A photo detection device comprising a contact layer through which light enters; an absorbing region positioned such that light admitted through the contact layer passes into the absorbing region; at least one diffractive element operatively associated with the absorbing region operating to diffract light into the absorbing region; the configuration of the at least one diffractive element being determined by computer simulation to determine an optimal diffractive element (or elements) and absorbing region configuration for optimal quantum efficiency for at least one predetermined wavelength detection range, the at least one diffractive element operating to diffract light entering through the contact layer such that phases of diffracted waves from locations within the photo detection device or waves reflected by sidewalls and waves reflected by the at least one diffractive element form a constructive interference pattern inside the absorbing region. A method of designing a photodetector comprises using a computer simulation to determine an optimal configuration for at least one wavelength range occurring when waves reflected by the diffractive element form a constructive interference pattern inside the absorbing region.