Ultrafast Dynamics of Vibrational Polaritons Probed with 2D-IR Spectroscopy

Abstract

Polaritonic chemistry—controlling chemical reactions by strongly coupling light modes with resonant reactant vibrational transitions—is a rapidly developing field. This fundamentally new approach to modifying chemical reaction rates and product yields promises to enable direct manipulation of chemical free energy surfaces, remotely controlled by external optical cavities. The strongly-coupled light-matter hybrid is referred to as “vibrational polaritons,” and understanding the mechanisms of this new type of chemical control method will be essential in defining the scope of manipulation available to develop of new reactions or discover novel materials for quantum information processing. Two-dimensional infrared (2D-IR) spectroscopy is a powerful tool to access vibrational structural dynamics, including population transfer and relaxation, as well as frequency fluctuations, and is thus ideally suited to the study of vibrational strong coupling systems. This dissertation focuses on how to use 2D-IR to understand basic polariton energy level structure and the time-dependent processes that occur following ultrafast excitation of light-matter hybrid polaritons. The use of a pulse shaper based 2D-IR apparatus has many advantages over alternative methods of implementing 2D-IR. Using a phase-cycling approach, it is demonstrated here that both “geometric” and “amplitude” based methods to obtain vibrational spectral diffusion dynamics are effectively equivalent. Both methods are applied to various organometallic complexes which serve as useful dynamical probes of solvation. The two analysis methods are essentially identical but manifest coherent, inherently non-equilibriium dynamics differently. One of the challenges in obtaining the polariton response using 2D-IR is a persistent bare molecule (i.e. non-polaritonic) background contribution. In real-world planar Fabry-Pérot cavities, not all of the molecules are coupled with the cavity, and due to the small vacuum Rabi splitting separating the lower and upper polariton states, there is an unavoidable 2D-IR signal due to the bare molecules. This background obfuscates the energy level assignment and dynamical information readout of the polaritons. This dissertation first demonstrates that this background contribution, which is present in most 2D-IR spectra published in the literature, is due to the uncoupled 2D-IR spectrum, and second, provides a means of suppressing the background by subtracting it from the measured cavity spectra. The nature of the hybrid polaritonic states is altered by changing the frequency of the cavity, which can be done either by modifying the length or by adjusting the angle of the incident light beam used for spectroscopy. Most of the ultrafast IR experiments done previously are based on probing the zero-detuning polariton level pairs. However, the contribution of different parts of energy dispersion is also worth investigating. To be able to use the angle-tuning approach with 2D-IR spectroscopy, it is possible to implement a fully collinear geometry to avoid complications from non-collinear beam geometries while changing the angle of cavity relative to the incoming beams. As an illustration of the benefits of using a fully-collinear geometry, an example is presented where two different vibrational modes are coupled to two different modes of the same optical cavity. These so-called dual-mode polaritons exhibit cavity-detuning-dependent relaxation dynamics.