  In this paper, we numerically and experimentally demonstrate negative
refraction of flexural waves in phononic crystal(PC) plates which is employed
for designing flat elastic lenses. We propose a thickness contrast-based plate
design to achieve refractive index equal to -1 at the interface of the PC and
host plate. The thickness contrast between the PC and host plate enables
matching their wave numbers in the all angle negative refraction (AANR)
frequency regime. The PC-lens design is then numerically and experimentally
verified to achieve the image of an omnidirectional subwavelength excitation
source. By changing the thickness contrast between the plate and PC, the
PC-lens can be tuned for wave focusing and subwavelength imaging at a desired
frequency.
