  Nano-fabrication in silicon, arguably the most important material for modern
technology, has been limited exclusively to its surface. Existing lithographic
methods cannot penetrate the wafer surface without altering it, whereas
emerging laser-based subsurface or in-chip fabrication remains at > 1
micrometer resolution. In addition, available methods do not allow positioning
or modulation with sub-micron precision deep inside the wafer. The fundamental
difficulty of breaking these dimensional barriers are two-fold, i.e., complex
nonlinear effects inside the wafer and the inherent diffraction limit for laser
light. Here, we overcome these challenges by using structured-laser-beams and
exploiting preformed subsurface structures as seed, in order to establish the
first controlled nano-fabrication capability inside silicon. We demonstrate
buried nano-structures of feature sizes down to 100 nm +- 25 nm, with
sub-wavelength and multi-dimensional control; thereby improving the
state-of-the-art by an order-of-magnitude. In order to showcase the photonic
capabilities, we fabricated the first in-chip nano-photonic elements, i.e.,
nano-gratings with record diffraction efficiency and spectral control. The
reported advance is an important step towards in-chip nano-photonic systems,
micro-/nano-fluidics & NEMS/MEMS, and 3D electronic-photonic integration.
