Detectors and Sensors

Conversion of physical signals into measurable electrical responses using detectors and sensors.

Conceptual context

A detector transforms a physical signal arriving from the environment into a response that can be measured, recorded, and compared. In photonics, the process begins when radiation reaches a sensitive material. The result depends not only on how much $P_opt$ arrives, but also on $\lambda$, conversion probability, and internal noise. Two detectors illuminated by the same source may therefore produce very different signals. The central question is not merely whether they \u201csee light,\u201d but how their $R_\lambda$ varies across the spectrum and which mechanisms set the boundary between a recognizable signal and a fluctuation that cannot be distinguished from the background.

🟢 Essential level

$R_\lambda$ tells us how much $I_ph$ the detector produces for each unit of received $P_opt$ at the given $\lambda$.

It is not constant for all light because it depends on the detector material, optical absorption, and the probability that generated carriers reach the contacts. Quantum conversion efficiency summarizes the fraction of incident photons that finally contribute useful charge. High response helps transform weak optical signals into measurable currents, but real sensitivity is also limited by electrical current noise. The detector may generate the substantial signal and still perform poorly when the background fluctuates strongly. Noise equivalent power level expresses the optical threshold associated with noise, while $D*$ supports comparison between devices by accounting for active sensor size. Correct interpretation therefore combines spectral response, noise level, and measurement conditions, and no single number about the detector alone can predict whether a weak signal will actually be detectable in practice, since the surrounding electronics and integration time both shape the final outcome.

🔵 Formal level

Basic formulas

In the linear regime, $I_ph$ is obtained by multiplying $R_\lambda$ by incident $P_opt$:

Photogenerated current

This relation assumes that the detector is not saturated and that the selected $R_\lambda$ corresponds to the actual $\lambda$. The microscopic link with $\eta$ is written through:

Responsivity and quantum efficiency

The relation contains $q$, $h$, and $c$. For fixed $\eta$, $R_\lambda$ increases approximately with $\lambda$ until material absorption becomes inefficient or the spectral cutoff is reached.

Real student questions

Why does responsivity increase with wavelength if photon energy decreases? For a fixed optical power, a longer wavelength implies more photons arriving per second. If the conversion efficiency is unchanged, this larger photon flux yields more photogenerated carriers per second, hence a larger current.

What does a low NEP mean physically? It means the detector is highly sensitive, requiring very little optical power to generate a signal that equals its internal noise floor.

Why is specific detectivity preferred over NEP? Because specific detectivity normalizes the detector performance with respect to active area and measurement bandwidth, allowing a direct comparison of devices with different sizes.

Final synthesis

Spectral responsivity represents the photon-to-electron conversion capability across the spectrum. The photocurrent scales with incident power, whereas the sensitivity limits are defined by NEP and specific detectivity based on active sensor size.