single electron effects at room temperature when studied by scanning tunneling microscopy. From these measurements, estimates for the electrical resistance of a single molecule can be obtained. The experimental values for this resistance are in reasonable agreement with theoretical calculations using the Landauer approach.

Acoustoelectric measurement of low carrier mobilities in highly resistive films

The acoustoelectric method of determining carrier mobility μ in semiconductor films has been modified to permit its application to high‐resistance, low‐mobility films. The conventional method requires knowledge of the acoustic loss caused by the mobile carriers; in high‐resistance films this loss becomes too small to be measured. We show that the required information may be derived from knowledge of the acoustic power and the device geometry. Our samples were amorphous hydrogenated Si and Si0.6Ge0.4 films on nonpiezoelectric substrates, separated by a convenient air gap (12.5 μm) from a LiNb03 slab carrying surface acoustic waves. One sample had 108 Ω/⧠ and μ=0.08 cm2/V sec, another sample 1010 Ω/⧠ and μ=0.5 cm2/V sec.

Unusual Parametric Effects on Line Acoustic Waves

Absrruct-Line acoustic wave (LAW) devices, in which waves are guided in a narrow region along an edge, permit one to achieve high local strains with relatively Little total power. One would expect to observe strong harmonic generation and parametric conversion, resulting from the fust-order elastic nonlinearity. However, first-order nonlinear effects are nulled out to a high degree by the symmetry properties of the LAW. Conventional pumping produces only small effects and second harmonic generation is weak. Pumping at one-half the usual frequency, however, interacts with the seconderder nonlinear elastic coefficient which is not subject to nulling and produces useful outputs which increase rapidly at higher frequencies. Experimental results obtained with a LiNb03 LAW device at 100 and 200 MHz are presented.

Analysis of Nonperiodic SAW Transducers Using a Circuit Model

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Line acoustic waves on cleaved edges

The feasibility of fabricating wedges suitable for wave propagation by cleaving LiNbO3 is demonstrated. This is a simple technique that affords excellent control over the wedge angle. The velocity and field distribution of line waves along the cleaved edge are predicted well from theory.

Surface acoustic waves in periodic structures

A noniterative technique is described for obtaining surface‐wave solutions in periodic layered media by expanding the field quantities in an orthonormal series of Laguerre functions perpendicular to the surface and in a Fourier series in the propagation direction. The principle is illustrated with uniform layered media, thin and thick metallic gratings, and grooved arrays. The technique is general and applicable to arbitrary periodic structures.

A model for multitrack nonperiodic multistrip couplers

A simple circuit model for the multistrip coupler is presented. The transmission matrix of a single strip relating the output‐ and input‐wave amplitudes in different tracks is obtained from circuit theory. The overall transmission matrix of the coupler is obtained by cascading the transmission matrices of successive strips. The model is shown to yield the correct results for common coupler configurations. This model because of its strip‐by‐strip approach makes it straightforward to analyze nonperiodic couplers which are difficult to analyze by usual techniques. Numerical solutions for a strip‐coupled UDT are presented in agreement with previous experimental results.

Tap weight enhancement for broad-band filters

The theory for the replacement of a series of taps having small sbengths with a single larger tap while maintaining filter selectivity is presented. This technique works for the general class of bandpass fiiters, including broad-band fiiters with low shape factors, where withdrawal weighting is ineffective. This tap enhancement procedure increases tap weight accuracy and reduces diffraction distortions by removing small taps. Experimental results for capacity weighted transducers verify the theory.