  Silicon is an excellent material for microelectronics and integrated
photonics with untapped potential for mid-IR optics. Despite broad recognition
of the importance of the third dimension, current lithography methods do not
allow fabrication of photonic devices and functional microelements directly
inside silicon chips. Even relatively simple curved geometries cannot be
realized with techniques like reactive ion etching. Embedded optical elements,
like in glass, electronic devices and better electronic-photonic integration
are lacking. Here, we demonstrate laser-based fabrication of complex 3D
structures deep inside silicon using 1 micrometer-sized dots and rod-like
structures of adjustable length as basic building blocks. The laser-modified Si
has a different optical index than unmodified parts, which enables numerous
photonic devices. Optionally, these parts are chemically etched to produce
desired 3D shapes. We exemplify a plethora of subsurface, i.e., "in-chip"
microstructures for microfluidic cooling of chips, vias, MEMS, photovoltaic
applications and photonic devices that match or surpass the corresponding
state-of-the-art device performances.

Optics