image: A PT-symmetric non-Hermitian photonic system composed of specifically designed metal structures is used to conduct arbitrary polarization transition from the equator and generate polarization vortices with exceptional point on the Poincaré sphere’s poles, which induces unidirectional hyperbolic polaritons propagation due to the break of mirror symmetry.
Credit: ©Science China Press
-Non-Hermitian photonics: definition and study both the intrinsic degrees of freedom of an open photonic system and the interactions with external environment. Special attention is given to the associated degeneracies also known as ‘exceptional points’ with biorthogonal eigenvectors.
-Rapid developments of these concepts over the past decade have already empowered us to revealing counter-intuitive optical phenomena, illuminating a host of thrilling possibilities for next-generation photonic technologies.
-In a recent paper published in Science Bulletin, the authors describe a non-Hermitian photonic system of tailored metal structures that are capable of manipulating field distribution on demand. This study is led by Dr. Zhiwei Guo, Dr. Hong Chen (MOE Key Laboratory of Advanced Micro-Structured Materials, School of Physics Science and Engineering, Tongji University) and Dr. Jiahua Duan (Center for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE), School of Physics, Beijing Institute of Technology). With the help of non-Hermitian photonics, they unveil a planar metasource that converts linear incident polarization into any polarization state on demand.
“Normally, electromagnetic wave propagation is intrinsically determined by the symmetry of crystal systems. For example, asymmetric propagation of tilted hyperbolic shear polaritons only emerges in low-symmetry monoclinic crystals.” The authors say, “An interesting question is how to achieve similar asymmetric propagation of polaritons in high-symmetry crystals with further flexible control over them.” To do that, Guo and Duan, together with Chen, sought to increase the degree of freedom of manipulation through the excitation source of hyperbolic waves. In MOE key laboratory of advanced micro-structure materials, they designed novel chiral dipoles and successfully imaged the asymmetric propagation of hyperbolic waves (i.e., the shear effect) without lowering the crystal symmetry.
“The chiral dipole constructed by exceptional point is a feasible scheme for exploring the near-field routing. We consider a tailored square resonator to inducing the coupling between the horizontal and vertical modes. Furthermore, we can change the chamfer size to modulate the coupling strength, enabling the exceptional point with self-orthogonal characteristic.” Chen says.
“In previous works, the square resonator without tailoring produces a normal dipole mode under linear incidence polarization (i.e., linear-polarized dipoles). However, in our non-Hermitian photonic system of tailored square resonator, we find that the bright and dark modes coalesce at the exceptional point, producing excitation with circular polarization. More interestingly, such design can control the polarization on demand without changing the incident polarization, i.e., Poincare engineering of polariton waves.” Guo says.
“What surprised me was that this resonance response can be tuned over an ultrabroadband frequency range from microwave to infrared regime by easily changing the resonator size. For example, chiral dipoles can promote asymmetric polariton propagation in boron nitride waveguides and thus selectively guide the mid-infrared energy flow at the nanoscale. These robust chiral sources will be useful for nanophotonic applications, e.g., polariton routing, photonic integration and (bio)-sensing.” Duan says.
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See the article:
Exceptional point empowered near-field routing of hyperbolic polaritons
https://doi.org/10.1016/j.scib.2024.09.027
Journal
Science Bulletin