We report results on a field-effect induced light modulation at ? = 1.55 ?m in a high-index-contrast waveguide based on a multisilicon-on-insulator (MSOI) platform. The device is realized with the hydrogenated amorphous silicon (?-Si:H) technology and it is suitable for monolithic integration in a CMOS integrated circuit. The device exploits the free carrier optical absorption electrically induced in the multistack core waveguide.