Incoherent cathodoluminescence emission occurs when the material is excited with an electron beam, where the primary electrons decelerate and deposit energy into the material.
An important property of incoherent CL is that there is no fixed phase relation between a photon that is emitted and incoming excitation electron. This CL type is spontaneous, it’s not phase correlated with the excitation, and it is unpolarised if the medium is homogeneous. Coherent cathodoluminescence is generated whenever there is a difference in refractive index in the propagating path of the electron. If material is in a vacuum, the interface between the material and vacuum would allow the generation of coherent CL. For example, if a high energy beam is exciting a gold crystal, it polarises the material at the interface, and because of this, an oscillating charge is created. Unlike in incoherent CL, the emitted photon has a fixed phase relation with incoming electron. In addition, the emitted light is always polarised. Coherent CL is always present but can be obscured by incoherent CL as it is a low probability event. For example, it is present in materials such as direct bandgap semiconductors, phosphors, and ceramics, but it is obscured by the higher probability of incoherent CL emission. In metals and indirect bandgap semiconductors, on the other hand, incoherent CL is suppressed by non-radiative channels, because of which coherent CL emission dominates.