Metasurfaces have become a rapidly growing field of research in recent years due to their exceptional abilities in light manipulation and versatility in ultrathin optical applications. They also significantly benefit from their simplified fabrication process compared to metamaterials and are promising for integration with on-chip nanophotonic devices owing to their planar profiles. The recent progress in metasurfaces is reviewed and they are classified into six categories according to their underlying physics for realizing full 2π phase manipulation. Starting from multi-resonance and gap-plasmon metasurfaces that rely on the geometric effect of plasmonic nanoantennas, Pancharatnam–Berry-phase metasurfaces, on the other hand, use identical nanoantennas with varying rotation angles. The recent development of Huygens' metasurfaces and all-dielectric metasurfaces especially benefit from highly efficient transmission applications. An overview of state-of-the-art fabrication technologies is introduced, ranging from the commonly used processes such as electron beam and focused-ion-beam lithography to some emerging techniques, such as self-assembly and nanoimprint lithography. A variety of functional materials incorporated to reconfigurable or tunable metasurfaces is also presented. Finally, a few of the current intriguing metasurface-based applications are discussed, and opinions on future prospects are provided.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/smtd.201600064

Fundamentals and Applications of Metasurfaces

This article describes the active control of terahertz waves using hybrid metamaterials containing embedded vanadium dioxide. VO${}_{2}$ undergoes an insulator-metal phase transition at around 68 \ifmmode^\circ\else\textdegree\fi{}C, causing its conductivity to change by about 5 orders of magnitude, which yields mode switching in the metamaterial. This mode switching, which is analogous to the resonant mode switching seen in plasmonics, can be realized experimentally using thermal, electrical, or optical stimuli. Such a diverse range of stimuli should be very useful for manipulating terahertz waves in practical applications, with a particular eye toward next-generation wireless communication.

Active Control of Terahertz Waves Using Vanadium-Dioxide-Embedded Metamaterials

Memory metamaterials are artificial media that sustain transformed electromagnetic properties without persistent external stimuli. Previous memory metamaterials were realized with phase-change materials, such as vanadium dioxide or chalcogenide glasses, which exhibit memory behaviour with respect to electrically/optically induced thermal stimuli. However, they require a thermally isolated environment for longer retention or strong optical pump for phase-change. Here we demonstrate electrically programmable nonvolatile memory metadevices realised by the hybridization of graphene, a ferroelectric and meta-atoms/meta-molecules, and extend the concept further to establish reconfigurable logic-gate metadevices. For a memory metadevice having a single electrical input, amplitude, phase and even the polarization multi-states were clearly distinguishable with a retention time of over 10 years at room temperature. Furthermore, logic-gate functionalities were demonstrated with reconfigurable logic-gate metadevices having two electrical inputs, with each connected to separate ferroelectric layers that act as the multi-level controller for the doping level of the sandwiched graphene layer.

/pdf/graphene-ferroelectric-metadevices-for-nonvolatile-memory-30mak4o4q8.pdf

Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations

We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.

Voltage switching of a VO2 memory metasurface using ionic gel

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-003737.pdf

Voltage switching of a VO$_2$ memory metasurface using ionic gel

Creation and control of spatial gradients in electromagnetic properties is a central theme underlying optical device design. In this work, we demonstrate that through modification of the spatial and temporal distribution of current, we can obtain increased control over the shape of these gradients. We are able to write spatially sharp gradients with ∼50% change in the index of refraction over length scales of only a few wavelengths as observed through diffraction limited terahertz spectroscopy. Furthermore, we assess the potentials for such gradients for beam-steering applications.

D. Barnas

Papers

Two-dimensional reconfigurable gradient index memory metasurface