It is possible change the dopant ratio in a photorefractive crystal in order to increase the non-linear response. But, whenever it happens, the transmission coefficient decreases. Hence, I doubt if the absortion increases, there will be more excited photoelectrons (the bands model) and higher will be the photorrefractive effect, or there is not any strain bettween the absortion and the excited photoelectrons.
Hi Rubén
The PR effect is triggered by photoexcitation of mobile charge carriers, which implies the photons to be absorbed in order to get some response. Therefore absorption is generally proportional to the donor concentration. Since the ratio betweeen donors and acceptors deternines the material conductivity under illumination, on one hand this determines the "speed" of the PR response, on the other it gives the maximum PR effect obtainable (in general, the higher the photoconductivity, the smaller the PR effect). In summary, absorbing materials are faster but show a small refractive index change, while transparent ones are slow and attain big changes.
If you want to have large PR changes whitout having absorption, you can try to play with other paramers, e.g. by cooling down your sample.
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