Quadratic-soliton-enhanced mid-IR molecular sensing

Optical solitons have long been of interest both from a fundamental perspective and because of their application potential. Both cubic (Kerr) and quadratic nonlinearities can lead to soliton formation, but quadratic solitons can practically benefit from stronger nonlinearity and achieve substantial wavelength conversion. However, despite their rich physics, quadratic cavity solitons have been used only for broadband frequency comb generation, especially in the mid-IR. Here, we show that the formation dynamics of mid-IR quadratic cavity solitons can be effectively leveraged to enhance molecular sensing. We demonstrate significant sensitivity enhancement while circumventing constraints of traditional cavity enhancement mechanisms. We perform experiments sensing CO$_2$ using quadratic cavity solitons around 4 $\mu$m and achieve an equivalent path length enhancement of 6000. Additionally, we demonstrate large sensitivity at high concentrations of CO$_2$, beyond what can be achieved using an equivalent high-finesse linear cavity by orders of magnitude. Our results highlight a path for utilizing quadratic cavity nonlinear dynamics and solitons for molecular sensing beyond what can be achieved using linear methods.