The sensor includes an optical waveguide defined in a light-transmitting medium. The waveguide includes a sensing portion and an non-sensing portion. The light-transmitting medium included in the sensing portion has defects that provide the light-transmitting medium with a deep band gap level between a valence band of the light-transmitting medium and a conduction band of the light-transmitting medium. The deep band gap level is configured such that the waveguide guiding light signals through the light-transmitting medium in the sensing portion causes free carriers to be generated in the light-transmitting medium. A detector is configured to detect the free carriers in the sensing region of the waveguide.