The goal of the fellowship is to build an ultrasensitive two-dimensional infrared spectrometer and apply it to detection of complex mixtures of trace amounts of volatile organic compounds (VOCs). Third-order spectroscopies using ultrashort pulses, such as 2D IR spectroscopy, are powerful tools for studying both structure and dynamics. They probe the evolution of state-to-state coherences between quantum states and evolution of state populations on femtosecond to nanosecond timescales, in between excitation by ultrashort optical pulses. In terms of molecular properties, 2D IR spectroscopy probes correlations between molecular bonds, which strongly depend on the structure of the molecule as a whole. Compared to linear spectroscopy, which is more bond-specific, 2D IR spectroscopy provides much greater selectivity. Compared to mass spectrometry methods, it is applicable to both small inorganic molecules and to VOCs and easily lends itself to quantitative analysis. 2D IR spectroscopy has not been used for trace-gas analysis up to now because of insufficient sensitivity. This project overcomes this problem by building first of its kind cavity-enhanced 2D IR spectrometer, with up to four orders of magnitude better sensitivity than the previous state of the art, and applying it to vibrational spectroscopy of VOCs. The potential for exploitation of the project outcomes includes breath analysis diagnostics, detection of explosives, narcotics and other trace-gas analysis problems. There are also many potential applications of the outcomes in basic science, in the field of ultrafast dynamics of optically dilute samples (e.g. cold molecular jets or sub monolayer films). Two notable examples include the problem of intramolecular vibrational energy redistribution and the dynamics of hydrogen bond networks. The expertise and unique skills gained during the outgoing phase will be used to establish a new research program in the host institution.