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CMOS compatible high-speed electro-optical modulator

Demand for a silicon (Si) based optical modulator is becoming more pressing as optical interconnects are starting to be considered seriously as replacement for conventional copper wires in electronic chips. Difficulties in realizing this device in Si are well known as are the stringent requirements on its performance in terms of size (~μm), power (~μW-mW), speed (>1 GHz) and operating voltage (<5 V). Here we present a detailed numerical design and analysis of a compact, high-speed silicon-on-insulator (SOI) waveguide electro-optical modulator. The device operates by tuning the reflection resonance of a microring resonator by means of field-effect generated free carriers in metal-oxide-semiconductor accumulation layers. Electrical and optical analyses are carried out by solution of Poisson's, charge continuity, and Maxwell's equations by finite-element method. Our simulations predict a ~0.5 nm shift in the spectral response of the resonator around 1550 nm. With an appropriate pre-biasing, this leads to ~80% modulation depth switching with voltage swing of 2 V. Field-effect induced generation of free-carriers allows for operating bandwidth >5 GHz while consuming a total dynamic power of < 500 μW. Use of the field effect results in extremely thin charge layers of very high carrier concentration. We show that an appropriate placement of these layers in the modal field of strong-confinement SOI waveguides greatly enhances the charge-field interaction. This enables significant improvements in size and modulation depth and allows the device to operate at CMOS compatible power and voltage levels. Present work adds to the design space explored in the previous works and aims to advance the field-effect based micro-resonator modulator as an active photonic device to be used in future generations of opto-electronic circuits.