Silicon photonics in midwave and longwave infrared

Optical data communication is not the only area where silicon photonics will have an impact. Silicon and related group 4 crystals have excellent linear and nonlinear optical properties in the midwave and longwave infrared spectrum [1-6]. These properties, along with silicon’s excellent thermal conductivity and optical damage threshold, open up the possibility for a new class of midwave and longwave infrared photonic devices. As potential platforms for this new regime, a wide range of applications from gas detection, sensing to free space communications can be realized on low cost, chip-level integration. To transfer the knowledge from near infrared and apply them in this new regime, detailed understanding of the material properties is essential. For passive devices, the optical transparency of the materials used for waveguide design has to be well chosen and studied. In the midwave infrared range, the waveguides are likely to be built on silicon-on-insulator (SOI), silicon-on-sapphire (SOS), silicon-on-nitride (SON) and germanium-on-SOI (Ge/SOI), whose low-loss transmission range extends out to the wavelength of 3.7 μm for SOI, 4.4 μm for SOS, 6.7 μm for SON and 14.7 μm for Ge/SOI. The SOI waveguide has a propagation loss of less than 2 dB/cm over the 1.1 to 2.5 μm and 2.9 to 3.6 μm bands, with fairly high loss over 2.5 to 2.9 μm. Except for SOS, these longwave waveguides are untested at present. For active devices, it is known that group 4 materials lack second-order optical nonlinearity due to the centrosymmetric atomic arrangements. Thus, the lowest-order nonlinearity – third-order susceptibility χ, which gives rise to the Kerr and Raman effects, is the key. For nonlinear optical processes at longer wavelengths, there has been experimental realization for applications recently such as Raman amplification [7], wavelength conversion, optical parametric waveguide gain, and cascaded four-wave mixing for multi-line infrared sources [8-10].