Relaxation polarization dependence of circular vector gratings in azobenzene glassy molecular films

Recording and relaxation of holographic gratings is studied in different azobenzene molecular glassy films by circularly orthogonally polarized 532 nm laser beams L and R. The readout was made by circularly polarized (R or L) 632.8 nm laser beam. Sandwich-type samples (glass-film-glass) were also studied. Maximum diffraction efficiency of 81% was achieved in sandwich-type AR-173 film. The following relaxation features were found: after reaching diffraction efficiency (DE) maximum no DE decay took place; DE read out by R-polarized beam was always higher than that by L-polarized beam; in sandwich-type samples DE decayed until zero if read out by R-polarization but DE was zero if read out by L-polarization. 50% relaxation times varied from 4 min to 44 min, and they mainly decreased if grating period was increased. The observed relaxation peculiarities can be understood if one assumes that volume birefringence grating (VBG) is recorded followed by volume density grating (VDG) and surface relief grating (SRG) recording. R-polarization "feels" all gratings, whereas L-polarization "feels" only VDG and SRG. At large exposures VDG and SRG dominate. These results confirm the conclusion made by J.Mikelsone in her 2018 PhD thesis that birefringence grating recording in azobenzene materials is a neccessary condition for SRG appearance.