Laser-induced plasmas in air for pulsed broadband cavity-enhanced absorption spectroscopy

Abstract

A pulsed laser-induced plasma (LIP) was generated in ambient air inside a high-finesse near-concentric optical cavity. The broadband optical plasma emission was successfully sustained within the cavity and the light leaking from the cavity was used to measure broadband absorption spectra of gaseous azulene through (i) time-dependent cavity ring-down (CRDS), and (ii) intensity-dependent incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) methodologies. Based on the broadband ring-down time of light within the cavity, cavity mirror reflectivities may be calibrated in situ, thus facilitating measurements of absolute absorption coefficients with the intensity-dependent (IBBCEAS) approach. Each of the two spectroscopic approaches are compared in terms of their overall performance and suitability to the experimental requirements. The IBBCEAS approach was found to possess numerous advantages over the CRDS approach in terms of measurement speed, and achievable signal-to-noise ratios and minimum detectable absorption coefficients. However, the IBBCEAS approach is limited by the presence of an artificial offset in the measured absorption that arises due to the absorption of light occurring between LIP formation and the commencement of data collection. This delay prior to data collection is necessitated by the non-Lambert-Beer conditions during plasma equilibration. For the given experimental conditions, the cavity output typically takes ~ 1.5 μs to begin mono-exponential decay. The time-dependence of the cavity output intensity was also investigated. Light propagating within the cavity is subject to direct absorption by the intra-cavity LIP, and a lensing effect (f ≈ −3.1 cm) by the LIP as a result of a localised nonlinear refractive index change induced by the high temperatures and strong electric fields associated with the LIP. This time-dependence was observed to be dependent on both the mirror separation and the pulse duration of the LIP formation laser. These dependencies come about as a consequence of the LIP lensing temporarily affecting both the cavity stability and the imaging of the light that is output from the cavity, with both of these aspects being dependent on the cavity geometry. In the case of fs-lasers, the increased pulse power density expectedly yields a more significant lensing effect when compared to LIP formation with ns-pulsed lasers.