Date of Award

Summer 8-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Program/Concentration

Physics

Committee Director

Hani E. Elsayed-Ali

Committee Member

Shukui Zhang

Committee Member

Volker Ziemann

Committee Member

Yuan Zhang

Committee Member

Gon Namkoong

Abstract

This dissertation presents a comprehensive experimental and computational investigation of nanostructured negative electron affinity (NEA) gallium arsenide (GaAs) photocathodes. This work was undertaken to achieve higher quantum efficiency (QE) and longer charge lifetime than conventional flat NEA GaAs electron sources commonly used in photoinjectors. Finite-Difference Time-Domain and charge-transport simulations of three types of nanostructure geometries reveal substantial QE enhancement over flat GaAs via Mie resonance mode excitation. Consequently, the truncated nanocone array (TNCA) photocathodes with heights of 400, 700, and 1000 nm were fabricated and NEA activated via Cs-NF₃ deposition. In a low-voltage ultra-high vacuum chamber with a base pressure on the order of 10-11 Torr, the NEA TNCA GaAs photocathode with periodicity and height of ~300 nm and ~700 nm, respectively, reached a maximum QE of ~13.6% at 590 nm (~1.44× higher than flat GaAs) and a maximum charge lifetime of 397.5 mC when activated at 50 °C, which is nearly 20× greater than the charge lifetime estimated for NEA flat GaAs. At the Jefferson Lab’s Gun Test Stand, the QE spectra for NEA TNCA GaAs photocathodes with periodicity of ~600 nm further demonstrated an overall enhancement in QE compared to flat GaAs, with a maximum enhancement of ~1.7× and ~2.5× at laser wavelengths of 532 nm and 780 nm, respectively.

Inside a 140 kV DC photogun, TNCA GaAs photocathodes showed a charge lifetime up to 26.8 C (~4.4× higher than flat GaAs) while sustaining 1 mA continuous operation without surface charge accumulation. The normalized emittance of TNCA GaAs increased by a factor of ~1.5–1.7 compared to NEA flat GaAs at a 532 nm laser wavelength. Additionally, electron spin polarization was ~18.8% for the ~1000 nm tall NEA TNCA GaAs, which is modestly lower than for flat GaAs. Collectively, these results establish that nanostructured GaAs photocathodes can simultaneously enhance QE and charge lifetime over flat GaAs, providing a viable pathway and clear geometric design criteria for future high-performance photocathode electron sources.

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DOI

10.25777/cz4v-gd87

ISBN

9798193214465

ORCID

0000-0002-8348-5204

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