Three-dimensional femtosecond laser nanolithography of crystals

So far, nanostructuring of hard optical crystals has been exclusively limited to their surface, as stress-induced crack formation and propagation render high-precision volume processes ineffective^1,2. Here, we show that the rate of nanopore chemical etching in the popular laser crystals yttrium aluminium garnet and sapphire can be enhanced by more than five orders of magnitude (from <0.6 nm h⁻¹ to ~100 µm h⁻¹) by the use of direct laser writing, before etching. The process makes it possible to produce arbitrary three-dimensional nanostructures with 100 nm feature sizes inside centimetre-scale laser crystals without brittle fracture. To showcase the potential of the technique we fabricate subwavelength diffraction gratings and nanostructured optical waveguides in yttrium aluminium garnet and millimetre-long nanopores in sapphire. The approach offers a pathway for transferring concepts from nanophotonics to the fields of solid-state lasers and crystal optics.