Abstract: A photoplethysmography (PPG) sensor configured to be worn on a user's body, the PPG sensor comprising: a sensing portion comprising an optical emitter configured to emit an emitted light toward the user's body, and an optical detector for detecting a detector light reflected or scattered from the tissue, such as to provide a PPG signal; and an interfacing portion comprised between the sensing portion and the user's body, the interfacing portion comprising a transparent interface element through which the emitted light is transmitted when travelling from the optical emitter toward the body, and through which the detector light is transmitted when travelling from the body to the optical detector, wherein the transparent interface element has an interface transmittance; the transparent interface element comprising an antireflective interface being configured such that an antireflective interface transmittance of the transparent interface element comprising the antireflective interface is larger than the interfac