Development and Applications of Shaper-Based Two-Dimensional IR and SFG Spectroscopy

2D IR spectroscopy has become a great tool for studying amyloid peptide and membrane protein structures. Due to its success on these research topics, it makes sense to further expand 2D IR spectroscopy into other complex system such as organic-inorganic interfacial charge transfer systems. Among all improvements to 2D IR spectroscopy, the integration of a mid-IR pulse shaper into the 2D IR setup is the most important for simplifying its alignment and facilitating fast data acquisition. This improvement is important since it provides the key for 2D IR spectroscopy to be extensively used to study chemical, biological and material science without large maintenance. Owing to the great impact that mid-IR pulse shaping made to 2D IR spectroscopy, other nonlinear optical techniques such as SFG spectroscopy could also benefit from utilizing the pulse shaper. Therefore, my PhD work focuses on a few topics: mitigating the lineshape distortion and improving signal-to-noise of the spectrum; demonstrating the unique capability of 2D IR spectroscopy to study charge transfer interface; designing and constructing the shaper-based time domain heterodyned SFG spectrometer and a shaper-based 2D SFG spectrometer. We used 2D IR as a probe to study the charge transfer dynamics of the Re1C dye on TiO2 thin film system, which is a model system of a dye-sensitized solar cell. Using a 400nm pulse to initiate the charge transfer process, we are able to show that the free electron background that dominates in the transient IR spectrum is eliminated in the transient 2D IR spectrum. The spectrum shows that the dye has an inhomogenous distribution when it is attached onto the surface and there are three major conformations involved in the charge transfer process. By inserting the 400nm beam in the waiting time period of the 2D IR pulse sequence, we show that the three conformations have different electron injection timescales. Thus, these conformations on the surface are keys to improve the charge transfer efficiency. To understand them, surface specific techniques such as SFG spectroscopy are needed. We built a shaper-based heterodyne frequency/time domain SFG spectrometer and show that heterodyne detection not only improves the signal strength but also allow us to obtain phase information and retrieve the non-resonance background free absorptive spectrum. Also, we demonstrated that time domain SFG spectroscopy has better resolution and lineshape fidelity compared to the widely used broadband frequency domain SFG spectroscopy. The implementation of pulse shaper facilitates phase control of the mid-IR pulse and allows us to update the IR time delay on a shot-to-shot basis, which makes the time needed to obtain a SFG spectrum is less than a second. The mid-IR pulse shaper also opens the possibility of implementing 2D SFG spectroscopy. By the end of my PhD career, we constructed the world's first 2D SFG spectrometer and show its ability to unwrap surface information that is not allowed before.