  Janus monolayers have long been captivated as a popular notion for breaking
in-plane and out-of-plane structural symmetry. Originated from chemistry and
materials science, the concept of Janus functions have been recently extended
to ultrathin metasurfaces by arranging meta-atoms asymmetrically with respect
to the propagation or polarization direction of the incident light. However,
such metasurfaces are intrinsically static and the information they carry can
be straightforwardly decrypted by scanning the incident light directions and
polarization states once the devices are fabricated. In this Letter, we present
a dynamic Janus metasurface scheme in the visible spectral region. In each
super unit cell, three plasmonic pixels are categorized into two sets. One set
contains a magnesium nanorod and a gold nanorod that are orthogonally oriented
with respect to each other, working as counter pixels. The other set only
contains a magnesium nanorod. The effective pixels on the Janus metasurface can
be reversibly regulated by hydrogenation/dehydrogenation of the magnesium
nanorods. Such dynamic controllability at visible frequencies allows for flat
optical elements with novel functionalities including beam steering, bifocal
lensing, holographic encryption, and dual optical function switching.
