Self-regeneration of volume gratings recorded inside polymethyl methacrylate (PMMA) after 70-100 days is demonstrated. First, volume gratings were made inside PMMA by femtosecond laser writing. The diffraction efficiency of the gratings reached the maximum-was regenerated-following an initial slow decrease within the first several days after the fabrication. Time-lapse measurements of the diffraction efficiency in both top-plane (as laser written) and side-plane illumination were used to monitor changes of the diffraction efficiency. The final efficiency was reaching values well in excess of the as-fabricated efficiencies, with a maximum diffraction efficiency of 90% for the side readout illumination. The regenerated volume grating is a possible candidate to achieve high diffraction efficiency in PMMA. The regeneration is consistent with the α-relaxation of the polymer structure and oxidation of the dangling bonds.
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