Potentiality of Metal–Oxide–Semiconductor Silicon Optical Modulator Based on Free Carrier Absorption

Abstract

Light propagation in a metal–oxide–semiconductor optical modulator based on free carrier absorption is analyzed theoretically. The analysis is based on Marcatili's approximation taking account of absorption by free carriers in the inversion layer. The gate voltage and wavelength dependences of propagation loss and extinction ratio are also evaluated. The free carrier absorption in the proposed modulator is appropriately confirmed by comparing theoretical results with the experimental results of fabricated optical modulators on silicon-on-insulator wafers; however, practical use of the device is limited since the extinction ratio remains small owing to a weak interaction between light and inversion carriers at 1.55-µm-wavelength light. By theoretical analyses, a large extinction ratio is obtained for possible applications in the deeper infrared regime because free carrier absorption increases with wavelength. A much larger extinction ratio is expected when the interaction between guided waves and surface plasmons occurs.