Atomically Thin Spatial Light Modulators with Excitonic Nanomaterials

Citation

Li, Melissa (2025) Atomically Thin Spatial Light Modulators with Excitonic Nanomaterials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5khv-t042. https://resolver.caltech.edu/CaltechTHESIS:02202025-192231690

Abstract

Achieving active control of light at the ultimate thickness limit—a single atomic layer—offers unprecedented opportunities for next-generation optoelectronic devices. The quest for ultrathin spatial light modulators has long relied on integrating tunable materials with plasmonic or high-index nanoantennas that serve as small, but three-dimensional optical resonators. As structures for controlling light become increasingly complex and compact, the geometrical constraints of these three-dimensional resonators will ultimately limit their scalability and versatility. A new avenue for device miniaturization emerges when harnessing electrically tunable resonances that are intrinsic to atomically thin materials.

This thesis explores how exciton resonances, specifically in two-dimensional (2D) van der Waals materials, can serve as the central building blocks for future spatial light modulators that are as thin as atoms. We start by characterizing the gate-tunable optical properties of a monolayer molybdenum diselenide (MoSe₂), a 2D transition metal dichalcogenide. By tuning the exciton resonances with voltage, we demonstrate over 200% modulation in the real and imaginary part of the complex refractive index. We attribute this large tunability to the interplay between radiative and nonradiative decay channels of the excitons. The index modulation gives rise to amplitude and phase modulation of the scattered light, which is then used to engineer an electrically tunable phase gradient across a single monolayer MoSe₂ flake to dynamically steer the reflected beam.

Next, we present a theoretical analysis of the complex frequency response of a generalized excitonic heterostructure. We show how the spectral positions of the phase singularities, e.g. zeros and poles, can be dynamically controlled, their impacts on the real frequency phase response, and how they can be used in active metasurface design. Finally, we evaluate excitons in quantum dots as an alternative platform for room temperature optical modulators and show how they present different challenges in designing phase modulators.

Overall, our work highlights the novel functionalities enabled by exciton resonances for advanced light manipulation, underscoring their potential for atomically thin light modulators.

Thesis Files