Mid-infrared difference-frequency generation in silicon waveguides strained by silicon nitride

To date, the development of coherent ultra-broadband (i.e., with a bandwidth exceeding a full octave of the spectrum) laser sources and associated frequency combs has spanned the spectral regions from the near-infrared (NIR) to the ultra-violet (UV). For extending the generation of coherent supercontinuum (SC) sources and frequency combs into the mid-infrared (MIR), the natural candidate is silicon. SC generation based on third order effects in silicon nanowires has been reported at NIR wavelengths [1], however the strong nonlinear two-photon absorption (TPA) and free-carrier absorption (FCA) limit the achievable spectral broadening to below half of an octave in this spectral range. Fortunately such limitation is absent for photon energies below the silicon half band-gap, that is for wavelengths > 2200 nm, as TPA and the associated FCA vanish in the MIR. Additionally, the symmetry-breaking occurring in silicon when strained by a silicon nitride overlayer introduces a significant bulk second-order nonlinear response, hence enabling the possibility of pure electro-optical phase modulation in silicon waveguides via the Pockels effect [2]. Recently, the presence of a significant χ
⁽²⁾
in silicon nitride strained silicon waveguides has been directly proved by second-harmonic generation (SHG) experiments [3].