A composite nanoparticle (1) comprises an inner core (3) made of a transparent dielectric material, a first coating layer (5) made of a plasmonic material which overlays the inner core (3) and a second coating layer (7) made of a semiconductor material overlaying the first coating layer (5). Incident light is absorbed by generating surface plasmons at a boundary (4) between the inner core (3) and the first coating layer (5) and at a boundary (6) between the first coating layer (5) and the second coating layer (7) in order to increase the light absorption and thus the exciton generation by the second coating layer (7). The structure of composite particle (1) also allows for tuning of the resonance of the surface plasmons which tunes the frequency of light, or other electromagnetic radiation, that is detected. A photodetector for detecting the absorbed light comprises a channel (25) which is a layer of a two-dimensional material between a source electrode (23) and a drain electrode (24), and a layer (27) of a plurality of composite particles (1). The layer (27) acts as a photogate of the field effect transistor (21).