  The mid-infrared (mid-IR) is a strategically important band for numerous
applications ranging from night vision to biochemical sensing. Unlike visible
or near-infrared optical parts which are commonplace and economically available
off-the-shelf, mid-IR optics often requires exotic materials or complicated
processing, which accounts for their high cost and inferior quality compared to
their visible or near-infrared counterparts. Here we theoretically analyzed and
experimentally realized a Huygens metasurface platform capable of fulfilling a
diverse cross-section of optical functions in the mid-IR. The meta-optical
elements were constructed using high-index chalcogenide films deposited on
fluoride substrates:the choices of wide-band transparent materials allow the
design to be scaled across a broad infrared spectrum. Capitalizing on a novel
two-component Huygens' meta-atom design, the meta-optical devices feature an
ultra-thin profile ($\lambda_0/8$ in thickness, where $\lambda_0$ is the
free-space wavelength) and measured optical efficiencies up to 75% in
transmissive mode, both of which represent major improvements over
state-of-the-art. We have also demonstrated, for the first time, mid-IR
transmissive meta-lenses with diffraction-limited focusing and imaging
performance. The projected size, weight and power advantages, coupled with the
manufacturing scalability leveraging standard microfabrication technologies,
make the Huygens meta-optical devices promising for next-generation mid-IR
system applications.
