What is a Photon Detector?

Photon or Quantum detectors generate a single response element for a single photon in response to the incoming photon flux. They respond in a quantized manner. Insufficiencies of response need to be accounted for, as it is hard to find perfect conditions in nature.

The single response elements can assume different forms, such as a photomultiplier tube; a photoelectron in a detector consisting of photoemissive sensors; an electron-hole pair separating in a junction photodiode, which is the principle of operation of silicon detectors; or an electron that is elevated from valance band to conduction band in photoconductors. This causes a change in voltage level or current flow, which can then be processed by electronics, such as amplifiers, into a recording or display.

Difference Between Thermal and Photon Detectors

Thermal detectors are responsive to heat, meaning that the level of response will be the same for 200nm UV photons delivering 1W, and 10µm infrared photons delivering 1W. Conversely, photon detectors, at most, generate only one response element per incident photon before any amplification.

The energy carried by each photon varies inversely with its wavelength. Hence, the photon flux in the UV is much lower than in the IR for the same radiant power. Consequently, the responsivity of the photon detector, in terms of V/W or A/W, is much lower in the UV region than in the IR region of the spectrum.

Detector Performance

Besides the responsivity curves, the quantum efficiency (QE) curves are equally important to be analyzed for evaluating the absolute performance a detector. The QE curve demonstrates the efficiency of the detector, providing the data about the amount of incident photon flux that is being transformed into electrical signals. Figure 1 illustrates a relative responsivity curve for a 100% efficient photon detector.

Relative spectral responsivities of perfect detectors

Figure 1. Relative spectral responsivities of perfect detectors

Figure 2 depicts a typical responsivity curve of a silicon detector and the relative QE curve. These curves are used to assess the inefficiencies in transforming the incoming photon flux into an electrical signal. An ideal silicon detector exhibits zero responsivity and quantum efficiency for photons, with energies below the band gap energy, or wavelength greater than roughly 1.1µm. Just below the long wavelength limit, the responsivity and QE must increase to roughly 0.9 A/W responsivity and 100% efficiency, relative to each of the incoming photons being transformed into a single electron worth of charge.

Silicon responsivity and quantum efficiency

Figure 2. Silicon responsivity and quantum efficiency

A 100% quantum efficiency with a responsivity behavior as shown in Figure 1 is expected for all the shorter wavelength photons. As can be seen in Figure 2, the onset of the responsivity is not as sudden as anticipated. Achieving 100% quantum efficiency is not possible, due to efficiency loss at short wavelengths.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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