Fabrication of 3D interconnected network of micro-channels inside silica by femtosecond irradiation and etching

We present, to the best of our knowledge, first demonstration of a direct three-dimensional (3D) microfabrication in the volume of silica glass. The microfabrication was carried out in two steps: i) recording 3-D patterns inside silica glass via silica damaging by focused femtosecond laser pulses (in multishot regime), and sample translation along X, Y, and Z directions, ii) etching the recorded patterns in HF based etchants. Comparative study of chemical etch rates in diluted HF, buffered HF, and a mixture of HF, H₂O and HNO₃ (P etch) reveals direct evidence of structural and/or stoichiometrical difference between damaged and "fresh" silica. 3D structures consisting of submicrometer size voxels (smallest optically damaged volume element per shot) were successfully fabricated in the silica glass. The presented technique allows fabrication of 3D channels as narrow as 10 μm inside silica, with arbitrary angle of interconnection and high aspect ratio (10 μm diameter channels in a 100 μm thick silica slab). This approach allows to speed up fabrication, and the resulting 3D structures are optically transparent, which is advantageous for optical characterization (transmission, photoluminescence, Raman scattering, etc.) with spatial resolution determined by focusing optics.