Femtosecond laser internal manufacturing of three dimensional micro-structure devices Chong Zheng1,2, Anming Hu1,2,*,Tao Chen1,*, Ken D. Oakes3, Shibing Liu1 (1) Institute of Laser Engineering, Beijing University of Technology, 100 Pingle Yuan, Chaoyang District, Beijing 100124, P. R. China; (Fax:+86-10-67392514; Email: [email protected]; [email protected]) (2) Department of Mechanical, Aerospace and Biomedical Engineering, The University of Tennessee, Knoxville, Tennessee, TN 37920; (3) Verschuren Centre, Department of Biology, Cape Breton University, P. O. Box 5300, 1250 Grand Lake Rd., Sydney, B1P 6L2 Canada ABSTRACT: Potential applications for three-dimensional microstructure devices developed rapidly across numerous fields including micro-optics, microfluidics, micro-electromechanical systems (MEMS), and biomedical devices. Benefiting from many unique fabricating advantages, internal manufacturing methods have become the dominant process for three-dimensional microstructure device manufacturing. This paper provides a brief review of the most common techniques of femtosecond laser three-dimensional internal manufacturing (3DIM). The physical mechanisms and representative experimental results of 3D manufacturing technologies based on multiphoton polymerization, laser modification, micro-explosion and continuous hollow structure internal manufacturing (CHSIM) are provided in details. The important progress in emerging applications based on the 3DIM technologies are introduced as well. Key words: Femtosecond laser direct writing (FsLDW); Three-dimensional internal manufacturing (3DIM); Multiphoton polymerization; Laser modification; Micro-explosion; Continuous hollow structure internal manufacturing (CHSIM); 1. Introduction of Femtosecond Laser Three-Dimensional Internal Manufacturing (3DIM) Femtosecond (fs) laser micromachining was first demonstrated by Srinivasan [1] in 1987 when a femtosecond laser was employed to ablate polymethylmethacrylate (PMMA) without inducing thermal damage. From then on, femtosecond laser was gradually noticed as a powerful tool in micromachining due to its excellent performance in high machining quality and precision [2] , especially in solid materials ablation, i.e., ablate 2D structures on metals [3-5] , polymers [6,7] , and crystals [8,9] . Meanwhile, 2D photonic devices manufacturing [10,11] , surface engineering [12-15] and the formation of novel polyynes, 1D molecular carbon wire [16] , nanojoining [17,18] , and casting [19] based on femtosecond laser irradiation are also well developed and attracted many scientific interests. Apart from the aforementioned 2D manufacturing applications of which functional features are fabricated either directly on the surface of the materials, or fabricated according to in-plane machining pattern, femtosecond laser micromachining is unique 1 in its 3D micro- /nano- structuring ability attributing to the nonlinear nature of the multiphoton absorption [20-22] : (1) the structure changes can be confined to the focal volume as the intensity distribution for multiphoton absorption is spatially narrower than linear absorption, providing an ideal tool for 3D manufacturing with high spatial resolution; (2) the absorption of laser energy is independent with materials, ensuring its wide applications in various materials; and (3) no thermal effect occurs during femtosecond laser irradiation since the lattice heating time (~10 ps) is much longer than the pulse duration of femtosecond laser (<1 ps), thus femtosecond laser machining is more precise than the fabrication with the lasers with longer pulse durations. Benefit by these advantages, four types of 3D manufacturing technologies were well developed in the past 15 years based on femtosecond laser direct writing (FsLDW), including: (1) multiphoton polymerization[22-24] , (2) laser modification [25-27] , (3) micro-explosion [28-31] and (4) continuous hollow structure internal manufacturing (CHSIM) [32-37] . For the reason that all the 3D structures fabricated by the aforementioned techniques are essentially obtained inside different materials with certain laser conditions, a summarized name of all these technologies is given here as femtosecond laser three dimensional internal manufacturing (3DIM). These four types of 3DIM methods are classified by the typical structural change following femtosecond laser micromachining. Among them, multiphoton polymerization is a process whereby monomers are polymerized into solid state macromolecules by laser irradiation; it is an example of additive manufacturing [22] . In this process, photopolymerizable monomers are used as basic elements to fabricate macroscopic structures, analogous to building a house with bricks. While, laser modification denotes a process that