A medical device for implantation on an epicardial surface of the heart. The hitting face of the club head has varying thickness, with a thickest portion being at or near the geometric center of the plate-like hitting face. The light introduced into the core layer travels substantially parallel to the axis of the parabola, and is reflected by the side surface so as to be converged at the focal point of the parabola and is then emitted from the apex portion.