  Hierarchical structures are abundant in nature, such as in the
superhydrophobic surfaces of lotus leaves and the structural coloration of
butterfly wings. They consist of ordered features across multiple size scales,
and their unique properties have attracted enormous interest in wide-ranging
fields, including energy storage, nanofluidics, and nanophotonics. Femtosecond
lasers, capable of inducing various material modifications, have shown promise
for manufacturing tailored hierarchical structures. However, existing methods
such as multiphoton lithography and 3D printing using nanoparticle-filled inks
typically involve polymers and suffer from high process complexity. Here, we
demonstrate 3D printing of hierarchical structures in inorganic silicon-rich
glass featuring self-forming nanogratings. This approach takes advantage of our
finding that femtosecond laser pulses can induce simultaneous multiphoton
crosslinking and self-formation of nanogratings in hydrogen silsesquioxane
(HSQ). The 3D printing process combines the 3D patterning capability of
multiphoton lithography and the efficient generation of periodic structures by
the self-formation of nanogratings. We 3D-printed micro-supercapacitors with
large surface areas and a remarkable areal capacitance of 1 mF/cm^2 at an
ultrahigh scan rate of 50 V/s, thereby demonstrating the utility of our 3D
printing approach for device applications in emerging fields such as energy
storage.
