  Ultra-compact, low-loss, fast, and reconfigurable optical components,
enabling manipulation of light by light, could open numerous opportunities for
controlling light on the nanoscale. Nanostructured all-dielectric metasurfaces
have been shown to enable extensive control of amplitude and phase of light in
the linear optical regime. Among other functionalities, they offer unique
opportunities for shaping the wave front of light to introduce the orbital
angular momentum (OAM) to a beam. Such structured light beams bring a new
degree of freedom for applications ranging from spectroscopy and
micromanipulation to classical and quantum optical communications. To date,
reconfigurability or tuning of the optical properties of all-dielectric
metasurfaces have been achieved mechanically, thermally, electrically or
optically, using phase-change or nonlinear optical materials. However, a
majority of demonstrated tuning approaches are either slow or require high
optical powers. Arsenic trisulfide (As$_2$S$_3$) chalcogenide glass offering
ultra-fast and large $\chi^{(3)}$ nonlinearity as well as a low two-photon
absorption coefficient in the near and mid-wave infrared spectral range, could
provide a new platform for the realization of fast and relatively low-intensity
reconfigurable metasurfaces. Here, we design and experimentally demonstrate an
As$_2$S$_3$ chalcogenide glass based metasurface that enables reshaping of a
conventional Hermite-Gaussian beam with no OAM into an OAM beam at
low-intensity levels, while preserves the original beam's amplitude and phase
characteristics at high-intensity levels. The proposed metasurface could find
applications for a new generation of optical communication systems and optical
signal processing.
