Self-collimation and bifurcation of elastic wave using phononic crystal structure with tapered units

What is it about?

If a plane harmonic elastic wave is launched from a line source in a thin crystal plate with anisotropic (direction dependent) elastic stiffness, the wavefront takes a circular shape. On the other hand, while performing experiments involving precise control of phonons (quanta of the elastic wave vibration), one needs to precisely collimate the elastic wave. Therefore, we designed a phononic crystal (periodic arrangement of inclusions with specific geometry on the thin crystal plate) which can self-collimate the elastic wave. We have used a tapered unit with gradually changing profile which alters the propagation of elastic wave by changing the group velocity. We have used technologically important Silicon to demonstrate this, but other materials satisfying certain condition can also be used.

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Photo by Jens Lelie on Unsplash

Photo by Jens Lelie on Unsplash

Why is it important?

The structure designed by us satisfies three requirements often demanded by several experiments a) finte thickness everywhere b) self-collimation c) self-collimation and bi-refraction or bi-refringence offered by the same structure. The later means, incoming wave with a single group velocity, being scattered by the phononic crystal transmit as two distinct waves with different group velocities.