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We have demonstrated the creation of polarization-dependent nanogratings with a period of about 250 nm on the surface of sapphire by scanning the femtosecond laser beam with appropriate irradiation conditions. Laser fluence range for nanograting self-formation was very narrow in the slow scan mode. The grating depth variation was observed with atomic force microscope image analysis. To see the composition distribution in the nanostructure, we carried out the Auger signal analysis. In addition, we found a new periodic structure with a period of a few μm and discussed the complex formation mechanism.

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We have fabricated broadband long-period gratings (LPGs) on hollow optical fibers (HOFs) by 15 corrugating slots with various widths by point-by-point exposure of a Ti:sapphire femotosecond laser. The corrugated LPGs showed unique broadband rejection whose FWHM extended to 190 nm with a low insertion loss of less than 1 dB. The maximum coupling strength was 8.5 dB, which is a significant improvement in comparison with previous HOF acousto-optic tunable filters (AOTFs). The center wavelength and coupling strength of the resonant peak could be systematically controlled by modulating the width of the corrugation.