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25 January 2012
A consortium of researchers in Italy has observed second-harmonic generation in a silicon waveguide. It is an important step towards making silicon devices in the mid- and far-infrared region of the spectrum, as well as extremely fast optical modulators and switches.
 The researchers say the effect is "as big as the one of the widely diffused second-order lithium niobate materials", which could lead to silicon replacing lithium niobate in some applications, advancing the cause of silicon photonic integration.
"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!" wrote the scientists in a statement. "In addition electro-optical effects which lead to low power and extremely fast optical modulator and switches can be engineered in silicon photonics."
The work was published in Nature Materials advance online publication (doi:10.1038/nmat3200).  Pauline Rigby spoke to Massimo Cazzanelli, lead author on the paper, about the development.
PR: Please explain what the difference between second- and third-order nonlinearity?
MC: At the first order the usual linear optics holds; at higher orders one enters into the realms of nonlinear effects. Depending on the intensity of the pump electrical field, one can exploit different levels of the expansion terms of the P-polarization field. Second-order nonlinear optics "operates", in principle, at lower powers than third-order nonlinear optics. The envisaged devices can work with lower consumption of energy and then lower production costs. In addition, second-order effects involve simpler conversion schemes when used to generate new frequencies.
PR: How was this work different to what had been done before?  In what sense was this a first?
FC: In fact, this work demonstrates for the first time that a bulk waveguide of deformed crystalline silicon can nonlinearly convert light beams propagating through it via a second-order nonlinearity mediated all-optical process: second harmonic generation. 
Previous works on nonlinear electro-optics are properly cited in our Nature Materials paper.
No one has ever observed second-harmonic generation in bulk silicon waveguides. In this sense it is the first.
PR: Why hadn’t it been done before?
MC: Probably due to the well-assessed idea that a centrosymmetric material cannot show bulk dipolar second-order nonlinear susceptibility. This is still true and in fact we found a way to break the internal crystalline symmetry of silicon: the deposition of a lattice-mismatched overlayer, which preserves the optical quality of the silicon waveguides themselves.
PR: Do you think that silicon devices could replace lithium niobate?
MC: A strained silicon waveguide can for sure compete with similar structures in lithium niobate. In the macroscopic case not for sure, both for obvious difficulties in inducing a nonlinear optical susceptibility in macroscopic silicon crystals and because silicon has an important third-order nonlinear susceptibility that can represent a competing nonlinear loss channel.
PR: Do you have any specific plans to commercialize the technology?  Or is more research needed, and if so what is the next step?
MC: The way to commercialize nonlinear optical silicon devices is in our mind, but still needs an important further step: to demonstrate the "time-reversed effect" of the second-harmonic generation process: the parametric down-conversion of near-infrared light.  This would allow silicon to generate mid-infrared light in a spectral region where silicon is very transparent, and where there is commercial interest: communication, security, bio-sensing, military apps, etc.
The collaboration involved researchers from the University of Trento, Bruno Kessler Foundation, University of Modena and Reggio Emilia, University of Brescia, and from the CIVEN-consortium.
By Pauline Rigby