Cathodoluminescence is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material such as a phosphor, cause the emission of photons which may have wavelengths in the visible spectrum. A familiar example is the generation of light by an electron beam scanning the phosphor-coated inner surface of the screen of a television that uses a cathode ray tube. Cathodoluminescence is the inverse of the photoelectric effect in which electron emission is induced by irradiation with photons.
Cathodoluminescence occurs because the pingement of a high energy electron beam onto a semiconductor will result in the promotion of electrons from the valence band into the conduction band, leaving behind a hole. When an electron and a hole recombine, it is possible for a photon to be emitted. The energy (color) of the photon, and the probability that a photon and not a phonon will be emitted, depends on the material, its purity, and its defect state. In this case, the "semiconductor" examined can, in fact, be almost any non-metallic material. In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way.
In geology, mineralogy and materials science and semiconductor engineering, a scanning electron microscope with specialized optical detectors, or an optical cathodoluminescence microscope, may be used to examine internal structures of semiconductors, rocks, ceramics, glass, etc. in order to get information on the composition, growth and quality of the material.
In these instruments a focused beam of electrons impinges on a sample and induces it to emit light that is collected by an optical system, such as an elliptical mirror. From there, a fiber optic will transfer the light out of the microscope where it is separated into its component wavelengths by a monochromator and is then detected with a photomultiplier tube. By scanning the microscope's beam in an X-Y pattern and measuring the light emitted with the beam at each point, a map of the optical activity of the specimen can be obtained. The primary advantages to the electron microscope based technique is the ability to resolve features down to 1 nanometer, the ability to measure an entire spectrum at each point (hyperspectral imaging) if the photomultiplier tube is replaced with a CCD camera, and the ability to perform nanosecond- to picosecond-level time-resolved measurements if the electron beam can be "chopped" into nano- or pico-second pulses. Moreover, the optical properties of an object can be correlated to structural properties observed with the electron microscope. These advanced techniques are useful for examining low-dimensional semiconductor structures, such a quantum wells or quantum dots.
Although direct bandgap semiconductors such as GaAs or GaN are most easily examined by these techniques, indirect semiconductors such as silicon also emit weak cathodoluminescence, and can be examined as well. In particular, the luminescence of dislocated silicon is different from intrinsic silicon, and can be used to map defects in integrated circuits.
Recently, cathodoluminescence performed in electron microscopes is being used to study Surface plasmon resonance in metallic Nanoparticles. Indeed, metallic nanoparticles can absorb and emit visible light because of surface Plasmons.
Although an electron microscope with a cathodoluminescence detector provides high magnification and resolution it is more complicated and expensive compared to an easy to use optical cathodoluminescence microscope which benefits from its ability to show actual visible color features directly through the eyepiece. Some systems combine both an optical and an electron microscope to take advantage of both these techniques. 
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- Attolight, a company offering commercial cathodoluminescence systems