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Fabrication of Gallium Nitride Grating by Interferometric Irradiation Using Focused Femtosecond Laser

We report the optical modification of gallium nitride by femtosecond laser irradiation. A 5-nm-band-gap shift, observed by spectral ellipsometry, results a reduction of 0.01 in the refractive index at a wavelength of 400 nm. An indistinct circular pattern was also observed using nano-beam diffraction. These results suggest that femtosecond-laser irradiation induces stress–strain, rather than slight amorphization. We also observed that the angle of first-order diffraction from the grating with a 1.0 µm period, fabricated by interferometric irradiation using a femtosecond laser without any damage, corresponds to the calculated angle. The first-order diffraction efficiency for a wavelength of 403 nm was estimated to be 4.0×10-3%.