SAW Phononic Reflector Structures

Abstract

This paper will present theory, analysis and experimental work on SAW phononic reflector structures. The term SAW phononic structures is used since the structures are composed of a-periodic or pseudo-periodic structures that are designed to affect the propagation of surface acoustic waves in a manner similar to the periodic potential in a semiconductor which affects the electron's energy in allowed or forbidden bands based on the crystal material. The simplest SAW reflective structure commonly used is the synchronous reflector, which provides a very narrow frequency stop-band. By building varying one-dimensional structures in the SAW propagation direction, it is possible to control and design numerous pass-and stop-bands, which provides frequency dependent transmission or reflection, and span large fractional bandwidths, unlike the simple SAW Bragg reflector. These types of a-periodic and pseudo-periodic structures have not been previously shown in the literature and may provide the possibilities of new SAW device embodiments. This work will discuss the basic theory which we use in the design of the phononic reflective structures. The approach uses the concept of orthogonal frequencies that allows the building of the a-periodic or pseudo-periodic structures. It is possible to design and predict various a-periodic structure performance quite well using an intuitive model. A coupling of modes (COM) model is then used to simulate the complex structure, and predict the overall frequency dependent reflectance or transmittance of the structure. The time domain COM responses can also be predicted and compared to the intuitive approach to provide insight. As expected, allowed and unallowed frequency bands are present. By changing the structures, the band characteristics can be changed in a predictive manner over a wide bandwidth. Experimental devices at 250 MHz are built on YZ LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> to demonstrate the SAW phononic structures and to confirm model predictions. The paper will present several phononic structures, theoretical analysis and simulations, and compare the predicted and measured results.