Refractive-index-modulated holographic gratings have been recorded for the
first time in the interior of polymer bulk, by a femtosecond laser at
800 nm. The polymer bulk was prepared by doping an azo dye, HPNA in
polymethylmethacrylate. This sample preparation process is quite
simple. Multi-photon absorption and, as a result, photoinduced changes in
isomer population were postulated as a brief discussion on the mechanism of
the grating formation.