| The mid-IR spectral region is of great spectroscopic interest as it corresponds to the energy scales associated with vibrational absorption resonances in molecules. Using the ultrafast mid-IR laser sources at the Stanford Picosecond Free Electron Laser Center, we have pursued two distinct research projects. In one project, we have extended the techniques of cavity ringdown spectroscopy (CRDS), an ultrasensitive linear absorption spectroscopy technique, to the study of condensed matter systems, in the form of thin films. We have compared two different approaches to thin-film CRDS: normal incidence coupled-cavity CRDS and Brewster's angle CRDS. Additionally, we demonstrated the viability of thin film CRDS as a calibrated ultrasensitve absorption measurement technique. In order to overcome the absorption sensitivity limitations imposed by poor optical throughput in CRDS, we have developed and demonstrated two throughput enhancement techniques, cavity-enhancement and pulse-stacking.; In the second project, we have developed a two-color mid-IR pump mid-IR probe technique for studying ultrafast temperature changes in the components of biological tissue. We have made measurements that establish the use of transient absorption as an ultrafast thermometer in both water and protein. We have used this method to measure the thermalization between the constituents of corneal stroma in order to examine the assumptions underlying the differential heating model of pulsed mid-IR ablation of soft tissue. |
