We present the first experimental demonstration of focusing ultrasound wavesthrough a flat acoustic metamaterial lens composed of a planar network ofsubwavelength Helmholtz resonators. We observed a tight focus ofhalf-wavelength in width at 60.5 KHz by imaging a point source. This result isin excellent agreement with the numerical simulation by transmission line modelin which we derived the effective mass density and compressibility. Thismetamaterial lens also displays variable focal length at different frequencies.Our experiment shows the promise of designing compact and light-weightultrasound imaging elements.
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