Metasurface is a kind of light field control device with flexible design and compact structure. It can flexibly manipulate the phase, amplitude, polarization and wavefront of light by precisely designing its micro-nano structural units of sub-wavelength size. Therefore, it holds significant application value in the fields of holographic displays and optical information encryption. However, due to the fixity of optical material properties and structures, the functionality of metasurface devices is limited. Consequently, the use of external stimuli (such as heat, electricity, force, magnetism, etc.) to achieve dynamically tunable metasurfaces has become a research focus in recent years. Since magnetic field control stands out for its sub-nanosecond ultra-fast response and non-invasive characteristics, endowing magnetically controllable metasurfaces with immense potential for dynamic light manipulation.
Magneto-optical (MO) metasurfaces are composite artificial microstructures that contain magneto-optical functional materials. Under the action of an external magnetic field, the magneto-optical materials endow them with the ability to dynamically control the phase, amplitude, and polarization of light. MO metasurfaces can provide multiple degree of freedom for light control via magneto-optical effects, such as the Faraday effect and Kerr effect. However, in terms of the MO modulation, it is challenging to make the nanostructure achieve a strong MO response due to the weak effect of the MO material. Therefore, most studies emphasis on using magnetoplasmonic or dielectric resonance to improve the modulation efficiency of the amplitude and polarization rotation of the MO metasurface. Nevertheless, few studies have focused on the phase modulation of MO metasurfaces by flexibly designing the nanostructure. Although it has been proved that magnetically controllable metasurface can realize superchiral light field manipulation and dynamic wavefront control, they are mainly applicable in the terahertz regime with relatively few reports on the use in optical holographic displays.
To solve the above problems, Yu Bi from Shandong Inspur Artificial Intelligence Research Institute Co., Ltd., and Professor Lingling Huang from Beijing Institute of Technology and the team members have jointly designed a MO metasurface composed of metal nanoantenna array-magneto-optical layer - metallic reflective layer. They have simulated and verified the function of magnetically controllable metasurface for multi-channel polarization multiplexed dynamic holographic display based on the binary amplitude holographic algorithm. The research results are published in Chinese Optics Letters, Vol. 22, Issue 4, 2024: Yu Bi, Lingling Huang, Tuo Li, Changhong Wang, Xiaofeng Zou, Lang Zhou, Guoguo Kang. Active metasurface via magnetic control for tri-channel polarization multiplexing holography[J]. Chinese Optics Letters, 2024, 22(4): 043601.
In this work, the researchers demonstrate the identical MO metasurface can realize switchable multichannel holographic display by magnetic control and suitable nanoantennas arrangement. When an external magnetic field is applied or not, the three different holographic images are reconstructed in the linear and circular polarization channel. The switchable reconstructed images such as the pattern "flower" (in the Exx channel), the Chinese character "中" (in the Ell channel), and the pattern "butterfly" (in the Erl channel) can be observed in the Fourier plane. This study provides a new technical approach for constructing actively tunable metasurface and enriches the technical achievements in phase modulation of magnetic controllable metasurface.
Schematic of the magnetically controllable multichannel holographic display based on MO metasurface.
This work will encourage researchers to explore different technical means in dynamic holographic display, and promote the development and application of magnetic controllable metasurface in optical information encryption. In the future, the team will focus on the field of "optical information display and encryption", dedicating their efforts to the study of the physical mechanisms and applications of magnetically controlled dynamically tunable metasurfaces for multi-dimensional information multiplexing. They will continuously explore new concepts, principles, and methods for light field manipulation.
