One example includes an optical device system. The system includes a waveguide that includes a fixed waveguide portion to propagate an optical signal, a semiconductor membrane layer, and a tunable air gap that separates the fixed waveguide portion and the semiconductor membrane layer. The system also includes an optical tuning system to move the semiconductor membrane layer with respect to the fixed waveguide portion in response to a control signal to control a separation distance of the tunable air gap to tune a characteristic of the optical signal.