why we use the bragg mirror under the piezeelectric guiding layer in resonators?
Dear Farouk,
you find in this link and attached Interesting works on the topic. I Wish that its can help you.
https://www.google.ch/patents/US8593234
B.regards.
The Bragg mirror is a succession of transparent planar surfaces with different refractive indices. It makes it possible to reflect, through phenomena of constructive interference, almost the totality of the incident energy. This is possible provided that the incident wave is close to the normal incidence. No other mirror can match this result because, the dielectric losses being lower than the metal losses for the optical wavelengths.
A simple Bragg mirror structure is a stack of several layers of different optical indices. Considering two materials, one with a low refractive index and the other with a stronger index, the maximum reflectivity is obtained for the so-called Bragg wavelength if the thicknesses are fixed at lambda / 4ni for each layer of index ni.
Indeed, for these precise thicknesses, the phase shift undergone by a wave having traversed the optical thickness lambda / 4 is exactly equal to Pi / 2. A Bragg mirror is thus constructed for a given wavelength and a given temperature, the thickness varying with it.
What is true for the optics (see previous message) remains true for the materials: the Bragg mirror consists of an alternation of two materials exhibiting a high impedance contrast (Z1 / Z2 as large as possible ), Which makes it possible to produce a very efficient mirror (reflecting 99.9% of the acoustic energy in a few pairs of layers). The thicknesses of the layers are equal to λ / 4.
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