Advances and Challenges in Polymer Three-Dimensional Photonic Integrated Circuits (Invited)

Significance Photonic integrated circuits (PICs) have been extensively researched and applied in optical interconnections, optical communication, and LiDAR. To further expand the scale and performance of photonic chips, three-dimensional (3D) PICs have emerged as a prominent research focus. 3D PICs represent an advanced type of PICs that achieve spatial expansion through coupling or three-dimensional waveguides, allowing light to propagate beyond a two-dimensional plane within the waveguide. Presently, most research and development in 3D PICs is focused on inorganic materials such as silicon, silicon nitride, and high-index silica. The preparation of 3D PICs on these material platforms necessitates polishing processes at the wafer level, which significantly increases fabrication complexity and cost. Among various optical platforms, polymer-based planar lightwave circuits stand out for their flexibility, low power consumption, and high performance. Polymers are cost-effective and can be processed using simple techniques such as spin coating and photolithography. Their fluidity allows for the creation of flat cladding without additional polishing, providing a foundation for multi-layer devices. This fluidity also simplifies hybrid integration with other material platforms. In the field of laser direct writing, many significant works have focused on waveguides based on inorganic materials. While these devices offer clear advantages in terms of propagation loss, they are limited in their capacity for modulation and reconfiguration, which impedes further functional expansion. In recent years, there has been the development of high-performance hybrid integrated photonic devices using unconventional materials such as.. V group material, lithium niobate, and lithium tantalate, in conjunction with conventional silicon waveguides. Three-dimensional integration is regarded as the inevitable path to achieving high-performance hybrid integrated photonic devices. The polymer photonic platform presents a flexible and cost-effective alternative for developing 3D hybrid integrated chips, thus expanding future functionalities. Progress Polymer-based 3D PICs are primarily fabricated using ultrafast laser inscription (ULI) and multi-layer stacking techniques. ULI offers precise machining and flexibility. For instance, Christian Koos's group introduced the concept of photonic wire bonding (PWB) in 2012, enabling the connection of polymer waveguides with three-dimensional geometries to bridge nanophotonic circuits across different chips ( Fig. 1). In 2018, they utilized PWB technology to connect indium phosphide (InP)-based horizontal-cavity surface-emitting lasers to passive silicon photonic circuits, achieving insertion losses as low as 0.4 dB (Fig. 2). PWB technology paves the way for hybrid photonic multi-chip assemblies that integrate known-good dies of different materials into high-performance hybrid multi-chip modules. Meanwhile, researchers at HHI have theoretically investigated and experimentally demonstrated the use of 3D PolyBoard PICs with multiple waveguide layers as a practical solution for realizing two-dimensional optical phased arrays ( OPAs) with end-fire waveguides ( Fig. 4). Similarly, Min-Cheol Oh's group has developed a design and fabrication process for 3D hybrid integration OPA using silicon nitride and polymer (Figs. 13 and 14). In our group, we focus on the functional integration of polymers using 3D polymer PIC fabrication technology. We have demonstrated a dual-layer optical encryption fluorescent polymer waveguide chip based on optical pulse-code modulation technique (Fig. 7) and a 3D optical switch with thermo-optical (TO) and electro-optical (EO) tuning effects (Fig. 12). In addition, we also researched on the fabrication technology and design of hybrid integration of organic and inorganic waveguides (Figs. 9 and 10). Polymer-based 3D PICs not only provide an expandable physical dimension but also offer an optical platform compatible with a variety of materials. Conclusions and Prospects Techniques like ultrafine laser processing, deposition, and lithography enable the preparation of complex 3D PICs. Current efforts focus on scaling up and enhancing functionalities such as phase shifters. Polymers offer potential for achieving gain amplification, optical nonlinearity, and other special properties, warranting further exploration to improve PICs'comprehensive performance parameters. Despite their advantages, polymers face challenges like temperature and humidity sensitivity, limiting their use in extreme environments. However, progress in addressing these stability issues is ongoing. Continued research and development in polymer materials for photonic platform, particularly in 3D configurations, promise advancements not only in traditional optical communication and optical interconnect but also in quantum and space optics, leveraging processing and performance characteristics.