基于拓扑优化的自由形状波分复用超光栅

Freeform wavelength division multiplexing metagrating based on topology optimization

超表面由亚波长尺度排列的人工原子阵列组成,在调控光场相位、振幅、偏振等方面具有巨大优势.受离散采样原理和周期性假设的限制,传统正向设计方法不可避免地存在设计误差,容易导致器件性能下降.本文采用基于伴随的多目标拓扑优化方法,逆向设计了一种具有大偏折角度、偏振不敏感特性的自由形状波分复用超光栅.仿真结果表明,相比于离散规则结构,拓扑优化的波分复用超光栅具有更优越的偏振不敏感性能.此外,该结构对510 nm入射光的偏折角度可达70.8°,其绝对偏折效率高达48%;对于852 nm入射光,其透射效率为98%.在此基础上,通过使用随机初始结构可将绝对偏折效率优化至70%以上.本文设计的自由形状波分复用超光栅具有偏折角度大、效率高和空间串扰低等优点,在光通信、微纳光场调控、基于里德堡原子的微波测量等领域具有潜在应用前景.

Metasurfaces consist of arrays of artificial atoms arranged on a subwavelength scale, and have significant advantages in modulating the phase, amplitude, and polarization of optical field. Limited by the discrete sampling principle and the assumption of periodicity, the conventional forward design method suffers unavoidable design errors, which easily leads the device performance to degrade. In this paper, a freeform wavelength division multiplexing(WDM) metagrating with a large deflection angle and polarization-insensitive characteristics is inversely designed by using an adjoint multi-objective topology optimization method. The simulation results show that the topology-optimized WDM metagrating has superior polarization in sensitivity compared with the discrete regular structure, with a deflection angle of 70.8° at 510 nm, an absolute deflection efficiency of 48%, and a transmission efficiency of 98% for 852 nm incident light. On this basis, the absolute deflection efficiency can be optimized to more than 70% by using a random initial structure. The freeform WDM metagrating designed in this paper has the advantages of large deflection angle, high efficiency, and low spatial crosstalk, and has potential applications in optical communication, micro and nano-optical field modulation, and Rydberg atom-based microwave measurements.