The development of metamaterials and metasurfaces with reconfigurable optical responses has become a visionary frontier, which will lead to unique functionalities or devices with unprecedented performance for applications such as sensing, imaging, and optical signal processing. Among the various tuning techniques, electrical tuning based on graphene has substantial technological potential in terms of response time, broadband operation, and compatibility with silicon technology and large scale fabrication, because graphene has high electrical and thermal conductivity, broadband widely-tunable electro-optical properties and good chemical resistance. Here we have demonstrated electrically tunable metamaterial absorbers by incorporating a tunable metasurface into an asymmetric Fabry-Perot resonator which has a total thickness less than λ0/10.
The device is based on a Fabry-Perot (FP) resonator with two mirrors, i.e. a tunable metasurface reflector as a front partially-reflecting mirror and a metallic back fully-reflecting mirror. The metamaterial absorber is fabricated on a silicon substrate, with an aluminum layer as the back reflector, an aluminum oxide (AlOx) as the insulator and a metasurface on graphene (grown via chemical vapor deposition) with a broad wavelength tuning range. As the gate voltage applied on the graphene sheet changes, the metamaterial absorber can be switched in and out of critical coupling condition. Optical modulators based on such metamaterial absorbers exhibit a maximum modulation depth of more than 95% and bandwidths (modulation depth > 50%) of up to 2 µm (5 µm to 7 µm). The highest modulation speed that can be achieved in our experiment is estimated to be >1 GHz, which can be further increased by shrinking the size of the device. Our strategy provides ultra-compact solutions for light modulation over broad wavelength ranges from the near infrared to the far infrared and even Terahertz.