Recent advances in femtosecond laser direct writing of three-dimensional periodic photonic structures in transparent materials

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Abstract
By periodically altering the optical properties (e.g., refractive indices and nonlinear optical coefficients) of transparent materials, a diverse range of periodic photonic structures can be created, including nonlinear photonic crystals (NPCs) and optical waveguide arrays.1 – 3 The significance of NPCs lies in their applications for frequency conversion and beam shaping,4 – 13 which are typically fabricated in nonlinear optical crystals using electrical poling techniques.14,15 However, these NPCs are limited to one-dimensional (1D) and two-dimensional (2D) configurations, as the electrical poling method cannot produce three-dimensional (3D) NPCs. This limitation has significantly restricted the diverse photonic applications of NPCs in 3D space for an extended period.4,16 Optical waveguides are fundamental components in the field of integrated optics,17 – 30 allowing light to be confined to very small areas (on the microscale or even nanoscale) for nondiffractive propagation. Techniques such as ion implantation,31,32 ion exchange,33,34 and Ti-in-diffusion35 – 37 have been employed to fabricate high-quality 1D and 2D optical waveguides in transparent materials; however, they are inadequate for creating 3D waveguide structures, particularly 3D optical waveguide arrays. Similar to the challenges faced in manufacturing 3D NPCs, conventional waveguide fabrication methods (e.g., ion implantation, ion exchange, and Ti-in-diffusion) lack the capability to produce 3D optical waveguide arrays, which hinders the advancement of high-performance and multifunctional 3D waveguide devices. The femtosecond laser direct writing technique presents a truly 3D processing approach,3,38 – 70 effectively addressing these challenges in the fabrication of both 3D NPCs and 3D optical waveguide arrays.
Funding Information
  • National Natural Science Foundation of China (12204274)
  • Key Technology Research and Development Program of Shandong Province (2024ZLGX02-3)
  • Natural Science Foundation of Shandong Province (ZR2022QA033)
  • Taishan Scholar Foundation of Shandong Province (tspd20210303)