Recently, Professor Dong Jianwen's team from the State Key Laboratory of Optoelectronic Materials and Technologies and the School of Physics at Sun Yat-sen University has discovered the pseudospin polarized topological properties of double-layer metasurface gratings. They elucidated that continuous domain bound states and unidirectional mode resonances are two distinct types of pseudospin images associated with topological optical modes. Moreover, asymmetric radiation under topological protection can be utilized for coherent perfect absorption with continuously tunable phase difference. These findings have been published in Physical Review Letters.
"Having a profound understanding and flexible control over asymmetric radiation behavior holds significant implications for applications in asymmetric light modulation," stated Zhuang Zepeng, the first author of the paper and a doctoral student at the School of Physics, Sun Yat-sen University. In recent years, far-field polarization vortices in periodic subwavelength structures, represented by photonic crystal slabs, have garnered widespread attention. Researchers have unveiled the topological properties behind two distinct asymmetric radiation phenomena: continuous domain bound states and unidirectional mode resonances. However, for general asymmetric radiation, encompassing arbitrary unidirectionality and phase differences, depicting the image of momentum-space polarization vortices proves challenging, thus greatly limiting the exploration of the evolution and modulation mechanisms of asymmetric radiation.
Building upon their previous studies on energy valley waveguides and angular mode microcavities, Professor Dong Jianwen's team introduced topological optical principles into the research on asymmetric radiation of metasurface gratings. They proposed a pseudospin physical image for general asymmetric radiation, mapping arbitrary asymmetric radiation to geometric parameters of pseudospin. By combining the Poincaré sphere, the team discovered that continuous domain bound states and unidirectional mode resonances correspond respectively to the poles of the sphere and the S2 axis, while general asymmetric radiation behavior can be fully described by the Poincaré sphere surface. Taking double-layer metasurface gratings as an example, the team further investigated the formation mechanism and evolutionary laws of pseudospin vortices in the synthetic parameter space (k_x - \Delta g). They found that continuous domain bound states induce pseudospin vortices with integer topological charges in the parameter space.
When breaking the spatial inversion symmetry, pseudospin vortices split into a pair of circular polarization points with half-integer values, enabling the coverage of a wide range of the Poincaré sphere surface. This allows independent control of radiation unidirectionality and phase difference based on double-layer metasurface gratings. Moreover, the team demonstrated a novel application of coherent perfect absorption with continuously tunable phase difference. Coherent perfect absorption mechanisms can greatly enhance the light absorption capability of weakly absorbing materials, also known as anti-lasers. The team theoretically analyzed that asymmetric radiation modulation can be applied to customizing the design of conditions for coherent perfect absorption.
In the preceding research, the conservation law of topological charges ensures the stable existence of a pair of circular polarization points in the parameter space, thereby guaranteeing continuous tunability of the phase difference of incident light for coherent perfect absorption within the range of (-\pi/2) to (\pi/2). Through theoretical calculations and full-wave simulations, Professor Dong Jianwen's team demonstrated that the magnitude of coherent absorption can be flexibly controlled by the phase difference of incident light, with peak values corresponding to phase differences customizable within the range of (-\pi/2) to (\pi/2).
Dong Jianwen, the corresponding author of the paper, stated that this work provides a new perspective on the topological properties of asymmetric radiation optical behavior in subwavelength structures and holds the potential to realize novel photonic devices for light control and asymmetric dynamic modulation, with broad applications in optoelectronic detection, thermal radiation, micro-LED light control, quantum emission, and other fields.
Related Paper Information: https://doi.org/10.1103/PhysRevLett.132.113801
