Intrinsic and free carrier absorption and effects of doping and temperature What is the difference between intrinsic and free carrier absorption coefficient?
For example in Optical properties of GaAs - Galium Arsenide are some graphs of intrinsic and free carrier absorption of GaAs dependent on energy, wavelength...
What is the reason why temperature and doping affects absorption in way it does?
The higher the temperature is the higher is the absorption.
The higher is the n-type impurity doping the lower is the absorption for photons with higher energy.
 A: The intrinsic absorption coefficient is about the interaction of the photons with the valence band electrons and is more commonly known as the single or multi photon absorption coefficient, depending on the material. The free carrier absorption coefficient describes absorption of photons by electrons.
The single and multiphoton absorption is mostly applied to semiconductors and dielectrics where the amount of free electrons at the process temperature is too low to dominate absorption. In metals on the other hand, the amount of free electrons in the conduction band is so large that free electron absorption dominates all other absorption processes. When semiconductors or dielectrics are processed with powerful pulsed lasers, both processes will actually become relevant as the single and/or multiphoton ionization will provide a large amount of free electrons such that avalanche ionization will become relevant.
Temperature affects the absorption in 2 ways. In semiconductors, the amount of thermally excited electrons increases which subsequently leads to an increase in free electron absorption while at the same time reducing the bandgap. In diëlectrics, the amount of thermally excited electrons will still be to small to cause any significant increase unless the temperature is increased several hundred degrees.
Doping will create extra energy bands between the conduction and valance band. These bands will allow electrons to be transferred via an 2 step process. In some semiconductors this may mean that instead of 2 photon excitation, 2x single photon excitation is required. This can have a significant impact on the ionization rate depending on the wavelength and material bandgap. 
