Author

Hemang Jani

Date of Award

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Optical Science and Engineering

Committee Chair

Lingze Duan

Committee Member

Don Gregory

Committee Member

Patrick J. Reardon

Committee Member

Seyed Sadeghi

Committee Member

Yu Lei

Subject(s)

Time-resolved spectroscopy., Reflectometer., Ultrashort laser pulses., Photoemission--Measurement.

Abstract

Over the last decade, advances in novel materials such as two-dimensional (2D) materials, metamaterials, plasmonics, and negative-electron-affinity (NEA) structures have triggered rapid growth in the development of ultrafast optoelectronic devices. This calls for simple yet effective approaches to characterize ultrafast dynamics in these materials and devices. An important aspect of this thesis research is to demonstrate one such approach by amalgamation of state-of-the-art in ultrashort light pulse source with a classical pump-probe methodology to develop a Few-Cycle Pump-Probe Reflectometer (PPR) system, which can offer both high temporal resolution and broad spectral coverage. As one of its applications, this work focuses on characterizing NEA semiconductor photocathode devices by offering a pre-photoemission investigation of photoelectron transport and relaxation in GaAs/AlGaAs based photocathodes. The results show the feasibility of using PPR in effectively characterizing photoemission performance not only based on the different doping architectures, but also based on different growth methods used for fabricating these photocathodes. A theoretical framework based on a carrier-diffusion model is presented to quantitatively verify the physical understanding of the pump-probe reflectivity measurements on these photocathodes and to better explain the transient behavior of the reflectivity of these structures. This particular work is a new and original approach to the long-standing problem of time resolved photoemission from such photocathodes. Preliminary results of a time-frequency spectroscopic study of the transient dispersion of GaAs near its bandgap are also discussed, to further illustrate the capability of the reflectometer and disentangle fundamental ultrafast processes happening in optoelectronic devices.

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