Black Phosphorus Nanodevices for Active Polarization Control

Author: Seah, Samuel Kai Wen

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Atwater, Harry Albert

Committee Members: Nadj-Perge, Stevan; Vahala, Kerry J.; Faraon, Andrei; Atwater, Harry Albert

Option: Applied Physics

Abstract

Manipulating the fundamental properties of light at the nanoscale represents a new paradigm of modern optics. Controlling polarization states on-demand, for example, is highly desirable in optical communications, bio-imaging and quantum information processing, where fast continuous switching, compactness, and ease of integration are essential. In this regard, anisotropic 2D quantum materials like black phosphorus (BP) provide an attractive platform with its electronically tunable anisotropy, strong excitonic absorption and direct bandgap, all at the ultrathin limit of atomic-scale thicknesses.

This thesis explores the capacity of BP for optically efficient, broadband and versatile polarization modulation by integrating the material into a variety of nanostructures. We first examine methods for optically efficient modulation with 2D materials via resonance wavelength detuning and high-Q cavities. By leveraging spatially engineered polarization gradients, we propose a tunable polarization beam splitter demonstrating the potential of BP for structuring light.

We then present a plasmonically-coupled black phosphorus metasurface capable of multispectral polarization control over several telecommunication bands. The device represents a 10-fold decrease in thickness and 5-fold decrease in gating voltage over similar modulators in the literature. Finally, we report a dual-gated cavity integrating two cross-aligned BP layers for comprehensive, all-electronic modulation across the Poincaré sphere surface. This is the first demonstration of independent, active two-parameter control via excitonic tuning in an optical cavity.

By concluding with several propositions of multi-dimensional parameter control using 2D quantum materials, this work illustrates the versatility of BP as a modulator, opening further possibilities for the arbitrary structuring of light.