The well-known characteristics of photodetectors including responsivity, noise equivalent power and detectivity are relevant for devices with dimensions larger than the wavelength. In this paper, we derive physically relevant merits for two-dimensional subwavelength photodetectors, for example, based on small flakes of graphene or transition metal chalcogenides. The frequently used current/voltage responsivity of a subwavelength detector with power normalized to the device area can be achieved by fabricating multiple identical detectors and combining them into parallel or sequential arrays. Noise equivalent power with power normalized to the device area is not achieved with any combination of detectors. Instead, with an optimal array size, a value greater by a factor of 2w/a is achieved, where w is the optical spot size and a is the size of an individual detector. The noise equivalent power of a subwavelength array decreases with area as {A^{ - 1/2}} for arrays with a large effective surface impedance, and as {A^{ - 3/2}} for arrays with a small effective surface impedance.
Svintsov et al. (Mon,) studied this question.