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Phase modulation is one of the key functionalities in an integrated photonics circuit. In the silicon photonics platform, several approaches have been undertaken. The thermooptic effect is slow and relatively power hungry, whereas a carrier-based approach is fast ( >; Gb/s) but lossy and weak. Integration of other (e.g., electrooptic) materials typically struggles with fabrication issues that limit the effect. Here, we present a nanoelectromechanical systems-based approach. By applying a voltage over a freestanding slot waveguide, the slot width will change, resulting in an effective index change and thus a phase change. Using a cascaded structure, the effect can be enlarged without reducing the speed. A phase change of 40° was observed for a voltage of 13 V over a cascade of three 5.8-μm-long freestanding slots. The expected speed is in the MHz range.