Continuous-wave cavity ringdown spectroscopy applied to solids: Properties of a Fabry-Perot cavity containing a transparent substrate

Cavity ringdown spectroscopy, or photon-trap spectroscopy for generality, is shown to be applicable to a sample m the solid phase by theoretical and experimental studies. In the technique investigated, a solid in a substrate form having optically flat parallel surfaces is inserted exactly normal to a light beam in a high-finesse optical cavity; the light reflected at the substrate surface is coupled back to the cavity and thus the optical loss is minimized. Thereby the trapping lifetime of photons in the cavity is measured to obtain total optical loss including absorption by the solid sample. As the solid substrate behaves as an extra cavity splitting the original cavity, the trapped photons are susceptible to an interference effect inherent to the triply coupled cavity. To elucidate this effect, the coupling efficiency of the incident light and the trapping lifetime of photons dissipating exponentially were analyzed theoretically for a Fabry-Perot cavity containing a transparent substrate as a model. An experiment was performed on a silicon substrate transparent in the mid-infrared range with a cw optical parametric oscillator based on periodically poled lithium niobate. The optical loss caused by insertion of the substrate was measured to be 2.3 × 10^-4 per round trip, which meets a low-loss requirement of the photon-trap technique. The trapping lifetime of photons was found to depend on the location of the substrate as predicted by theory. By optimizing the experimental conditions, the present technique provides a high sensitivity to optical absorption associated with a trace amount of dopants in solids and adsorbates on surfaces.