Deep and fast free-space electro-absorption modulation in a mobility-independent graphene-loaded Bragg resonator

Deep and fast electro-optic modulation is critical for high-speed near infrared signal processing. We combine the electro-absorption tunability of graphene with the high-Q resonance of a Bragg based Fabry-Perot resonator at lambda=1550 nm and show that 100% free-space signal modulation at small insertion loss and GHz speed can always be achieved independently of graphene quality (mobility), provided that the device operates in the reflection mode and is tuned in critical coupling with graphene. Remarkably, the critical coupling mechanism produces a higher extinction ratio for lower graphene mobility. We use practical considerations to optimize the device architecture and operation as a function of graphene mobility. With a small modification, this scheme can be turned into a very sensitive acousto-absorption modulator with an extinction ratio of 30 dB/˚A or an index sensor with a sensitivity of 10^7 %/RIU. These designs can be extended throughout the IR spectrum by appropriate material selection and scaling of layer dimensions.