https://www.osti.gov/pages/biblio/1730956-direct-laser-writing-volumetric-gradient-index-lenses-waveguides
Direct laser writing of volumetric gradient index lenses and waveguides (Journal Article) | OSTI.GOV
The U.S. Department of Energy's Office of Scientific and Technical Information
direct laser writing
https://3dprint.com/tag/femtosecond-two-photon-direct-laser-writing/
femtosecond two-photon direct laser writing Archives - 3DPrint.com | Additive Manufacturing Business
direct laser writing