Finding Ferrons

Taketo Handa, Ph.D., Columbia University  

Abstract:  In this talk, I will share my story on the experimental finding of coherent ferrons, i.e., collective modes of ferroelectric order predicted over 60 years ago. This results from a combination of new experiments, sheer luck, and re-reading of textbooks.

Our research has focused on quantum materials, particularly their elementary excitations in the low-energy terahertz (THz) region. While two-dimensional (2D) materials and their van der Waals (vdW) interfaces have emerged as the most versatile platform for exploring fascinating quantum phenomena, determining their THz responses has been very challenging due to a severe mismatch between sample sizes (1–10 µm) and wavelengths (0.1 – 1 mm) of THz radiation. A conventional far-field THz spectroscopy typically requires sample sizes ranging from millimeters to centimeters [1]. To overcome the size mismatch problem, we developed near-field THz spectroscopy based on a vdW THz emitter [2]. We discovered that the 2D vdW in-plane ferroelectric NbOI₂ emits intense and broadband THz radiation via optical rectification, with an efficiency 20-50x higher than that of the current standard emitter, ZnTe. This 2D vdW THz emitter can be easily integrated with vdW structures for in situ THz spectroscopy and microscopy, while uniquely providing a broad spectral range and polarization sensitivity. 

Then came the big surprise: in addition to broadband THz emission due to optical rectification, we observed extremely narrowband and intense THz radiation from NbOI₂ at the ferroelectric transverse optical (TO) phonon frequency for its thinner limit. This serendipitous discovery puzzled us: why were we the first to observe THz emission from the TO phonon? We solved this puzzle recently [3]. It turned out to be the long-predicted coherent ferron, i.e., collective excitation of the ferroelectric order. From the Maxwell equations, we show that the narrowband THz emission is driven by the ferroelectric TO phonon that coherently modulates the macroscopic polarization. Another key observation is the unidirectional propagation of coherent ferrons along the polar axis at an extremely hypersonic velocity exceeding 10⁵ m/s, in agreement with modeling based on the Landau-Khalatnikov-Tani equation. The observation of coherent ferrons opens the door to ferronics for novel, energy-efficient information processing and transport.

[1] T. Handa et al., Sci. Adv. 10, eadj4060 (2024).

[2] T. Handa et al., Nat. Mater. 24, 1203–1208 (2025).

[3] J. Choe^*, T. Handa^*,†, et al., submitted. (^* equal contributions; ^† co-correspondence).

Bio: Taketo Handa is currently a postdoctoral researcher at Columbia University working with Prof. Xiaoyang Zhu, where he investigates quasiparticle dynamics and collective excitations in 2D materials and van der Waals structures using high-sensitivity terahertz spectroscopy and microscopy. Taketo received his B.S. in Physics from Nagoya University in 2015 and his Ph.D. in Physics from Kyoto University in 2020 working with Prof. Yoshihiko Kanemitsu. His work at Kyoto focused on the spectroscopic studies of optical properties of metal halide perovskites and photovoltaic devices. He was supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship at Kyoto and the JSPS Overseas Research Fellowship at Columbia.