Dissertations, Theses, and Capstone Projects

Date of Degree

9-2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Physics

Advisor

Matthew Y. Sfeir

Committee Members

Vinod Menon

Swapan K. Gayen

Johannes Flick

Juan-Pablo Correa-Baena

Subject Categories

Atomic, Molecular and Optical Physics | Optics

Keywords

Photonics, Ultrafast Spectroscopy, Nonlinear Optics, Photochemistry

Abstract

Ultrafast spectroscopy is a powerful tool for investigating phenomena that occur on extremely short timescales and for probing ultrabroadband spectral responses enabled by its ultrashort laser pulses. These capabilities are essential for advancing both the fundamental understanding of light–matter interactions and the development of next-generation optoelectronic technologies. The first part of this thesis investigates the ultrafast dynamics of organic exciton–polaritons and their application to controlling molecular excited-state processes using momentum-resolved pump–probe spectroscopy. A major challenge in these systems is the dark-state problem, where photoexcitation predominantly populates long-lived excitonic reservoir states that obscure the in trinsic dynamics of the hybrid light–matter states. Using carefully designed open photonic cavities, selective excitation of exciton–polaritons is achieved, enabling direct observation of their sub-100 fs dynamics and demonstrating a route to overcome the dark-state problem. Building on these find ings, the second part employs Bloch surface wave polaritons, whose favorable dispersion and longer lifetimes (100–400 fs) enable efficient photoinduced charge transfer between donor and acceptor molecules. By selectively exciting these hybrid states, the energetic driving force for charge sepa ration is reduced by up to 0.5 eV compared with bare exciton excitation while achieving an internal quantum efficiency of 0.77. Together, these results demonstrate that appropriately engineered pho tonic platforms enable both direct observation of ultrafast polariton dynamics and precise control of photochemical charge-transfer processes, providing a pathway toward tunable photochemistry and improved optoelectronic devices. The second part of this thesis focuses on the development of ultrabroadband terahertz time domain spectroscopy (THz-TDS) for material characterization. A Modified Transfer Matrix Method (MTMM)is developed to accurately extract the complex refractive index from experimental THz TDS measurements using a new transfer matrix formalism. The method is broadly applicable to a wide range of material properties, sample structures, and frequency ranges, providing reliable opti cal constants across a broad terahertz spectrum. In parallel, an ultrabroadband two-color air-plasma THz-TDS system is designed, implemented, characterized, and automated. To maximize the usable bandwidth and signal-to-noise ratio, the system incorporates ultrafast pulse compression, two-color air-plasma THz generation, optimized electro-optic detection, high-speed data acquisition, and auto mated instrument control. Together, these computational and experimental developments establish a versatile ultrabroadband THz-TDS platform for accurate material characterization. Overall, this thesis demonstrates how ultrafast time-domain and broadband frequency-domain spectroscopic techniques can be combined to investigate complex material systems. The methods developed in this work have broad applications in organic photochemistry, optoelectronics, funda mental light–matter interaction studies, and material characterization, providing powerful tools for understanding and engineering ultrafast physical processes.

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