2026 Theses Doctoral

Polarization Waves in Layered van der Waals Ferroelectrics

Huang, Chun-Ying

Ferroelectric materials have garnered significant interest for next-generation electronic and optoelectronic technologies. These applications include, but are not limited to, the mature ferroelectric random access memory (FeRAM), ferroelectric field effect transistor (FeFET), nonlinear optics, and ferroelectric-based photonic integrated circuit (PIC). However, as the demand for nanometer-scale components rises, the integration of conventional oxide ferroelectrics faces substantial challenges, such as dead interfacial layers, thickness limitations for maintaining the ferroelectric phase, and reduced efficiency of nonlinear optical processes at the thin limit. Two-dimensional (2D) van der Waals (vdW) ferroelectric materials offer a promising route to overcome these obstacles due to: (1) the absence of surface reconstruction in heterostructure devices because of the vdW nature; (2) robust ferroelectricity at the 2D limit; and (3) exceptionally large nonlinear optical coefficients at the atomic-thin limit. Nevertheless, fundamental studies on 2D ferroelectrics remain scarce, primarily due to the limited number of confirmed materials in this category.

In this thesis, we explore the fundamental physics of a recently identified 2D vdW ferroelectric, NbOI2, which exhibits room-temperature in-plane ferroelectricity with a parallel alignment between vdW layers. In Chapter 1, we briefly introduce 2D vdW materials and the essential physics of ferroelectricity. In Chapter 2, we provide an overview the principles and experimental setups of the optical techniques extensively used in this thesis.

In Chapter 3, we investigate the fundamental optical properties of NbOI2, including anisotropic electronic transitions, phonons, and their interactions. Using polarization-angle resolved Raman spectroscopy and transient reflectivity spectroscopy, we identified a ferroelectric soft phonon mode at 3.1 THz that interacts with the above-gap electronic transition.

In Chapter 4, we examined the THz generation of NbOI2 via optical rectification. We demonstrate its exceptional THz generation efficiency and its full potential for integration into on-chip heterostructures for a near-field THz probe. Furthermore, we identify a long-lived, single-frequency coherent oscillation persisting beyond a sampling time of 20 ps, a striking contrast to the benchmark THz emitter, ZnTe, whose signal damps within 3 ps. The Fourier transform of this coherent wave reveals an extremely narrowband emission centered at 3.1 THz, corresponding to the frequency of the ferroelectric phonon mode in NbOI2.

In Chapter 5, motivated by the observed long-lived THz radiation and the formal similarities between spin waves in magnets and polarization waves in ferroelectrics, we further explore the nature of the polarization waves in NbOI2. Through transient reflectivity spectroscopy with various pump-probe spot separations, we reveal the propagation characteristics of these waves in NbOI2, a unidirectional transport along the crystallographic polar axis at an ultrasonic speed of approximately 100 km/s. Detailed simulations further establish a connection between these propagating polarization waves and canalized hyperbolic phonon polaritons.

Finally, in Chapter 6, we explore the frequency tunability of these polarization waves. Through halogen substitution, alloying, and THz emission spectroscopy, we demonstrate wide tunability within the NbOX2 family (X = Cl, Br, I), with frequencies ranging from 3.1 to 5.8 THz, limited by the current detection window. Combined with DFT results, we establish a firm link between the spontaneous polarization and phonon eigenvectors. This connection leads to a theoretical phononic framework, providing a quantitative understanding of the origin of the narrowband THz radiation. We conclude that the vdW ferroelectric NbOX2 family represents a promising class of materials for the next-generation on-chip optical and information technologies.