The prosperous development of nanotechnology and photonics built their solid foundations on the design and invention of novel materials. We are targeting at the research area that covers materials science, nanotechnology and photonics, where new opportunities lie.

a. Twist structure in photonic crystal.

Photonics is the art of controlling photons. Recently, we are particularly interested in utilizing photonics as a powerful means to mimic the behaviors of electrons in condensed matter. We discovered that 2D photonic crystals that are stacked together at a twist angle may also exhibit novel band properties, such as the formation of flat photonic bands. Our group established a theoretical framework to rigorously solve the optical modes in a twisted photonic crystal system. This theory is based on the coupled mode theory in combination with a continuum model. With this new approach, we numerically demonstrated the photonic flat band and non-Anderson type of localization of light in twisted bilayer photonic crystals. Our generic approach can be widely utilized in future analysis of similar photonic structures.

b. Controllable growth of helical van der Waals crystals.

We developed a completely new approach that enables the direct growth of twisted 2D materials. By introducing screw dislocations to the vapor phase deposition of a typical 2D material GeS, we successfully achieved spiral growth of the 2D materials. Most interestingly, there is a twist between the adjacent layers. Such twists are so-called Eshelby twists and can be tuned during the growth process, giving us a great opportunity in exploring the twist degree of freedom. This work was recently reported in Nature.

c. Tuning nonlinear optical properties of twisted van der Waals materials.

We also looked further into the optical response of twisted 2D materials. In fact, the twisting of neighboring layers may break the inversion symmetry of the material, for example, in the twisted bilayer graphene system (tBLG). Unlike a monolayer of graphene which has inversion symmetry, tBLG allows the generation of second harmonics. More importantly, strong resonance at inter-band transitions near the van Hove singularities in tBLG enabled substantial enhancement of the second harmonic generation. The tunable resonant frequency depending on the twist angles also brings highly practical applications.

d. World's thinnest magnet.

The recent discovery of ferromagnetism in two-dimensional van der Waals crystals has provoked a surge of interest in the exploration of fundamental spin interaction in reduced dimensions. However, existing material candidates have several limitations, notably lacking intrinsic room-temperature ferromagnetic order and air stability. In our lab, we demonstrated room-temperature ferromagnetism in Co-doped graphene-like Zinc Oxide, a chemically stable layered material in air, down to single atom thickness. Through the magneto-optic Kerr effect, superconducting quantum interference device and X-ray magnetic circular dichroism measurements, we observe clear evidence of spontaneous magnetization in such exotic material systems at room temperature and above. Transmission electron microscopy and atomic force microscopy results explicitly exclude the existence of metallic Co or cobalt oxides clusters. X-ray characterizations reveal that the substitutional Co atoms form Co2+ states in the graphitic lattice of ZnO. By varying the Co doping level, we observe transitions between paramagnetic, ferromagnetic and less ordered phases due to the interplay between impurity-band-exchange and super-exchange interactions. Our discovery opens another path to 2D ferromagnetism at room temperature with the advantage of exceptional tunability and robustness.

e. Direct access of non-degenerate valleys in anisotropic materials at room temperature.

The field of valleytronics has promised greater control of electronic and spintronic systems with an additional valley degree of freedom. However, conventional and two-dimensional valleytronic systems pose practical challenges in the utilization of this valley degree of freedom. Recently we experimentally showed the valley effect in a bulk, ambient, and bias-free model system of Tin(II) sulfide. We elucidate the direct access and identification of different sets of valleys, based primarily on the selectivity in absorption and emission of linearly polarized light by optical reflection/transmission and photoluminescence measure- ments, and demonstrate strong optical dichroic anisotropy of up to 600% and nominal polarization degrees of up to 96% for the two valleys with band-gap values 1.28 and 1.48 eV, respectively; the ease of valley selection further manifested in their non-degenerate nature. Such discovery enables a new platform for better access and control of valley polarization.