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Free carrier absorption occurs when a material absorbs a photon, and a carrier (electron or hole) is excited from an already-excited state to another, unoccupied state in the same band (but possibly a different subband). This is different from interband absorption because the excited carrier is already in an excited band, such as an electron in the conduction band or a hole in the valence band, where it is free to move. In ''interband'' absorption, the carrier starts in a fixed, nonconducting band and is excited to a conducting one. It is well known that the optical transition of electrons and holes in the solid state is a useful clue to understand the physical properties of the material. However, the dynamics of the carriers are affected by other carriers and not only by the periodic lattice potential. Moreover, the thermal fluctuation of each electron should be taken into account. Therefore a statistical approach is needed. To predict the optical transition with appropriate precision, one chooses an approximation, called the assumption of quasi-thermal distributions, of the electrons in the conduction band and of the holes in the valence band. In this case, the diagonal components of the density matrix become negligible after introducing the thermal distribution function, This is the famous Fermi-Dirac distribution for the distribution of electrons energies . Thus, summing over all possible states (l and k) yields the total number of carriers ''N''. ==The optical susceptibility== Using the above distribution function, the time evolution of the density matrix can be ignored, which greatly simplifies the analysis. The optical polarization is With this relation and after adjusting the Fourier transformation, the optical susceptibility is 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Free carrier absorption」の詳細全文を読む スポンサード リンク
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