  Acoustic focusing plays a pivotal role in a wide variety of applications
ranging from medical science to nondestructive testing. Previous works have
shown that acoustic metagratings can overcome the inherent efficiency
limitations of gradient metasurfaces in beam steering. In this work, we propose
a new design principle for acoustic metalenses, based on metagratings, to
achieve efficient ultrasonic focusing. We achieve beam focusing by locally
controlling the excitation of a single diffraction order with the use of
adiabatically varying metagratings over the lens aperture. A set of
metagratings is optimized by a semi-analytical approach using a genetic
algorithm, enabling efficient anomalous reflection for a wide range of
reflection angles. Numerical results reveal that our metalens can effectively
focus impinging ultrasonic waves to a focal point of FWHM = 0.364{\lambda}. The
focusing performance of the metalens is demonstrated experimentally, validating
our proposed approach.
