An optical device includes a first conductive layer, a first junction layer, a light absorption layer, a second junction layer, and a second conductive layer. The first junction layer is disposed on the first conductive layer. The light absorption layer is disposed on the first junction layer, wherein the light absorption layer includes a plurality of unit cells, each of the unit cells includes a plurality of pillar structures, and the pillar structures of each of the unit cells are different sizes. The second junction layer is disposed on the light absorption layer. The second conductive layer is disposed on the second junction layer.