As quasiparticles formed by the interaction of photons and matter, polaritons provide a unique way to use and manipulate light, which is of great significance to many research fields such as photophysics, photochemistry, and light information.
Metals are able to form surface plasmon Polaritons (SPP) through the action of their free electrons and photons, but SPP has a lower lifespan and many limitations in application. Compared with SPP, the two-dimensional material can not only support the propagation of polarization excitons, but also has less polarization exciton loss in the material, and has broad application prospects in integrated and ultra-thin nanophoton devices.
However, the polarization excitators in two-dimensional materials are subject to the regulation of dispersion when applied. Researchers can effectively regulate the dispersion of polarization excitons through structured two-dimensional materials such as graphene strips and hBN strips, as well as bilayer torsional graphene strips and α-MoO3 to achieve customizable polarization excitons, which is a key step toward on-demand control of polarization excitons.
However, the current existing regulatory methods lack dynamic tunables, and the active tuning of anisotropic polarization radicals is a major problem that scientists urgently need to solve. Recently, the team of Professor Chen Huanyang of the School of Physical Sciences and Technology of Xiamen University cooperated with the team of Professor Qiaoliang Bao, the team of Professor Dai Zhigao of China University of Geosciences and the team of Professor Qiu Chengwei of the National University of Singapore to bring a solution to this problem.
According to the introduction, "they achieved a tunable hybrid polarization excitation and its optical topology transformation at the heterostructural interface of the isotropic material graphene and the natural biaxial crystal α-MoO3, and experimentally verified the presence of hybrid polarization excitators in the graphene/α-MoO3 heterostructure with different thicknesses α-MoO3." ”
The paper, titled "Tailoring Topological Transitions of Anisotropic Polaritons by Interface Engineering in Biaxial Crystals," was published on the cover of Nano Letters, professor Qiaoliang Bao, Professor Chen Huanyang and Professor Dai Zhigao served as corresponding authors.
Figure | Current cover of Nano Letters (Source: Nano Letters)
It is understood that in the mid-infrared band, graphene can excite isotropic SPP, while α-MoO3 can excite phonon polarization excitator PhP (Phonon polaritons) in its remaining ray band.
This time, based on the excitation wavelength of PhP overlap in graphene SPP and α-MoO3, the team adjusted the dispersion of hybrid polarization excitators through different interface engineering of α-MoO3 and graphene, and obtained a customized anisotropic polarization excitator and its tunable optical topology transformation.
Optical topology transition refers to the change of the iso-frequency curve in the surface of the polarized excitator from a closed curve such as a circle and ellipse to an open curve. For isotropic materials, the in-plane isobar curve is circular, and the polarized exciton has the same propagation wave vector in all directions; and for anisotropic materials, when the dielectric constant is the same, its in-plane isometric curve is elliptical, and the polarized exciton has different wave vectors related to the direction of propagation; when the dielectric constant is different, its in-plane isobar curve is hyperbolic, and the polarized exciton not only has different wave vectors related to the direction of propagation, but also can support the wave forward propagation of infinitely large wave vectors, and such materials can be applied to negative refraction and super-resolution imaging.
Figure | Real-space imaging of graphene/α-MoO3 heterostructures (Credit: Nano Letters)
To verify hybridized polarization excitons in the graphene/α-MoO3 heterostructure, the researchers processed samples of them and α-MoO3. They found that in α-MoO3, the PhP excited by the boundary can only propagate in the 100 direction and not in the 001 direction, which is determined by its hyperbolic dispersion properties. Hybridized polarization excitons in the graphene/α-MoO3 heterostructure can propagate in the PhP forbidden direction in α-MoO3 and have anisotropic wave vectors in all directions.
At the same time, the team concluded that the wavelength of the polarized excitator is related to the excitation frequency, so the wavelength and wave vector of the polarized excitation element can be adjusted by changing the excitation frequency.
Figure | Tunable hybrid plasma-phonon polarization excitons in graphene/α-MoO3 heterostructures (Source: Nano Letters)
In addition, the team said that in the heterostructure it proposes, the interlayer coupling between graphene and α-MoO3 can be dynamically adjusted at the Fermi energy level. With the increase of graphene Fermi energy level, the wave vector of graphene SPP gradually decreases, and the influence of graphene SPP on hybrid excitons at low k gradually increases, so that the dispersion curve of hybridized polarization excitators changes from hyperbolic to elliptical, and the wavefront changes from open to closed.
The team then adjusted the thickness of α-MoO3 in the graphene/α-MoO3 heterostructure to try to control the dispersion of hybrid polarization excitants. The results show that the PhP excitated by α-MoO3 will have less and less influence on the hybridized polarization excitator as the thickness of α-MoO3 decreases, which reduces the anisotropy of the hybridized polarization excitator in the heterogeneous structure, and the dispersion curve eventually becomes an ellipse.
Figure | Thickness-related hybrid polarization excitons in graphene/α-MoO3 heterostructures (Source: Nano Letters)
Finally, the team calculated the life of hybridized polarization excitators in graphene/α-MoO3 heterostructures of different α-MoO3 thicknesses and was pleasantly surprised to find that the polarized excitons in the graphene/α-MoO3 heterogeneous structures had much longer lifespans than the PhP in α-MoO3.
The team said, "This research has laid the foundation for tunable photonic devices, optical signal processing and neuromorphic photonic circuits based on low-loss polarization excitons, and has been strongly supported by the National Natural Science Foundation of China, the Key Research and Development Program, the National Scholarship Council and the 111 Wisdom Introduction Project." ”
-End-
Reference:
1、Yali Zeng et al. Tailoring Topological Transitions of Anisotropic Polaritons by InterfaceEngineeringin Biaxial Crystals.Nano Letters（2022）https://doi.org/10.1021/acs.nanolett.2c00399