@phdthesis{12712,
  author = {Samuel Johnson},
  title = {Ultrafast Infrared Nano-Imaging of Strongly Coupled Phonon and Carrier Dynamics in Quantum Materials},
  abstract = {Infrared (IR) vibrational scattering scanning near-field optical microscopy (s-SNOM) has advanced to become a powerful nano-imaging and -spectroscopy technique to probe molecular and lattice vibrations, low-energy electronic excitations and correlations, and related collective surface plasmon, phonon, or other polaritonic resonances. In combination with scanning probe microscopy, near-field infrared nano-spectroscopy and -imaging enables the study of complex heterogeneous materials with simultaneous nanoscale spatial resolution and quantum state spectroscopic specificity. IR s-SNOM has also been extended to studying dynamics in the time domain, where ultrafast vibrational and electronic spectroscopy unravels mechanisms underlying functionality in quantum materials. Femtosecond-to-picosecond dynamics are convolved with multiscale spatial heterogeneities, ranging from microscopic defects to the macroscopic domain level. This technique opens the door to elementary processes and interactions in functional materials with full spatio-temporal-spectral resolution. In this thesis, I will describe my work in advancing s-SNOM through the design, testing, and implementation of ultrafast hyperspectral imaging modalities on both molecular and quantum systems.

Following the introduction, I will summarize the fundamentals of s-SNOM as advancements detailed here are based on these starting points. Then, I will detail three distinct method advancements made to improve the image quality, acquisition speed, and information content. By utilizing knowledge of the resonance being probed, the bandwidth of the excitation source, and the desired spectral resolution, I can transition from the stationary frame to the rotating frame to reduce the Nyquist maximum frequency cut-off. With the necessary maximum frequency reduced, fewer data iii points are needed along the reference arm axis compared to conventional hyperspectral imaging, enabling faster data acquisition of a higher resolution image over a higher field of view with automatic drift correction. I further have theoretically demonstrated that prior knowledge about the limited frequency range of the optical source in a nano-FTIR measurement and the fact that samples are typically composed of only a few different molecular constituents allows for faster detection algorithms. Specifically, compressive sensing and matrix completion techniques can be used to reduce the necessary data points from the Nyquist limit by a factor of 10. Further, this technique can be applied to active systems for adaptive algorithms that use previous measurements to inform future measurements during acquisition. Finally, I will detail advances made in ultrafast pump-probe nano-imaging and -spectroscopy, where pump modulation with selective interferometric detection isolates the difference between the excited and ground state. This advancement enables higher signal to noise ratios for low excited state contrast systems, especially necessary for the case of low repetition rate, far-from-equilibrium pump excitation parameter regimes. Next, I will discuss my work performing spectroscopy with broad band infrared (IR) light sources, such as synchrotron radiation, fs broadband lasers, and ps narrowband lasers. This variety of light sources proved extremely useful for a variety of material systems. Synchrotron infrared nano-spectroscopy (SINS) has been used as a consistent diagnostic tool for molecular systems, like the self assembled monolayer 4-NTP, hydrocarbon and mineral systems, like oil filled pores in shale, and quantum systems, like strongly coupled heterostructures composed of multi-quantumwells and gold antennas. The advancements and discoveries here represent the successes of applying infrared spectroscopy to a variety of systems to discern inter-molecular coupling, spatially resolved hydrocarbon mapping, and phase resolved hybridization controlled weak to strong coupling.

Lastly, I will present ultrafast nano-FTIR on 2D transition metal dichalcogenide (TMD) systems, interfacial energy transfer, lattice expansion and phonon softening, and spatially inhomogeneous electron dynamics. Here, I performed visible pump, infrared probe nano-imaging spectrscopy, where I pumped the interband electronic state in WSe2, producing free carriers that thermalize through phonon emission within the TMD and then dissipate heat through the substrate. We iv perform ultrafast nano-FTIR on a phonon mode of the substrate as a means of monitoring local temperature. We find that the electron population in the TMD relaxation is also heterogeneous as an increase in carriers from a hot phonon bottleneck leads to an Auger recombination and stronger bright state population for faster excited state photo-relaxation. I will close with a short summary of my work, and an outlook to future experimental efforts.},
  year = {2021},
  journal = {Department of Physics},
  volume = {Ph.D.},
  pages = {168},
  month = {2021-09},
  publisher = {University of Colorado Boulder},
  address = {Boulder},
}