Focusing of ultrasonic waves by negative refraction in phononic crystals

Negative refraction and focusing phenomena in phononic crystals is reviewed, starting with their initial discovery over 10 years ago in flat three-dimensional (3D) phononic crystals. This work soon led to direct observations of negative refraction in 2D phononic crystals, and an extensive series of experiments, simulations and theoretical predictions to explore and optimize focusing by flat phononic crystal lenses. More recently, the emphasis has been on demonstrating how super-resolution focusing that beats the diffraction limit can be achieved. Ultrasonic experiments, in combination with theory and simulations, have played an important role in developing a detailed understanding of these phenomena.