Semiconductors, bulk resonant nonlinearity

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... 

The interest in semiconductor QD s as NLO materials has resulted from the recent theoretical predictions of strong optical nonlinearities for materials having three dimensional quantum confinement (QC) of electrons (e) and holes (h) (2,29,20). QC whether in one, two or three dimensions increases the stability of the exciton compared to the bulk semiconductor and as a result, the exciton resonances remain well resolved at room temperature. The physics framework in which the optical nonlinearities of QD s are couched involves the third order term of the electrical susceptibility (called X )) for semiconductor nanocrystallites (these particles will be referred to as nanocrystallites because of the perfect uniformity in size and shape that distinguishes them from other clusters where these characteriestics may vary, but these crystallites are definitely of molecular size and character and a cluster description is the most appropriate) exhibiting QC in all three dimensions. (Second order nonlinearites are not considered here since they are generally small in the systems under consideration.)... 

For devices the important classes of nonlinear materials are (i) molecular and polymeric materials, (ii) multiple quantum well semi-conductors, (hi) bulk semiconductors and (iv) semiconductor doped glasses. Of these classes, organic polymeric structures have shown the largest non-resonant x - Important nonlinearities observed in inorganic semiconductor materials are in a narrow wavelength region determined by excitonic resonances. Therefore, these nonlinearities are resonant, slower in response and produce thermal effects. 

This factor is taken into account in (3.5)-(3.8) above, but it can have an even greater importance in nonlinear effects, since the second-order and third-order nonlinear optical coefficients, and respectively, are affected by factors and /, respectively, as compared with the bulk material of the nanoparticle. Hence, for large /, a nanostructured material can have a larger optical nonlinearity than its bulk constituents. For typical semiconductor-doped matrices, > and /< 1. However, particularly strong local-field enhancements are observed for metal nanoparticles in the vicinity of the plasmon resonance [3.75]. 

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