Silicon-chip mid-infrared frequency comb generation

Optical frequency combs represent a revolutionary technology for high precision spectroscopy due to their narrow linewidths and precise frequency spacing. Generation of such combs in the mid-infrared (IR) spectral region (2-20 um) is of great interest due to the presence of a large number of gas absorption lines in this wavelength regime. Recently, frequency combs have been demonstrated in the MIR in several platforms, including fiber combs, mode-locked lasers, optical parametric oscillators, and quantum cascade lasers. However, these platforms are either relatively bulky or challenging to integrate on-chip. An alternative approach using parametric mixing in microresonators is highly promising since the platform is extremely compact and can operate with relatively low powers. However, material and dispersion engineering limitations have prevented the realization of a microresonator comb source past 2.55 um. Although silicon could in principle provide a CMOS compatible platform for on-chip comb generation deep into the mid-IR, to date, silicon's linear and nonlinear losses have prevented the realization of a microresonator-based comb source. Here we overcome these limitations and realize a broadband frequency comb spanning from 2.1 um to 3.5 um and demonstrate its viability as a spectroscopic sensing platform. Such a platform is compact and robust and offers the potential to be versatile and durable for use outside the laboratory environment for applications such as real-time monitoring of atmospheric gas conditions.