5 Photodetectors

Photodetectors convert photons into an electric current or voltage with a magnitude proportional to the number incident photons, thereby enabling quantitative measurement of light intensity for various spectroscopies.

UV-Visible-NIR Detectors

These detectors are generally sensitive to ultraviolet (UV), visible, and near-infrared (NIR) wavelengths of light, although each detector differs in wavelength sensitivity and useful range.

Photomultiplier Tube (PMT). This detector is an evacuated glass or quartz tube that contains a photocathode, anode, and multiple intermediate electrodes called dynodes. When a photon hits the photocathode, an electron is ejected via the photoelectric effect. Under the high voltage applied to the PMT, the electron is accelerated to the first dynode and produces additional electrons upon impact. These new electrons are then accelerated to the next dynode, where more electrons are produced, and so on, until all the electrons are collected at the anode. The magnitude of the electric current is proportional to the incident light intensity, with the dynodes amplifying the signal by a factor of 10⁶–10⁹. Some PMTs are able to count single photons. The useable wavelength range is ca. 200–800 nm.

Photodiode. This detector is based on a p-n junction of semiconductor materials. The choice of semiconductor material determines the wavelength range to which the photodiode is sensitive. Silicon is used for visible light (ca. 400–1000 nm) and InGaAs is common for near-infrared (ca. 0.7–1.7 µm). At the interface between the p-type and n-type semiconductors, there is a depletion of charge carriers (i.e. conduction-band electrons and valence-band holes). The absorption of light in the depletion region excites an electron from the valence band to the conduction band, resulting in charge carriers that can be measured as an electric current under applied voltage. The magnitude of the current is proportional to the light intensity. Photodiodes are much smaller, more rugged, and less expensive than PMTs, but less sensitive. Linear arrays with a few up to a thousand closely spaced photodiodes are available.

Avalanche Photodiode (APD). The operation of APDs is similar to that of conventional silicon photodiodes, except that the applied voltage is high enough to accelerate charge carriers to the point that they produce new charge carriers upon collision (i.e. impact ionization) with the semiconductor lattice. Depending on the design and intended use, the amplification factor can be 10²–10³ or 10⁵–10⁶, the latter enabling counting of single photons.

CCD and CMOS Cameras. These cameras are two-dimensional arrays of silicon photodetectors, where each detector unit is a camera pixel. The charge-coupled device (CCD) and complementary-metal-oxide-semiconductor (CMOS) designs differ in their fabrication and readout electronics, but their principles of light detection are largely analogous. Both CCD and CMOS cameras detect light through the photogeneration, accumulation, and readout of conduction band electrons in a depletion region (i.e. p‑n junction) at each pixel. The useable wavelength range is ca. 400–1000 nm. CMOS cameras are gradually displacing CCD cameras in many applications.

Cameras with InGaAs photodetectors (pixels) are also available for detection of light with wavelengths between 900–1700 nm, although not exactly CCD or CMOS in design.

EM-CCD Camera. Conceptually, electron multiplying-CCD cameras are the imaging version of an APD, taking advantage of the same impact ionization amplification mechanism when the electrons accumulated at each camera pixel are read out. These cameras are capable of single photon detection.

Infrared Detectors

MCT Detectors. A mercury cadmium telluride (MCT) detector is composed of a HgCdTe semiconductor and responds to near- and mid-infrared light. Absorbed infrared light (ca. 1–16 µm) excites electrons from the valence band to conduction band and the resulting current (under applied voltage) is proportional to the intensity of light. MCT detectors need to be cooled to ~77 K but are more sensitive than DTGS detectors. They can be manufactured as camera-like two-dimensional arrays (known as FPAs or  “focal plane arrays”).

D(La)TGS Detectors. Deuterated (lanthanum α alanine doped) triglycine sulfate is a pyroelectric material. The charge on the surface of the triglycine sulfate crystal changes as a function of temperature and is measurable as an electrical capacitance. IR light incident of a DTGS detector heats the crystal, changing the capacitance and inducing a voltage change. These detectors are operated with pulses of light to allow heating and cooling cycles for the crystal, with a useful wavelength range between ca. 1–28 µm.

Connections

Light is useful for chemical analysis and other measurements only if its intensity can be measured quantitatively.

• Photodetectors are how light is transduced (Ch. 1) into an electrical signal and partly determine the achievable analytical figures of merit.
• Detectors for visible light are commonly used for measurements of molecular and atomic absorbance (Ch. 7, Ch. 11), fluorescence (Ch. 9), atomic emission (Ch. 11), and Raman scattering (Ch. 15).
• Detectors for infrared light are commonly used for measurements of infrared absorbance (Ch. 13).
• Imaging detectors are used for imaging samples (e.g. microscopy) and for imaging the output of polychromators (Ch. 4).

Post-Reading Questions

1. Classify all of the photodetectors into two categories: visible detection or infrared detection.
2. Classify all of the photodetectors into two categories: amplified or non-amplified.
3. Classify all of the photodetectors into two categories: imaging or non-imaging.

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Summarize the operating principles of various photodetectors.
• Draw diagrams that illustrate the design and operating principles of selected photodetectors.
• Summarize the operating principles for amplifying signal and/or increasing signal-to-noise ratio for applicable photodetectors.
• List the limitations of photodetectors.
• Decide when measurements do and do not require correction for the limitations of photodetectors.
• Devise useful combinations of light sources, wavelength selectors, and detectors for a given measurement requirement.