Second-order nonlinearity paves way to new silicon photonics devices

A consortium of researchers in Italy has observed second harmonic generation in a silicon waveguide – the first time this effect has been seen in a silicon waveguide, they say. This is an important step towards making integrated silicon devices in the mid- and far-infrared region of the spectrum, as well as extremely fast optical modulators and switches.

The consortium, led by Prof. Lorenzo Pavesi at the University of Trento in Italy, has been working on strained silicon. Under normal circumstances, silicon is a centrosymmetric material that cannot display second-order nonlinearities. However, by deforming the silicon structure with an over-layer of silicon nitride, the researchers have been able to demonstrate second-order optical nonlinearities, which they say are "as big as the one of the widely diffused second-order lithium niobate materials."

Up until now, silicon photonics relied on high-order nonlinear effects, which only occur at high optical powers. Second-order nonlinear activity has a much lower power threshold, and, more interestingly, allows silicon to be considered as a true nonlinear crystal, where direct nonlinear down-conversion or other optical parametric processes can be achieved.

"All the ingredients to get mid-infrared or far-infrared parametric optical sources are now available and the race to demonstrate such exciting possibilities is open!" the scientists wrote. "In addition electro-optical effects which lead to low power and extremely fast optical modulator and switches can be engineered in silicon photonics."

The collaboration involved researchers from University of Trento, Bruno Kessler Foundation, University of Modena & Reggio Emilia, University of Brescia, and from the CIVEN-consortium.

The work was published in Nature Materials advance online publication (doi:10.1038/nmat3200).