Room-temperature quantification of $^{14}$CO$_{2}$ below the natural abundance with two-color cavity ring-down spectroscopy

Radiocarbon's natural production, radiative decay, and isotopic rarity make it a unique tool to probe carbonaceous systems in the life and earth sciences. However, the difficulty of current radiocarbon ($^{14}$C) detection methods limits scientific adoption. Here, two-color cavity ring-down spectroscopy detects $^{14}$CO$_{2}$ in room-temperature samples with an accuracy of one-tenth the natural abundance in 3 minutes. The intra-cavity pump-probe measurement uses two cavity-enhanced lasers to cancel out cavity ring-down rate fluctuations and strong one-photon absorption interference (>10,000 1/s) from hot-band transitions of CO$_{2}$ isotopologues. Selective, room-temperature detection of small $^{14}$CO$_{2}$ absorption signals (<1 1/s) reduces the technical and operational burdens for cavity-enhanced measurements of radiocarbon, which can benefit a wide range of applications like biomedical research and field-detection of combusted fossil fuels.