Regeneration of a Grating in PMMA Inscribed by Femtosecond Laser Bessel Beam

We demonstrate recovery of diffraction efficiency of the grating in PMMA inscribed by direct femtosecond laser writing. We investigated time-lapse measurements of the diffraction efficiency of volume gratings in PMMA after inscription with femtosecond Bessel beam. Use of the side-plane as an incident plane for diffraction measurements enables control of the volume grating thickness by the translation length of the sample. Hence optically thick volume gratings can be made. Bessel beam with an axicon lens produces a larger modification length, and is favorable for diffraction measurements. The first-order diffraction efficiency of the grating begins to increase after fabrication. Volume Bragg gratings with the thickness of 2 mm reached diffraction efficiency of approximately 90% after three days (the readout wavelength was 633 nm). Diffraction efficiency decays afterwards on the time scale of tens-of-days. Interestingly, a recovery of the grating and its diffraction efficiency was observed after 100 days. A glass-rubber relaxation under an internal strain and oxidation of laser-produced dangling bonds can explain the observation.