Aruna Kumarasiri

Topic

Second-Order Nonlinear Optics as an Orientation-Independent Probe of Molecular Environments at Interfaces

Department of Chemistry

Date & location

• Monday, April 20, 2026
• 9:30 A.M.
• Elliott Building, Room 226

Examining Committee

Supervisory Committee

• Dr. Dennis Hore, Department of Chemistry, University of Victoria (Supervisor)
• Dr. Matt Moffitt, Department of Chemistry, UVic (Member)
• Dr. Reuven Gordon, Department of Electrical and Computer Engineering, UVic (Outside Member)

External Examiner

• Dr. Luis Velarde, Department of Chemistry, State University of New York at Buffalo

Chair of Oral Examination

• Dr. Jean Buckler, School of Exercise Science, Physical and Health Education, UVic

Abstract

Interfacial chemistry is fundamentally governed by a unique molecular environment, within which the electronic structure changes drastically across distances of only the order of angstroms, yet it profoundly controls much of the macroscopic behavior in materials science, catalysis, and biological systems. With such critical importance, the need for interface-specific analytical techniques has become increasingly apparent. As a result, nonlinear optical techniques, such as vibrational sum frequency generation (SFG), have become powerful tools for probing interfaces, particularly due to their inherent surface specificity. Even though these techniques show great promise for such applications, the electronic structure information that SFG provides is inherently coupled with the molecular orientational distribution of the molecules, such that, extracting structural information often has to be done under the assumption of a particular orientational distribution function. This, in return, introduces significant uncertainties into quantitative interfacial analysis that could potentially lead to inaccurate interpretations. In this dissertation, a theoretical and experimental framework is introduced for the SFG process to improve the accuracy of interfacial analysis by addressing this matter, to extract the electronic structure information at the interface without committing to any particular orientational distribution.

The first part of the dissertation demonstrates a novel theoretical framework to determine the surface hyperpolarizability ratio—one of the most fundamental parameters in interfacial chemistry as it directly reflects the molecular electronic response at the interface—completely independent of, and therefore unaffected by, the orientational distribution of the molecules. In this way, the results of this work leverage the best features of the SFG technique, that is, its surface specificity, while expanding its practicality toward what is typically accessible only through bulk isotropic measurements. Another critical challenge to address in this type of work is experimental accuracy across the different polarization combinations of the SFG signal. To address this, building on our theoretical foundation, the second part of this dissertation introduces a null-angle-based calibration method for SFG analysis. This calibration procedure also helps extend this orientational distribution-free framework into buried interfaces. Finally, in the third part of the dissertation, the orientational characteristics across different interfaces were analyzed with the aid of the previously developed theoretical and experimental framework, and the importance of using surface-specific parameters for accurate orientational analysis was demonstrated, strengthening the use of polarization-resolved SFG as an accurate quantitative probe of interfaces.