  Metasurfaces enable a new paradigm of controlling electromagnetic waves by
manipulating subwavelength artificial structures within just a fraction of
wavelength. Despite the rapid growth, simultaneously achieving
low-dimensionality, high transmission efficiency, real-time continuous
reconfigurability, and a wide variety of re-programmable functions are still
very challenging, forcing researchers to realize just one or few of the
aforementioned features in one design. In this study, we report a subwavelength
reconfigurable Huygens' metasurface realized by loading it with controllable
active elements. Our proposed design provides a unified solution to the
aforementioned challenges of real-time local reconfigurability of efficient
Huygens' metasurfaces. As one exemplary demonstration, we experimentally
realized a reconfigurable metalens at the microwave frequencies which, to our
best knowledge, demonstrates for the first time that multiple and complex focal
spots can be controlled simultaneously at distinct spatial positions and
re-programmable in any desired fashion, with fast response time and high
efficiency. The presented active Huygens' metalens may offer unprecedented
potentials for real-time, fast, and sophisticated electromagnetic wave
manipulation such as dynamic holography, focusing, beam shaping/steering,
imaging and active emission control.
