An aspect comprising an optical sensor is disclosed. The optical sensor comprises stacked layers comprising: a window layer configured to allow the passage of photons; a sensing layer configured to generate charges upon impinging of the photons through the window layer; and a bottom electrode layer comprising at least one bottom electrode for receiving charges generated in the sensing layer. The sensing layer is sandwiched between the window layer and the bottom electrode layer. The at least one bottom electrode of the bottom electrode layer comprises conductive material with reflectivity higher than 0.7 to reflect back received photons into the sensing layer; and the at least one bottom electrode is obtained by semiconductor device fabrication techniques.