An apparatus comprises a transparent substrate (3), at least one sensor (5) for the detection of electromagnetic radiation (31), and for each sensor a corresponding mirror having a reflective surface (11). The reflective surface (11) is shaped so that electro-magnetic radiation (31) incident on the transparent substrate (3) at a specific angle, passing through the transparent substrate (3) and being reflected by the reflective surface (11) is directed towards the sensor (5). The sensor (5) comprises a two dimensional material like graphene and may be a quantum dot functionalised graphene field effect transistor. The present invention enables the incident electromagnetic radiation (31) to be focussed onto the at least one sensor (5) without the use of additional optical components like lenses or microlenses. This may enable focussed images to be obtained by the apparatus.