Step 1. Charge photogeneration in the spatial regions where constructive interference occurs. Equal numbers of positively and negatively charged carriers are produced, since the electric neutrality of a material as a whole must be maintained.

Step 2. Transport of the photogenerated charges, with one type of the carriers more mobile than the other, in the static, externally applied, electric field. The holes are shown to be more mobile, which is the more common case for organics. Note that the condition of having one type of charge carriers more mobile than the other is necessary, since charge separation is essential for the space-charge field formation. If both carriers were equally mobile, they would recombine resulting in zero space-charge field and hence no PR effect.

Step 3. Charge trapping occurs at trapping sites, which are local regions of the material where the mobile charge can be trapped, i.e., prevented from participating in transport for some period of time.

Step 4. Change in the optical index of refraction of the material in response to the local electric field. A spatial modulation of the index of refraction results from the sinusoidally varying space-charge field.

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