Ultrafast direct laser writing of 2D materials for multifunctional photonics devices

Ultrafast direct laser writing of 2D materials

Two-dimensional (2D) materials usually refer to materials consisting of mono or a few layers of atoms, with thicknesses varying from one atomic layer to more than 10 nm. Various 2D materials have been successfully isolated, including graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDCs), black phosphorus (BP), and perovskite. 2D materials exhibit exotic physical and chemical properties such as atomic thickness, strong nonlinear optical properties, magnetic properties, and excellent mechanical strength that are different from their bulk counterparts, opening new opportunities for nanodevices, especially photonics applications.

Micro/nanostructures and functional devices in 2D materials have been proposed and fabricated by various fabrication techniques in order to fulfil the intriguing properties of 2D materials. Ultrafast direct laser writing (DLW) with the advantages of rich light-matter interaction mechanisms and dynamics; unique three-dimensional (3D) processing capability; arbitrary-shape design flexibility; and minimized thermal effect, which enables high fabrication resolution of tens of nanometers, has been widely used in 2D materials patterning, modification, and functionalization, demonstrating versatile capabilities.

This timely review written by the research group from Swinburne University of Technology captures some of the most exciting advancements in the field and provides an in-depth summary and understanding of the latest functional photonic devices enabled by 2D materials and the DLW method. The review has been published in Chinese Optics Letters, Vol 18, Issue 2, 2020 (Tieshan Yang, Han Lin, Baohua Jia. Ultrafast direct laser writing of 2D materials for multifunctional photonics devices [Invited][J]. Chinese Optics Letters, 2020, 18(2): 023601).

First, a briefly overview of the physical property changes of 2D materials upon laser exposure was provided. Laser-matter interactions may involve several processes: single/multiphoton absorption, material ablation under laser exposure, phase change of nanostructures, and chemical/physical properties modifications. These processes lead to different physical and chemical property changes of materials, like tuning the refractive indices (n) and extinction coefficients (k), bandgap engineering, conductivity changes, and surface wetting properties (hydrophilic or hydrophobic), which are the fundamental basis for various functional photonics or optoelectronics device designs. Furthermore, light and 2D materials interactions can enable micro/nano-patterning, laser thinning, and laser doping of various 2D materials, which are critical for high-resolution processing and functionalization of 2D materials down to a monolayer accuracy. Then, the advantages and limitations of 2D materials functional photonic devices were elucidated, including ultrathin flat lenses, graphene metamaterials, perfect absorbers, holographic displays, etc. fabricated by DLW toward practical applications. Moreover, ultrafast DLW has been used to locally change the nonlinear properties of 2D materials, which offers a new flexibility in directly converting the conventional photonic devices into highly nonlinear systems by simply integrating a layer of 2D materials, leaping the device performance for ultrafast, all-optical communication devices. Finally, the challenges, opportunities, and perspectives in this field were provided.

"Ultrafast DLW is an indispensable tool to fabricate 2D material functional photonics devices with excellent performance." says the corresponding author Professor Baohua Jia from the research group, " Ultrafast lasers can drive a wide range of processes for the patterning and functionalization of 2D materials with a high resolution, accuracy and cost-effectiveness that can be used for scalable processing and realization of the next-generation high-performance portable, integratable, and flexible devices."

Combining the ultrafast DLW technique with the parallel writing and super-resolution methods, is promising to develop a novel laser fabrication platform enabling multifunctional and scalable 2D material integrated devices for multidisciplinary research and applications.

超快激光直写二维材料制备多功能光子学器件综述

二维材料超快激光直写示意图

二维材料通常指含有单层或多层原子的材料，其厚度从单原子层到数十纳米。各种各样的二维材料，如石墨烯、氮化硼、过渡金属硫化物、黑磷和钙钛矿都已经被成功地分离出来。二维材料表现出奇异的物理化学性质，比如具有原子层厚度、强烈的非线性光学性质、磁性以及优异的机械性能。这些与其对应的块体材料迥异的性能为二维材料应用于纳米器件，尤其是在光子学中的应用创造了新的机遇。

最近各种二维材料微/纳米结构和功能性器件不断被提出，并通过多种加工手段设计出来，以实现其优异的性能。超快激光直写技术由于其丰富的光和物质相互作用的机制和动力学，特殊的三维制造能力，任意结构设计的灵活性以及最小的热效应，使其加工精度高达数十纳米，已被广泛应用在材料图案化、修饰、功能化当中，充分展示出其卓著的加工制造能力。

近日，来自澳大利亚斯威本科技大学的研究团队综述报道了用激光直写技术在二维材料中制备功能性光子器件的最振奋人心的进展，提出了深层次的总结和理解。综述文章发表在Chinese Optics Letters 2020年第18卷第2期 (Tieshan Yang, Han Lin, Baohua Jia. Ultrafast direct laser writing of 2D materials for multifunctional photonics devices [Invited][J]. Chinese Optics Letters, 2020, 18(2): 023601)。

该综述文章首先简洁地总结了材料在激光激发下的物理性质变化。激光和材料相互作用可能包含多种作用机制：单光子/双光子吸收，激光照射下材料烧蚀，纳米材料结构相变以及化学/物理性质修饰。这些过程导致材料不同的物理化学性质变化，比如折射率、消光系数、带隙、电导率和表面浸润特性（亲水性或疏水性）等。这些特性改变是功能性光子器件或光电子器件设计的基础。光和材料相互作用可实现二维材料微米/纳米图案化、激光减薄以及激光掺杂，这些特性改变在二维材料高分辨器件的制备和功能化中是至关重要的。之后，文章分析了用激光直写技术制备功能性光子器件，如超薄透镜、石墨烯超材料、完美吸收体以及全息显示等的优劣势。此外，超快激光直写技术还可用于二维材料局部非线性性质改变，通过在传统光子器件中简单地加入二维材料层，将其转化成高非线性系统。这种方法具有极大的灵活性，可以实现超快的速度，并极大地提升全光通信系统的性能。最后，文章指出了该领域的机遇和挑战，并对未来进行了展望。

该研究团队的贾宝华教授指出：“超快激光直写技术是制备高性能二维材料功能性器件必不可少的工具。超快激光可以应用于多种二维材料的图案化和功能化过程，其超高的分辨率和加工精度、简单灵活的加工方法和优异的性价比对于下一代大面积、高性能、便携式、集成化和柔性器件的制备是不可或缺的。”

通过将超快激光直写技术和并行加工以及超分辨技术相结合，有望开发出新颖的激光制备平台，用于多功能、可拓展的二维材料集成器件的制备，从而拓展其在多学科的研究和器件的应用中去。