image: Figure 1 | Charge-transfer Mott insulator and the extreme weak interlayer coupling.a, schematic of the charge-transfer Mott insulator state using the Hubbard U treatment, where the parameters Δ and U indicate the charge-transfer energy and on-site Coulomb energy, respectively. b, structure of 2D VOCl along the a-axis. c, orbital-projected band structures for V 3d orbitals. d, Band gap values as a function of the number of layers in VOCl flakes. e, calculated electronic structures for monolayer (left) and bulk (right) VOCl. f, THG process-dependent on excitation intensity at 1558 nm. g, variation of χ(3) with thickness.
Credit: Zheng Liu et al.
Nonlinear optical processes and their anisotropy are foundational to advanced optics, directly governing the function and efficiency of on-chip photonics and nano-optical devices. With integrated photonics surging, there is a pressing need for 2D materials that combine strong nonlinearity with pronounced optical anisotropy. Owing to correlation-induced carrier localization and a charge-transfer gap, Mott insulators couple electronic, spin, and orbital degrees of freedom to yield excitations and optical phenomena distinct from conventional band insulators.
In a new paper published in Light: Science & Applications, a team of scientists, led by Prof. Zheng Liu and Prof. Qi Jie Wang (Nanyang Technological University), together with Prof. Xingji Li (Harbin Institute of Technology) and collaborators, reports that the 2D charge-transfer Mott insulator VOCl exhibits a strong nonlinear response and a record nonlinear optical anisotropy: under 1280-nm excitation, the third-harmonic generation (THG) anisotropy ratio reaches ρTHG = 187, the highest known among van der Waals materials. VOCl also shows nearly thickness-independent third-order susceptibilities (χ(3) ~10-19 m2/V2), highlighting its extremely weak interlayer coupling.
The study attributes this colossal anisotropy to the synergistic effect of strong electron correlations in the charge-transfer Mott insulating state and intrinsic C3 symmetry breaking. These unique characteristics make VOCl a promising building block for integrated photonic devices such as polarization beam splitters, polarized ultrafast lasers, and upconversion photodetectors.
“This discovery not only expands our understanding of correlated 2D Mott insulators but also provides a new platform for high-performance nanophononics,” said Prof. Zheng Liu, the corresponding author from NTU.
Article Title
Colossal infrared nonlinear optical anisotropy in a 2D charge-transfer Mott insulator