Probing surfaces and interfaces by nonlinear optical spectroscopy with time, energy, and phase resolution
Creators: Nelson, Cory A.
Publisher: name: Columbia University
Year: 2015
Subjects: Chemistry
Power resources
Optics

description: Surfaces and interfaces are a ubiquitous part of nature. They influence the behavior of devices and are essential components in charge transfer and charge trapping. While surfaces and interfaces are important studying them is difficult because they consist of only the first few layers of a material. Therefore, surface-specific techniques are needed to investigate their properties and dynamics. Perhaps the most common surface electronic surface characterization techniques are electron spectroscopies which have become the standard for determining surface electronic band structure. However, these spectroscopies require ultra high vacuum which precludes the study of surfaces at ambient pressures and buried interfaces. Ambient pressures and interfaces are precisely the conditions under which most devices operate. Therefore there is a need for a technique which can reveal information about electronic states and their dynamics of buried interfaces at ambientconditions.
      This thesis describes the implementation of broadband time-resolved second harmonic generation and the recovery of the time-resolved amplitude and phase by employing spectral interferometry. The even order nonlinear process allows the measurement to be surface specific which the spectral amplitude and phase reveal information about surface state transitions and couplings. The first chapter motivates the study of surface and interfaces while chapters 2 and 3 cover background information about surfaces and nonlinear optics to help understand the experiments presented in the following two chapters.
      Chapter 4 presents a broadband time resolved spectral SHG technique whose usefulness is demonstrated on gallium phosphide passivated undoped gallium arsenide. In this case the spec-tral features are due to the E 1 resonance in GaAs and the dynamics are assigned to band gap renormalization. Chapter 5 details a method to recover the time resolved amplitude and phase and then demonstrates the recovery of the amplitude and phase from SH emitted from n- and p-type GaAs. The spectra reveal a discreet surface state ascribed to defect formation specific to n-type GaAs. The asymmetric line shape of this state indicates that it is coupled to a continuum; most likely a surface projected bulk band. We found that this coupling can be controlled by changing the azimuthal angle. However, p-type GaAs does not show distinct features in the second harmonic spectrum.
      Experiments on bilayers consisting of p-type GaAs and copper pthalocyanine (CuPc) are also presented in chapter 5. No changes in the signal are observed for either the constituents alone. However, when CuPc is deposited on GaAs a transient state forms at 200 fs delay between the pump delay which also exhibits an asymmetric line shape. This indicates the formation of a new state at the heterojunction that was not present before and may be evidence for a charge transfer state.
      Chapter 6 closes the thesis with concluding remarks which suggest improvements in the experimental design and implementation of time-resolved second harmonic spectral interferometry.