Efficient couplers are critical for achieving high-performance on-chip photonic devices. Current couplers for on-chip lasers and amplifiers lack flexibility, as they are limited to chip edges or corners, reducing system integration and chip utilization. A key challenge lies in effectively coupling multi-wavelength free-space light into arbitrary locations on a chip-based system. Common coupling methods include edge couplers​ and ​grating couplers. Edge couplers exhibit relatively low loss, broad bandwidth, low polarization dependence, but are restricted to chip edges and require edge polishing. Grating couplers, while offering flexibility in placement, suffer from higher loss, narrow bandwidth, and strong polarization dependence. Metasurface structures, capable of manipulating light’s amplitude, phase, polarization, and frequency, present an opportunity to address multi-wavelength coupling challenges. Recent studies have demonstrated phase-gradient metasurfaces as couplers, but their efficiency remains limited. We propose an ​inverse-designed broadband meta-coupler​ based on intelligent algorithms. Compared to forward design methods with constrained parameter spaces, our coupler achieves high coupling efficiency, broad bandwidth, and enables efficient coupling of broadband free-space light to arbitrary on-chip locations.

Figure 1. Schematic illustration of compact broadband meta-coupler by inverse design. (a) Schematic diagram of the principle of meta-coupler. (b) Unit of metasurface. (c) Meta-coupler on thin film LN without inverse design. (d) Meta-coupler on thin film LN with inverse design. (e) Enlarged view of the meta-coupler without inverse design. (f) Enlarged view of the meta-coupler with inverse design. (g) Cross-section diagram of the meta-coupler on thin film LN

The schematic illustration of compact broadband meta-coupler by inverse design is shown in Figure 1. The efficiency of meta-coupler with(out) inverse design is shown in Figure 2. Using the generalized Snell’s law, we designed a meta-coupler optimized for 1550 nm wavelength. Through inverse design, we developed couplers for both 1460 nm and 1550 nm wavelengths. Without inverse design, the coupling efficiency at 1460/1550 nm under TE plane wave illumination is 32%/85\% and with inverse design, the coupling efficiency at 1460/1550 nm under TE plane wave illumination is 77%/83%, with a ​3-dB bandwidth of 300 nm​ and ​1-dB bandwidth of 138 nm.

Figure 2. Efficiency of meta-coupler with(out) inverse design. The efficiency of the meta-coupler with inverse design under TE plane wave illumination is plotted in black. The efficiency of the meta-coupler without inverse design under TE plane wave illumination is plotted in cyan. The efficiency of the meta-coupler with inverse design under TM plane wave illumination is plotted in red. The efficiency of the meta-coupler without inverse design under TM plane wave illumination is plotted in green. The data at a wavelength of 1460 nm is marked by green points and the data at a wavelength of 1550 nm is marked by yellow points. The S(short)/ C(conventional)/L(long) band is highlighted by translucent yellow/purple/red areas.

This work was published in Journal of the Optical Society of America B: Zhiwei Wei, Jiangwei Wu, Chengyu Chen, Wenjie Wan, Yuping Chen, and Xianfeng Chen, "Inverse-designed broadband meta-coupler with high efficiency on thin-film lithium niobate,"J. Opt. Soc. Am. B​42, 597-602 (2025). Selected as a **"Spotlight on Optics"** by Optica Publishing Group:Spotlight Summary: https://opg.optica.org/spotlight/summary.cfm?id=568192.

Link：https://doi.org/10.1364/JOSAB.545358