Invited Speaker: Jun JiangTitle: Two-color cavity ringdown spectroscopy*

In this talk, we introduce the two-color cavity ringdown (2C-CRD) technique. The method utilizes cavity-enhanced pump power and probe absorption pathlength, and offers generally applicable, high-sensitivity, high-selectivity optical detection. With the pump radiation switched off during every other probe ringdown, the net 2C signal from the difference between the pump-on and pump-off decay rates is immune to drifts of the CRD rates and spectral overlaps from non-target one-photon transitions.

The measurement capabilities of our 2C-CRD method are demonstrated with the first-ever room-temperature optical detection of radiocarbon dioxide (¹⁴CO₂) below its natural abundance concentration. The baseline compensation of the 2C technique allows us to simultaneously achieve high detection sensitivity (k_min~5×10^-13 cm^-1) and orders-of-magnitude reduction in spectral congestion compared to one-color detection. Two quantum cascade lasers are used to excite a pair of ν₃=1←0 and ν₃=2←1 rovibrational transitions of ¹⁴CO₂. With 2 minutes of averaging, the technique differentiates, with an accuracy of 8 parts-per-quadrillion (10¹⁵) (equivalent to 0.7% of the ¹⁴CO₂ natural abundance), six combusted CO₂ samples with ¹⁴CO₂ concentrations ranging from zero to approximately 1.5X of the contemporary abundance. Room temperature optical detection of ¹⁴CO₂ at our demonstrated accuracy is not possible with any other existing techniques.

In addition to its use in ultra-trace analysis, our cavity-enhanced 2C technique is well-suited for rovibrational-state-resolved measurements in chemical dynamics and high-resolution spectroscopic studies. The sensitivity and selectivity of our 2C technique could potentially enable spectroscopic investigations of chemical species at internal energies and molecular conformations that are inaccessible with other methods. One potential extension of our cavity-enhanced pump-probe technique is the use of broadband radiation as the probe, such as a mid-IR frequency comb, to achieve rapid, multiplexed detection with high sensitivity and selectivity.