Low-loss silicon platform for broadband mid-infrared photonics

Broadband mid-infrared (mid-IR) spectroscopy applications could greatly benefit from today’s well-developed, highly scalable silicon photonics technology; however, this platform lacks broadband transparency because of its reliance on absorptive silicon dioxide cladding. Alternative cladding materials have been studied, but the challenge lies in decreasing losses while avoiding complex fabrication techniques. Here, in contrast to traditional assumptions, we show that silicon photonics can achieve low-loss propagation in the mid-IR from 3 to 6 μm wavelength, thus providing a highly scalable, well-developed technology in this spectral range. We engineer the waveguide cross-section and optical mode interaction with the absorptive cladding oxide to reduce loss at mid-IR wavelengths. We fabricate a microring resonator and measure an intrinsic quality (Q) factor of 106 at wavelengths from 3.5 to 3.8 μm. This is the highest Q demonstrated on an integrated mid-IR platform to date. With this high-Q silicon microresonator, we also demonstrate a low optical parametric oscillation threshold of 5.2 mW, illustrating the utility of this platform for nonlinear chip-scale applications in the mid-IR.