Flat lens for Lamb waves focusing

National audienceFirst proposal of negative index materials by V.G. Veselago [Sov. Phys. Usp. 10 (4), 509514 (1968)] has triggered interest for flat lens design. In 2000, J.B. Pendry proposed [Phys. Rev. Lett. 85, 3966-3969 (2000)] to take advantage of negative refraction and flat lens geometry to achieve subwavelength focusing. An alternative is based on photonic crystals. Such materials reveal strong anisotropic dispersion relation that may lead to band folding and negative refraction. Flat lens behavior is observable in a narrow spectral range called the All Angle Negative Refraction (AANR), where all incident wave vectors are negatively refracted. Negative refraction in phononic crystal has been recently demonstrated for elastic waves in a solid, and for Lamb waves at high frequencies in a plate with a square lattice of square holes [Appl. Phys. Lett. 96, 101905 (2010)]. This paper presents the experimental study of bending wave focusing by a solid flat lens in a thin plate with a 45°-tilted square lattice of circular holes. The dynamics of the focusing effect is observed below the first stop band in the acoustic dispersion surface at the carrier frequency predicted by theory