  We study experimentally and theoretically a double-peak fine structure of
Surface Relief Gratings in azo-functionalized poly(etherimide). For the
theoretical analysis we develop a stochastic Monte Carlo model for photoinduced
mass transport in azobenzene functionalized polymer matrix. The long
sought-after transport of polymer chains from bright to dark places of the
illumination pattern is demonstrated and characterized, various scenarios for
the intertwined processes of build-up of density and SRG gratings are examined.
Model predicts that for some azo-functionalized materials double-peak SRG
maxima can develop in the permanent, quasi-permanent or transient regimes.
