Laser-written nanograting deep inside silicon*

Functional optical elements fabricated on the surface of silicon (Si) constitute fundamental building blocks of Si-photonics. For fabrication of these elements, generally conventional lithography and etching techniques are used. We recently demonstrated a laser-writing method that enables direct creation of buried or “in-chip” optical elements inside the wafer [1]. This direct laser-writing method had enabled 3D optical elements at 1-µm resolution [1]. Here, we expand the technique for demonstrating the first in-chip nanogratings crated in Si, realized without damaging the wafer surface. In order to achieve this, we exploit the ‘non-diffracting’ nature of Bessel beams, and infrared laser pulses of ~5 ns, 1.55 µm where Si is transparent. The crystal structure is modified in the form of periodic rod-like structures with structure-widths 250 nm to 1-micrometer, resulting in grating efficiencies up to 45%. These in-chip optical elements constitute the first examples of a larger class of emerging in-chip nano-photonic components, that can potentially lead to novel infrared elements at the nanoscale.

[1] Tokel et al, “In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon,” Nature Photonics, 11, 639, 2017.