Design of Tunable Nanophotonic Devices
Conference on Lasers and Electro-Optics
This tutorial addresses design of tunable nanophotonic arrays, enabling dynamic, active control of the properties of light -amplitude, phase, wavevector, wavelength and polarization -opening new applications such as optical beam steering, focusing and wavefront engineering. Introduction Achieving versatile dynamical control of the key constitutive properties of light at the nanoscale is a grand challenge for nanophotonics. In the last several years, metasurfaces have shown extraordinary promise
... traordinary promise to achieve such comprehensive control over the characteristics of scattered light. Metasurfaces can be viewed as artificially designed arrays of subwavelength optical scatterers, where each scatterer introduces abrupt changes to the phase, amplitude or polarization of the reflected or transmitted electromagnetic waves 1-3 . Thus, metasurfaces offer the ability to control the wavefront of the scattered light, thereby creating new flat optics and ultrathin optoelectronic components 4, 5 . To date, metasurfaces have been used to demonstrate a number of low-profile optical components with important capabilities including focusing 6-9 , polarization control and detection 10-12 , holograms 13-15 , and quantum light control 16, 17 . Among the large volume of experimental reports about metasurfaces, most demonstrated so far are passive. For passive metasurfaces, the light scattering characteristics are defined by the geometry and arrangement of subwavelength scatterers, fixed at the time of fabrication. In contrast to passive metasurfaces, actively-tunable metasurfaces can realize multiple functions 18-20 , serving as low-profile nanophotonic devices capable of beam steering, active polarization switching, and formation of reconfigurable metalenses. So far, a number of different methods have been used to realize tunable metasurfaces, commonly by incorporating an active material into the metasurface structure. The dielectric permittivity of the active material is then dynamically controlled via application of an external stimulus such as an electrical bias 21, 22 , laser pulse 23 , or heat input 24 . Reconfigurable metasurfaces, which are based on incorporating active materials into otherwise passive antenna arrays, are hereafter referred to as hybrid metasurfaces. For example, incorporation of monolayer graphene into a plasmonic metasurface can enable active tuning of the spectral response by electrically tuning the Fermi level of the graphene sheet  . Electrical tuning of the coupling between metasurface resonances and intersubband transitions in multiple-quantum-wells (MQWs) has also been explored for applications such as tunable filters 28 and optical modulators at mid-infrared wavelengths 29 . To achieve active metasurface performance at visible and nearinfrared (NIR) wavelengths, the integration of metasurfaces with phase-change materials or liquid crystals has enabled the demonstration of phase modulation and active beam switching. Modulation of the dielectric permittivity near the epsilon-near-zero (ENZ) transition in doped transparent conducting materials can yield large optical modulation of the scattered light, and to date the ENZ transition in indium tin oxide and titanium nitride has been exploited to electrically tune the properties of scattered/emitted light. These hybrid metasurfaces operate by spectrally overlapping the geometrical antenna resonance and the ENZ permittivity regime, and also spatially overlapping the metasurface element mode profile with the tunable permittivity transparent conducting material. To enable a widely tunable optical response, strong local field confinement and enhancement in the active material is required. Prior research has also combined tunable metasurface optics with microelectromechanical systems (MEMS) technology to demonstrate varifocal lenses. Moreover, previous work has shown that fabricating metasurfaces on elastomeric substrates may yield adaptive metalenses, strain-multiplexed meta-holograms, and an active control of the structural color. However, in MEMS-based and mechanically stretchable substrate modulation approaches, control of the optical response is achieved by changing the distance between either adjacent metasurface elements or entire element arrays, and requires a mechanical transducer, which limits the frequency bandwidth. While interesting, these approaches are not able to yield versatile active control over the scattered light wavefront over a wide frequency range. This condition can only be realized by electronic tuning the optical response of each metasurface element. Fabricating metasurface elements directly in an active material could substantially simplify the metasurface design and facilitate the fabrication process. For example, prior research has used phase-change materials as metasurface building blocks to achieve actively tunable optical responses. The ability to rewrite metasurface patterns incorporating phase-change materials with a pump laser has enabled the demonstration of multiple functions when using a single sheet of either GST or VO2. However, the tuning speed of the phase-change-material-based tunable Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on September 22,2020 at 00:44:22 UTC from IEEE Xplore. Restrictions apply.