Optical absorption coefficient doping effects

Hall and Williams [96] doped thin films of lead azide with T1 and Bi. There was no marked effect on the photodecomposition efficiency at 330 nm as compared to undoped films. However, both the spectral dependence of the rate and the optical absorption were altered by thallium. The incorporation of T1 (10 mole fractions) removed the 375 nm peak from the optical absorption spectra while the incorporation of Bi left the peak unaltered. Partial decomposition of films (0.1%) also removed the 375 nm peak (dotted curve. Figure 32). The results are consistent with the fact that the Tl " impurities require anion vacancies for charge compensation. This is equivalent to partial decomposition. They concluded that the peaks in the optical absorption curve and spectral photodecomposition curves are probably a result of charge-transfer excitons. Furthermore, peak separations may arise because of differences in the interaction energies of inequivalent lead and azide ions in the unit cell. The selective removal with decomposition of the 375 nm peak may indicate selective decomposition of the azide site having the highest valence band energy. The selective decomposition would reduce the density of states and thus the extinction coefficient for electronic transitions from that particular azide band. 

An interesting and important point is that, at low optical power, the orientational effect actually is the dominating one, even in such highly absorptive material. In other recent studies " of dye-doped nematic liquid crystals, observed nonlinearities have approached the so-called supranonlinear scale characterized by refractive index coefficients n2 1 cm /W. The observed nonlinearity is orders of magnitude larger than the so-called giant optical nonlinearity of pure liquid crystals. In particular, methyl-red dye-doped nematic liquid crystals were first observed to have a nonlinear index... 

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