Superfocusing of a spherical wave: Theory and experiment

The possibility of focusing a spherical wave within a region considerably smaller than the wavelength is studied theoretically and experimentally. The coefficient of reflection of a spherical wave from a small rigid or soft sphere is-1. Consequently, the total field near such scatterers remains finite as the radius of the sphere tends to zero. The power of the acoustic field concentrates in this case in a region whose radius is proportional to the acoustic wavelength. The field can be enhanced by choosing an appropriate reflection coefficient. For example, if the reflection coefficient is made equal to 0, the sound pressure will be created by the converging wave alone. As the observation point approaches the center, this field increases without limit as 1/r, where r is the distance from the center. Structures that provide the absorption of the converging wave are analyzed. The experimental setup is described, and the experimental results demonstrating a strong absorption (the reflection coefficient does not exceed 0.2) of the converging wave are presented. The main result of the study is that it experimentally corroborates the possibility of focusing the wave in a region whose dimensions are much smaller than the wavelength.