Focusing Ultrasound with an Acoustic Metamaterial Network
This material research
was stimulated by the opportunity to develop an artificial media with
negative refractive index and the application in superlens which allows
super-resolution imaging. High-resolution acoustic imaging techniques
are the essential tools for nondestructive testing and medical
However, the spatial resolution of the conventional acoustic
imaging methods is restricted by the incident wavelength of ultrasound.
This is due to the quickly fading evanescent fields which carry the
subwavelength features of objects.
By focusing the propagating wave and recovering the
evanescent field, a flat lens with negative-index can potentially
overcome the diffraction limit. We present the first experimental
demonstration of focusing ultrasound waves through a flat acoustic
metamaterial lens composed of a planar network of subwavelength
Helmholtz resonators. We observed a tight focus of half-wavelength in
width at 60.5 KHz by imaging a point source. This result is in excellent
agreement with the numerical simulation by transmission line model in
which we derived the effective mass density and compressibility.
This metamaterial lens also displays variable focal length at
different frequencies. Our experiment shows the promise of designing
compact and light-weight ultrasound imaging elements.
Figure 1. left, Schematic
showing the experimental setup; right, photo of real sample.
Pseudo colormap of the normalized
pressure field distribution at 60.5 KHz. (a) Measured and (b) simulated
field map of the acoustic NI metamaterial and (c) Line plot of pressure
field cross the focal plane parallel to interface.