An optical sensor includes a silicon-rich dielectric photosensitive device and a read-out device. The silicon-rich dielectric photosensitive device includes a first electrode, a second electrode, and a photosensitive silicon-rich dielectric layer disposed therebetween. The photosensitive silicon-rich dielectric layer includes a plurality of nanocrystalline silicon crystals therein. The read-out device is electrically connected to the first electrode of the silicon-rich dielectric photosensitive device for reading out opto-electronic signals transmitted from the photo-sensitive silicon-rich dielectric layer.