Two-color cavity ringdown spectroscopy: Optical detection of room-temperature $^{14}$CO$_2$ samples with accuracy at parts-per-quadrillion concentration levels

Detection sensitivity is one of the most important attributes to consider during selection of spectroscopic techniques. However, high sensitivity alone is insufficient for spectroscopic measurements in spectrally congested regions. Two-color cavity ringdown spectroscopy (2C-CRDS), based on intra-cavity pump-probe detection, simultaneously achieves high detection $sensitivity$ and $selectivity$. Thanks to its built-in compensation for instrumental baseline fluctuations and one-photon absorption signals, 2C-CRDS enables mid-IR detection of radiocarbon dioxide ($^{14}$CO$_2$) in $room$-$temperature$ CO$_2$ samples, with better than 10 parts-per-quadrillion (10$^{15}$) (ppq) $sensitivity$ and $accuracy$ (4 ppq on average). These highly-reproducible measurements, which are the most sensitive and accurate in mid-IR, are accomplished in the presence of orders-of-magnitude stronger, one-photon, hot-band signals from other CO$_2$ isotopologues. Room-temperature detection of $^{14}$CO$_2$ is not possible, even at the natural abundance ($\sim$1200 ppq), by any existing one-photon-absorption techniques, because of these overwhelmingly large background signals. 2C-CRDS measurements of room-temperature $^{14}$CO$_2$ samples at the record sub-10-ppq concentration represent a major technological achievement in laser spectroscopy. A compact and low-cost 2C-CRDS sensor for $^{14}$CO$_2$ benefits a wide range of scientific fields that utilize $^{14}$C for dating and isotope tracing. The 2C-CRDS technique significantly enhances the general utility of high-resolution mid-IR detection for analytical trace measurements and fundamental chemical dynamics studies.