The Relationship Between Real and Virtual Excitation Mechanisms for Nonlinear Refraction
It is well known that the nonlinear refraction that arises from real incoherent excitation in semiconductors is several orders of magnitude larger than that arising from virtual excitation. In a semiquantitative way this can be understood using a two level model. Under CW illumination in the tail of the absorption peak the model predicts1,2,3 that the refractive index change for real excitation is a factor 2T1/T2 larger than that for virtual excitation where T1 is the energy relaxation time (lifetime) and T2 is the dephasing time. Because T1 is typically in the order of nanoseconds and T2 in the order of picoseconds or femtoseconds the ratio 2T1/T2 can account, at least in part, for the relative efficacy of real and virtual excitation. The main purpose of this paper is to highlight the physical origin of the 2T1/T2 enhancement factor.
KeywordsEnhancement Factor Refractive Index Change Optical Intensity Fall Rate Occupation Probability
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