Nanograting Bragg responses of femtosecond laser written optical waveguides in fused silica glass

Multiple Bragg nanograting stop bands are reported for the first time in single and multi-mode optical waveguides generated by femtosecond laser direct writing in bulk fused silica glass. The stop bands observed in the guided broadband light spectra originated with the orthogonal alignment of volume nanogratings co-generated with the waveguides. Rapid shifting of stop bands across the near UV and visible spectrum was sensitively controlled by laser exposure and sample scanning direction. Bragg periods anticipated from the observed stop bands concurred with the nanograting structural pitches revealed by scanning electron microscopy. The spectroscopic characterization of nanogratings along macroscopic-scale (12.5 mm long) waveguide sections constitutes a nondestructive, convenient and sensitive approach to examine long-range order and uniformity of the self-organized periodic structures that may assist to unravel the laser-glass interaction physics of nanograting formation.