transmission volume phase holographic gratings were fabricated in bulk photo-thermo- refractive glass using zero-order femtosecond laser bessel beams and subsequent thermal treatment. microstructures composed of nano-sized crystals were observed in the exposed regions. the concentration of nano-crystals depended on the writing power, but the size of the nano-crystals was found to be quasi power-independent. low writing power led to sparse nano-crystals, while optimized writing power achieved dense nano-crystals distribution and high refractive index change. effects of the gratings thickness, writing laser power and thermal treatment on the diffraction efficiency were investigated. a maximum diffraction efficiency of 94.73% was achieved (at 532 nm testing wavelength) with 1 mm grating thickness at period of 5 mu m. (c) 2016 optical society of america