Fig. 4: Sensing behaviors of quadratic cavity solitons.

a Measured output power as a function of CO₂ concentration for different number of times above threshold, N. A high sensitivity of 4.1 mW/ppm is measured near threshold, emphasized using the solid trend lines. b Simulations of the simulton response to the addition of CO₂ at various number of times above threshold exhibit good qualitative agreement with the experimental data. c Equivalent path-length enhancement calculated for neighboring points in the experiment, showing a measured enhancement as large as 6000. Solid lines show the expected asymptotic enhancement corresponding to the linear fits in (b), with dashed lines extending these fits to enable extrapolation of detector-limited enhancements for detection bandwidths of 1 MHz (x’s) and 1 Hz (stars). d Simulated change in output power as a function of CO₂ concentration with a linear fit (dashed line) showing good linearity over a dynamic range of 10⁷. e Measured sensitivity as a function of CO₂ concentration in direct comparison with linear sensing (light blue), demonstrating orders of magnitude sensitivity improvement over linear methods at high sample concentrations.