only the optical [38] and/or chemical [39] properties change within the irradiated region, and no significant structural additions or losses occur during this process. In other words, only specific material properties are modified, without inducing remarkable microstructural changes. It should be noticed that the term “laser modification” is broadly used in many other scientific contexts whenever a laser beam is employed on a material to elicit either structural or optical/chemical/mechanical properties changes. It is a much broader usage of the term than is strictly used in this context of femtosecond 3DIM, since the formation of microvoids are sometimes also described as a result of “laser modification” [30] . Whereas, here in this review, microvoids are always manufactured by femtosecond-laser-radiation-induced micro-explosion or micro-dissociation when the laser power exceeds the threshold of optical damage [40] . Compared with multiphoton polymerization processes, microexplosion techniques can be considered as a “reductive manufacturing” process, during which single microvoid formed due to extremely high temperature or pressure caused by laser irradiation. Another reductive manufacturing technology in 3DIM is continuous hollow internal structure manufacturing (CHISM). As its literal meaning of the name, CHISM identifies the process of producing characteristic 3D continuous hollow structures directly inside a bulk material, either using liquid assisted femtosecond laser selective etching [41] or by laser direct-write fabrication [37] . It is different from the micro-explosion processing because the fabricated structures by CHISM method are either long hollow channels or large chambers fabricated by laser continuously 3D scanning, while microexplosions are only utilized to create dispersed microvoids array by fs laser single spot or parallel exposure. Besides, the methods underlying CHISM are considerably different from those utilized by microexplosions. Further unique attributes defining these four 3DIM methods, including relevant physical tools, procedures, and experiment setups, will be described in detail within subsequent sections of this review. 2 As there are already many review articles and pioneer works on femtosecond laser [42] , non-linear processes [8,43] , femtosecond laser ablation [44-46] , and surface micromachining [47] , we will only focus on femtosecond laser three dimensional internal manufacturing technologies in this review. Consequently, we examine the fabrication processes, experimental conditions and physical mechanisms of each 3DIM technology. In addition, the advanced applications and the most recent progresses of 3DIM technologies are also introduced in this review. 2. 3D Microstructure Device by Femtosecond Laser Processing Although 3DIM technologies are classified into four types due to the fabricated structures, it should be noted that there are only two typical physical processes for laser-material interaction, that is: light-induced polymerization (multiphoton polymerization) and optical breakdown (laser modification, laser microexplosion and CHISM). The differences between the technologies caused by optical breakdown are attributed to the different laser parameters and post-processing methods, but they share a similar physical procedure in laser-material interaction. Generally, laser-induced optical breakdown can be described by three major stages [21,48] : (i) Multi-photon ionization and/or tunneling ionization cause the excitation of the electrons to the conduction band; (ii) Sufficient seed electrons provided by multi-photon ionization and tunneling ionization initiate the avalanche ionization; (iii) High density free electron plasma absorbs laser energy by free-carrier absorption and then transfers the energy to the lattice leading to permanent structural changes in the material. Later in this article, the basic physical mechanisms underlying each technology will be distinguished in details. In light of the unique advantages of femtosecond laser 3D manufacturing as we talked above, microstructured device applications, especially with complicated 3D structures such as micro-sculptures [24,49] , micro-components [50-52] , micro-lens arrays [53,54] , waveguides [26,55] , gratings [56] , photonic crystals [29] , and microfluidic chips [57,58] are developing rapidly with diverse applications in recent years. The unique applications of each technology will be also provided in the later illustration. 2.1 Multiphoton polymerization Photopolymerization refers to the process by which small unsaturated molecules (monomer or oligomer units) such as unsaturated polyester (UPR) are converted from liquid to solid state macromolecules using light as an energy source driving polymerization