Beam deflection is of fundamental importance in optical beam manipulation. It has potential applications in many fields, such as the LiDAR systems, advanced chips, free-space optical communication, and high-sensitivity sensing. Therefore, it is of great significance to achieve the large-angle and high-efficiency beam deflection for the design of the optical components.
The general ways to deflect the beams are based on the principles of phase accumulation or linear phase gradient, but they are too big in size and hard to integrate, or have low efficiency when the deflection angle is large. Recently, metagrating, one of the emerging sub-wavelength optical structure, has been proposed. It offers a new way to realize high-efficiency and large-angle beam deflection, by combining the Mie scattering behaviors of tailored meta-atoms with the diffraction effect of the grating. However, most of them are constructed by complicated structure with time-consuming design algorithm or extreme nano-parameters for fabrication. Besides, the metagratings based on the metallic rods may suffer from the intrinsic ohmic metallic loss. Consequently, designing metagrating with simple structure, low-loss and high-efficiency is necessary and beneficial to large-scale production and the application of high-performance wavefront shaping.
The research group led by Professor Jianwen Dong from Sun Yat-sen University proposed a rectangular dielectric metagrating to achieve large-angle and high-efficiency beam deflection. The results have been published in Chinese Optics Letters, Vol.18, Issue 7, 2020 (Wei-Yi Shi, Wei-Min Deng, Wei-Nan Liu, et al. Rectangle Dielectric Metagrating for Perfect Diffraction with Large-Angle Deflection[J]. Chinese Optics Letters, 2020, 18(7):073601).
They concentrated almost all the energy to a specific diffraction order and suppress others by the simple rectangular units. The deflecting angle reaches to 90°, and highest efficiency obtained is 97.55%. Moreover, the efficiency maintains over 90% for a wide range of incident angles (31° to 64°), acting as an element with wide-angle tolerance. This phenomenon is explained by the waveguide-mode expansion and the scattering-matrix method. Besides, they also considered the practicability of the grating and found that the diffraction efficiency could still maintain the high value after considering the influences of the material absorption, material dispersion and the substrate.
Schematic illustration of the rectangular dielectric metagrating for beam deflection with high diffraction efficiency
"Metagrating is of great significance for the spectrum tailoring or beam manipulation. People can not only substantially decrease the volume and weight of the conventional gratings, but also achieve some novel properties that the conventional elements do not have by utilizing the resonant properties of these sub-wavelength units." says the corresponding author Professor Jian-Wen Dong.
The metagrating in this work has the excellent properties of simple structure consisting, large-angle deflection and high diffraction efficiency. It will be a fundamental optical element and play an important role in the fields of spectral detecting, high-resolution imaging, and the planar optics.
