A photodetector comprising an optical waveguide structure comprising at least three stripes spaced from one another such that a slot is present between each two adjacent stripes of the at least three stripes. A graphene absorption layer is provided over or underneath the at least three stripes. There is an electrode for each stripe, over or underneath the graphene absorption layer. The photodetector is configured such that two adjacent electrodes are biased using opposite polarities to create a p-n junction effect in a portion of the graphene absorption layer. In particular the portion of the graphene absorption layer is located over or underneath each respective slot between said each two adjacent stripes.