title: Nonlinear Optical Beam Processing in 3D Nonlinear Photonic Crystals
Creators: name: Liu, Shan
Publisher: name: The Australian National University
Year: 2025

description: For decades, scientists have focused on how to efficiently facilitate various nonlinear optical processes in the field of nonlinear optics, particularly second harmonic generation. Limited by the dispersion properties of natural nonlinear materials and the resulting phase mismatch between light waves, only a portion of these nonlinear processes can be efficiently conducted. However, artificial manipulation of nonlinear materials can achieve Quasi-phase matching (QPM), significantly expanding the application range of nonlinear material. Such artificially processed materials are known as nonlinear photonic crystals (NPCs). The traditional method of fabricating NPCs is the electric field poling technique, which involves preparing microelectrodes of specific shapes on the surface of the nonlinear material wafer and applying high voltage on both sides of the wafer to locally alter the material's properties. This method processes the material from underneath the electrodes right through to the other side, thus making it unsuitable for fabricating complex three-dimensional structure NPCs. The latest technique, however, utilizes femtosecond laser direct writing. This technique focuses femtosecond laser pulses inside the nonlinear material, where multi-photon absorption and the material's interaction at the focal point allow for artificial manipulation. This technique is highly innovative, and both the technology itself and its applications are worthy of further research. This thesis focuses on the aforementioned issues, exploring the design, fabrication, and characterization of various types of 3D NPCs fabricated by the femtosecond laser direct writing technique. Specifically, this thesis is organized as follows. In Chapter 1, we review the development history and significant research breakthroughs in the field of nonlinear photonic crystals. Some important concepts are also discussed. The purpose of this chapter is to provide readers who are unfamiliar with this field with a general understanding. Those already familiar with the subject may choose to skip this chapter. Chapter 2 delves into more specific physical concepts and models required in the field of nonlinear optics. It also introduces essential mathematical tools like Fourier transformation, which will be utilized in subsequent chapters. From Chapter 3 onwards, we present specific research projects. Chapter 3 describes our use of femtosecond laser direct writing to fabricate 3D NPCs capable of nonlinear wavefront shaping. This work experimentally demonstrates the superiority of 3D NPCs over 2D NPCs. Chapter 4, building upon the foundation laid in Chapter 3, improves the structure of the 3D NPCs. Using the theory of nonlinear volume holography, we designed and fabricated 3D NPCs with high nonlinear conversion efficiency capable of nonlinear wavefront shaping. Chapter 5 systematically studies the mechanism of laser poling and optimizes this technique, enabling us to fabricate larger and more efficient 3D NPCs. Chapter 6 showcases large-volume 3D NPCs achieved through the improved technique and mature 3D NPC structural design theory. These samples have a nonlinear conversion efficiency of up to 4.6%.