We realized the erasable holographic storage in visible region for the thin layers of PMMA matrices doped with N-salicylideneaniline material.An argon laser beam of 488.0nm and a He-Ne laser beam of 632.8nm were used for writing and reading

respectively.With the reading light of 0.5mW

we studied the relationship between the intensities of the two writing beams and the intensity of the diffraction signal.The results and explanations were as following.For a given intensity of the writing beams

the intensity of the diffraction signal for the writing process increased at first

then decreased with the increase of time.A possible explanation is that pressure-driven mass transport induced from photo-isomerization which causes a change on the surface to form a surface-relief grating.However the surface-relief grating partly counteracts the phase-grating resulting from the two gatings with phase difference of π.We also found that the intensity of the diffraction signal depends on the intensities of the two writing beams.Higher intensity of the diffraction signal was observed with higher power of the writing beams.It is believed that the writing beams with higher intenisties cycle more SA molecules through a keto-enol-keto cycles.So the phase grating formed by the oriented molecules through photo-isomerization is stronger than that formed with the writing beams with lower intensities.Because the intensities of the writing beams have a more important influence on the phase grating than on the surface-relief grating.So the resultant grating formed by the counteraction of the surface-relief grating and the phase grating strengthened

we could observe the diffraction signal with higher intensity.To confirm the surface-relief grating

under same writing condition

we carried out the erasing process and investigated the relationship between the intensity of the erasing light and the erasing rate.For a given power of erasing light

we also observed that the intensity of the diffraction signal increased firstly

then decreased with the increase of time.It is considered that one erasing beam would homogeneously distributed on the recording spot

without the gradient of the light intensity.So there would no isomerization pressure and the mass transport in the recording region would stop

which made the surface-relief grating kept and restrained.At the same time

because there exists a dynamic equilibrium of photo-isomerization

the phase grating would reach the equilibrium state firstly

then would grow weak because of the change of the refractive indexes(Δ

n

)between the regional bright and dark region become small with the erasing light.So the resultant grating formed by the two grating would strengthen at first

then would weak.So we could observed the phenomenon.Higher erasing rate was observed with higher power of the erasing beams

which can be contributed to the more quick decrease of Δ

n

with higher intensity of the erasing light.In summary

we believe that the mechanism of an isomerization-driven pressure led to the formation of the surface-relief grating and these phenomena are due to the change of the resultant grating formed through the counteraction between the surface-relief grating and the phase grating.