Experimental exploration of imaging properties of a two-dimensional flat lens made of phononic crystals

We investigate negative refraction of acoustic waves in a two-dimensional phononic crystal (PC) made of stainless steel rods arranged in a triangular lattice filled with methanol. The theoretical dispersion curves of this PC exhibit a branch with group and phase velocities with opposite signs yielding to negative refraction. Moreover, in the frequency range of interest, the equi-frequency contours are circular around the high symmetry point T leading to a negative effective refractive index (ERI) constant at a given frequency whatever the incident angle. An experimental study is performed in a way to evaluate the ultrasonic beam deviation when the wave is transmitted through a slab of PC. This allows the determination of the experimental ERI as a function of the frequency. Furthermore, the transmission coefficients are obtained versus the incident angles. Finally, the property of All-Angle-Negative-Refraction (AANR) of the required PC for imaging is discussed.