Semiconductors, bulk band-filling model

For bulk semiconductors at room temperature, the mechanism for the resonant nonlinearity can be described by the band-filling model [82,87]. This is shown schematically in Figure 16b for a direct gap semiconductor such as CdS. Absorption of photons across the band gap, g, generates electrons and holes which fill up the conduction and valence band, respectively, due to the Pauli exclusion principle. If one takes a snap shot of the absorption spectrum before the electrons and holes can relax, one finds that the effective band gap, , increases (Figure 166), since transitions to the filled states are forbidden. The bleaching efficiency per photon absorbed can be derived as... 

For large semiconductor particles with bulk properties (e.g., larger than several hundred angstroms for CdS), the origin of the optical nonlinearity should be the same as that of bulk semiconductors. The band-filling model is in principle applicable [91,92] if defects in the sample can be minimized. When the cluster size is smaller than the exciton size ( 60 A for CdS), the band-filling model becomes fundamentally invalid. In this size regime, there... 

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