Applying High-Sensitivity Ultrafast Spectroscopy Techniques to Extract Optoelectronic Properties of Optical Energy Conversion Heterostructures

Author

• Anton Kyrylenko, CUNY Graduate CenterFollow

Date of Degree

9-2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Physics

Advisor

Matthew Y. Sfeir

Committee Members

Carlos Meriles

Maria C. Tamargo

Alexander Punnoose

Matthew C. Beard

Subject Categories

Atomic, Molecular and Optical Physics | Condensed Matter Physics | Electromagnetics and Photonics | Electronic Devices and Semiconductor Manufacturing | Nanoscience and Nanotechnology | Nanotechnology Fabrication | Optics | Quantum Physics | Semiconductor and Optical Materials

Keywords

Metasurfaces, Semiconductors, Photonics, Nanotechnology, Plasmonics, Solid-State Physics

Abstract

A fundamental challenge in the field of heterogeneous solar water-splitting (photoelectrolysis) is the optimization of visible and near-infrared absorbing semiconductor heterostructures to maximize solar-to-hydrogen efficiency. We posit that a key limitation at longer optical wavelengths results from a mismatch of the characteristic length scales for charge transport, determined by the depletion width, and absorption. Thin film heterostructures serve as a foundation to improve bulk-based photoelectrochemical (PEC) cell performance. We present a framework for tailoring light absorption by patterning thin films into nano-resonator arrays (metasurfaces) of conventional semiconductors to surpass the optical absorption limits of thin films. We optimize the magnetic dipole (MD) Mie resonance photonic mode that preferentially generates charge carriers near the liquid-solid interface, enabling short transport distances and higher conversion efficiencies.

To quantify the effect of the MD mode on generated carrier profiles, we use broadband pump-probe spectroscopy. We demonstrate the results of our methodology using hydrogenated amorphous silicon (a-Si:H) films that are patterned to create a metasurface with an MD resonance at 580 nm. We discuss the modeling, simulation, and nanofabrication of the thin film and metasurface heterostructures. We apply the relevant intrinsic optical properties of a-Si:H to study photoexcited carrier densities and dynamics of the metasurfaces and quantify the improved optical absorption against thin films using a developed optical model. We demonstrate that metasurfaces enhance the spatially-averaged carrier generation rate (⟨G_rate⟩) by a factor of ∼5.2 compared to films of equivalent thickness. Furthermore, transient absorption decay kinetics validate that the local photoexcited carrier density exceeds a 5.2x increase, confirming that ⟨G_rate⟩ is dominated by the local carrier density formed by the MD mode.

In addition, we extend our methodology to gold thin films, an essential metal for plasmonic applications. Pump-probe measurements reveal novel intraband carrier dynamics that challenge the current understanding of non-thermal photoexcited carrier distributions near the X symmetry point of gold's Brillouin zone. Specifically, the presence of a p-band saddle point suppresses the density of states, hindering bidirectional intraband transition rates. Consequently, even a relatively small photoinduced depletion of the electronic population from the Fermi level (E_F) has a noticeable effect near the X point, reducing the transient non-equilibrium electronic temperatures. This proposed mechanism successfully accounts for the overestimation of electronic temperatures in standard two-temperature model (TTM) approaches.

Recommended Citation

Kyrylenko, Anton, "Applying High-Sensitivity Ultrafast Spectroscopy Techniques to Extract Optoelectronic Properties of Optical Energy Conversion Heterostructures" (2026). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/6808