  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 due to 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 - 6 um 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 10^6 at wavelengths from 3.5 to 3.8 um. 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.
