Responsivity

Responsivity is a measure of how effectively a sensor converts an input stimulus into an electrical output. In photodetection, it is typically defined as the ratio of the output electrical signal to the incident optical power, expressed as R = Iout / Popt for current output or Vout / Popt for voltage output. Spectral responsivity describes how R varies with wavelength. For a biased photodiode, the fundamental relation is R(λ) = η(λ) q / (hc/λ) = η(λ) q λ / hc, where η is the quantum efficiency, q is the elementary charge, h is Planck’s constant, c is the speed of light, and λ is the wavelength.

Responsivity depends on wavelength, device structure, and operating conditions. It can be enhanced by internal gain mechanisms, as in avalanche photodiodes, or by external amplification such as transimpedance amplifiers. Temperature, bias, and optical wavelengths influence η and thus the overall responsivity.

Related concepts include noise and detectivity. The noise-equivalent power (NEP) represents the input optical power required to produce a signal equal to the noise level in a 1 Hz output bandwidth, with NEP = sqrt(Sn)/R, where Sn is the input-referred noise spectral density. Detectivity D* combines NEP with detector area and bandwidth.

Responsivity also relates to dynamic performance. The speed of a detector, described by its bandwidth or rise/fall times, is governed by electrical and optical time constants, independent of the static gain captured by R. Applications span imaging, optical communications, spectroscopy, and sensing, with examples including photodiodes, photomultiplier tubes, and bolometers, each using a distinct definition of responsivity (current, voltage, or power-based) suited to its readout scheme.