Saturated-absorption cavity ring-down (scar) spectroscopy for optical radiocarbon detection
Radiocarbon (14C), the “natural clock” for dating organic matter, is a very elusive atom. Its concentration is about one part per trillion. About 40 years ago, accelerator mass spectrometry (AMS) was adopted as the standard method for dating organic samples via radiocarbon. AMS requires a smaller carbon mass and shorter measurement times than the former method of liquid scintillation counting (LSC). However, AMS requires huge, expensive and high-maintenance experimental facilities. We have developed a laser spectroscopy technique that is sensitive enough to detect radiocarbon dioxide molecules at very low mole fractions with an all-optical setup that is orders of magnitude more compact and less expensive than AMS. The new approach, named saturated-absorption cavity ring-down (SCAR), makes use of molecular absorption saturation to enhance the sensitivity with respect to conventional cavity ring-down spectroscopy. By combining SCAR with a couple of quantum cascade lasers (QCLs) emitting narrow-linewidth mid-IR radiation tunable around at 4.5 µm, we could achieve an unprecedented limit in trace gas detection, down to a few parts-per-quadrillion (10-15) mole fraction. SCAR-based results are currently about a factor of three less precise than AMS, but there is still room for improvement. Moreover, SCAR has a wider dynamic range than AMS, encompassing more than 5 orders of magnitude in measurable concentration values.
A feasibility study for radiocarbon dating by infrared laser spectroscopy was published over 40 years ago. In 2011 we succeeded, for the first time, in measuring the spectral area of the selected target absorption line of the 14C16O2 isotopic species, thus retrieving the absolute radiocarbon dioxide concentration. We accomplished this by exploiting the unique features of our experimental apparatus and relying on strong molecular absorption in the mid IR. The SCAR technique starts with an optical cavity filled with CO2, which is illuminated with an intense CW laser tuned to excite a target molecular transition of 14C16O2. When the laser is turned off, photons stored in the cavity decay due to 14C16O2 absorption and mirror leakage. Because the high intensity of the intra-cavity light saturates the ability of the 14C16O2 to absorb it, the initial instants of decay are affected by losses from the mirrors only. Once we subtract that background, we can determine the absolute quantity of 14C16O2 from the linear molecular absorption encoded in the decay tail. Our results pave the way to a brand-new all-optical dating technique, moving from detection of high-energy (MeV) accelerated ions to more comfortable detection of low-energy (0.25 eV) absorbed photons. This technique can be used to detect extremely rare molecules with myriad of applications in many fields: dating of cultural heritage, certification of biomaterials, pharmacology, nuclear safety, etc.