超透镜除了可以实现传统透镜的聚焦和成像功能之外，还可通过对超构单元的设计实现对光场偏振、波长和振幅等的多维度操控，由于体积薄、重量轻、成本低、易集成，其在光电子器件领域中开始崭露头角，已经成为当前的研究热点和重要方向。本文采用时域有限差分算法（Finite?difference time?domain，FDTD）设计并优化了基于InGaAs雪崩探测器原位集成的超透镜，同时估算了超透镜的聚焦效率和透射率。仿真结果表明，超透镜将入射光会聚至探测器的光敏区中，透射率达到82.8%，并且在目标焦距150 μm、超透镜半径50 μm时聚焦效率达到 84.89%。为进一步提高透射率，在超透镜表面增加抗反射层（AR Layer）。结果表明，300 nm的SiO₂层透射率达到最大值86.6%，250 nm的SiN层透射率达到最大值87.6%，透射率比未增加AR层时分别增加了3.8%和4.8%。最后计算得出集成超透镜的探测器吸收区光场能量比未集成超透镜的探测器吸收区光场能量提升了250.96倍，将极大提升探测器的响应度。本文提出了单片集成超透镜的雪崩探测器设计方案，将雪崩探测器光敏区之外的入射光会聚至光敏区，在不损失带宽前提下提升探测器的量子效率，为高响应度、带宽雪崩探测器的设计提供了新思路。

A high-efficiency inverse design of “digital” subwavelength nanophotonic devices using the adjoint method is proposed. We design a single-mode 3 dB power divider and a dual-mode demultiplexer to demonstrate the efficiency of the proposed inverse design approach, called the digitized adjoint method, for single- and dual-object optimization, respectively. The optimization comprises three stages: 1) continuous variation for an “analog” pattern; 2) forced permittivity biasing for a “quasi-digital” pattern; and 3) a multilevel digital pattern. Compared with the conventional brute-force method, the proposed method can improve design efficiency by about five times, and the performance optimization can reach approximately the same level. The method takes advantages of adjoint sensitivity analysis and digital subwavelength structure and creates a new way for the efficient and high-performance design of compact digital subwavelength nanophotonic devices, which could overcome the efficiency bottleneck of the brute-force method, which is restricted by the number of pixels of a digital pattern, and improve the device performance by extending a conventional binary pattern to a multilevel one.

With the rapidly increasing bandwidth requirements of optical communication networks, compact and low-cost large-scale optical switches become necessary. Silicon photonics is a promising technology due to its small footprint, cost competitiveness, and high bandwidth density. In this paper, we demonstrate a 12×12 silicon wavelength routing switch employing cascaded arrayed waveguide gratings (AWGs) connected by a silicon waveguide interconnection network on a single chip. We optimize the connecting strategy of the crossing structure to reduce the switch’s footprint. We develop an algorithm based on minimum standard deviation to minimize the port-to-port insertion loss (IL) fluctuation of the switch globally. The simulated port-to-port IL fluctuation decreases by about 3 dB compared with that of the conventional one. The average measured port-to-port IL is 13.03 dB, with a standard deviation of 0.78 dB and a fluctuation of 2.39 dB. The device can be used for wide applications in core networks and data centers.

We propose and experimentally demonstrate a novel ultracompact dual-mode waveguide crossing based on subwavelength multimode-interference couplers for a densely integrated on-chip mode-division multiplexing system. By engineering the lateral-cladding material index and manipulating phase profiles of light at the nanoscale using an improved inverse design method, a subwavelength structure could theoretically realize the identical beat length for both TE0 and TE1, which can reduce the scale of the device greatly. The fabricated device occupied a footprint of only 4.8 μm×4.8 μm. The measured insertion losses and crosstalks were less than 0.6 dB and 24 dB from 1530 nm to 1590 nm for both TE0 and TE1 modes, respectively. Furthermore, our scheme could also be expanded to design waveguide crossings that support more modes.