Can an “impulse response” really be defined for a photoreceiver?

• We critically review the fundamental concept of “impulse response” of a photodetector.
• We unveil the problems of the Shockley–Ramo formalism as is commonly understood.
• The true nature and validity of several types of impulse response are elucidated.
• The flaws of the accepted model of single-photon detection are analyzed.

In this paper we examine the validity of the concept of impulse response employed to characterize the time response and the signal-to-noise ratio of p-i-n and similar photodetecting devices. We analyze critically the way in which the formalism of analog linear systems has been extrapolated, by employing results from macroscopic electromagnetic theory such as the Shockley–Ramo theorem or any equivalent approach, to the extreme case of a single-photon detection. We argue that the concept of “response to an optical impulse” is ill-defined in the customary terms it is envisioned in the literature, this is, as an output current pulse having a certain predictable, calculated temporal shape, in response to the detection of an optical “Dirac delta” impulse, conceived in turn as the absorption of a single photon.