Femtosecond Laser Internal Manufacturing of Three Dimensional Micro-Structure Devices

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 reactions [59] . The theoretical framework underlying the development of multiphoton polymerization was conceptualized in 1931 by Göppert - Mayer [60] who postulated that the medium molecules would be excited from the ground state to the excited state by simultaneously absorbing two photons when irradiated at a high light intensity. Notably, the first laser in the world was not invented until 30 years following this prediction, delaying validation until 1961 by W. Kaiser et al. [61] who observed the two-photon absorption (TPA) process for the first time. Building on the mechanism underlying two- or multiphoton absorption, three dimensional structures with complex planar or spatial structures could be produced at low cost by multiphoton polymerization [22] . The emergence of the application of two-photon polymerization (TPP) starts from 1997 when

3 Maruo et al. [62] demonstrated that the photopolymerizable resins (SCR500) could be polymerized by simultaneously absorbing two photons by using a near-infrared wavelength femtosecond laser (~ 1 nJ for 0.85 NA focusing of 790 nm, 200 fs pulses with a repletion rate of 76 MHz). The chemical reactions resulted from the two-photon photopolymerization can be described as follows [62,63] : (1) In the initiation process, a photo-initiator (PI) molecule simultaneously absorbs two photons and ionizes into two free radicals (R.) at the beginning of this process. The disassociated free radicals then recombine with other monomers-oligomers (M) and form a chained radical (R-M.); (2) In the propagation phase of the process, the chained radicals

(R-Mn.) continuously combine with additional monomers and form even longer chained radicals

(R-Mn+1。); (3) Finally, in the termination phase, two chained radicals (R-Mn.) are polymerized to form a polymer. The theoretical foundation of TPP is multiphoton absorption, in which TPA is the most common form. Generally, the TPA process in TPP process is the degenerate TPA [64] , during which the initiator simultaneously absorbs two photons with the same frequency. Since TPA is a third-order nonlinear process, the absorption rate can be expressed as [63,64] :

dW 8π ω22 = I 2 Im(χ )3( ) (2) dt nc 22 where, ω the frequency of incident light; I the intensity of light; c the speed of light; n the refractive index of the medium; Im(χ(3)) the imaginary portion of third-order optical susceptibility.

To measure the TPA effectiveness of material, TPA cross-section (δ2) is defined as follows [64] : dn photon = δ NF 2 (3) dt 2 where, dnphoton/dt represents the number of photons absorbed per unit time; N the density of the molecules that are absorbing the photons; F=I/hν denotes the photon flux of laser beam (h represents Planck’s constant; ν the incident light frequency). This concept is proposed deriving from the cross section of one-photon absorption (δ1), which is approximate to the geometrical size of the cross section of the absorbing molecules, ΔS ~ 10-16 cm2 , and the estimated virtual state lifetime in degenerate TPA process based on the uncertainty principle τ = 10-16 s [65] . Thus the cross-section of N-photon absorption can be calculated as [65] : NN −1 cm 2 s NN −1 . As a result, δ N = 1 τδ [ ]

TPA cross-section of commonly used photopolymerizable materials is about 10-46 ~ 10-52 cm4s/photon [66,67] . Accordingly, the cross-section of the three photon absorption is around 10-80 ~10-82 cm6s2/photon2 [68,69] . Three photon polymerization becomes significant when lasers intensity is higher than 1011 W/cm2. While, higher order absorption are usually not take into consideration since their cross-section is too small to play a significant role in multiphoton polymerization process. Also, even though nonlinear nature of multiphoton absorption contributes to a high fabrication resolution, it is not practical to pursue high-order nonlinear absorption since lasers an extremely high peak power are needed, which would counteract the resolution improvement induced by the nonlinearity. Compare with the lasers with longer pulse durations, i.e. picosecond (ps) lasers and continuous wave (cw) lasers, femtosecond laser is superior in polymerization efficiency when applied in multiphoton polymerization [70] , and also the thermal effects associate with fs laser is the lowest. These advantages leads to the fact that femtosecond

4 laser becomes the most commonly used equipment for multiphoton polymerization. With the technology of femtosecond laser multiphoton polymerization, advanced applications in micro-machines and microfluidics were widely studied in the past decades. Witzgall et al. [23] reported a two-photon exposure experiment of the commercial photoresist SU-8 with a single-shot of 120 fs, 800 nm pulses in 1998. The exposure and damage thresholds for SU-8 were also accurately determined as 3.2×1012W/cm2 and 8.1×1012W/cm2, respectively. Their work demonstrated that 3D structure can be fabricated in a sub-diffraction-limit resolution based on a voxel-by-voxel pattern, offering significant insight for other scientists. Based on their work, later in 2001, a remarkable work was subsequently reported by Kawata and Sun et al. in which a ‘micro bull’ sculpture was created by using fs laser TPP method at a sub-diffraction-limit spatial resolution of 120 nanometers [24] . The smooth skin, sharp horns and all of other nanometer-scaled features represented a fine 3D fabrication ability of femtosecond laser multiphoton polymerization [24,71] (Fig. 1(a)). Building upon these earlier studies, additional novel applications employing TPP methods developed rapidly throughout the following decade (mid-2000s) [72-75] . Periodic structures fabricated by TPP methods aroused great scientific interest including photonic crystal (PhC) as introduced in ref. [72-74] , as shown in Fig. 1(c) that a photonic crystal woodpile structure was fabricated in negative SU8 resist by TPP method. Similarly, Kato et al. developed TPP methods to fabricate periodic structures by utilizing a micro-lens array to generate multiple focused spots for parallel fabrication [75] . More than 200 spots were simultaneously fabricating 3D structures in the photopolymerizable resin SCR500 under optimized the exposure conditions. A microletter array of ‘N’ and a self-standing micro-spring array were written in 5 min and 30 min, respectively. Developed on the parallel TPP fabrication techniques, periodic metallic micro/nanostructures was realized by electroless plating of silver nanoparticles on the polymer structures by Formanek et al. [76] . Periodic structures like scaffolds for growing cells, or microlens are also obtainable using TPP methods as introduced in ref. [77] and ref. [78] . With sub-diffraction-limit micromachining resolution, TPP method is also an advisable choice for making micro-optical devices. As reported by Guo et al. in 2006, micro-ball lens array (Fig. 1(b)) and Fresnel lens with smooth surfaces were successfully fabricated with TPP method [53] . Similar efforts were reported by Hu et al. that aspheric micro-lens arrays were fabricated by holographic femtosecond laser-induced photopolymerization [79] . Cojoca et al. demonstrated that various microstructures with good optical properties can be fabricated on the top of optical fibers, providing a competitive mean for on-fiber fabrication [80] . Apart from these, there are still some other approaches for micro-optical devices fabrication in photopolymerizable resins by TPP method, which further illustrate the practicability and flexibility for TPP fabrication [54,81-83] . Moreover, two-photon polymerization methods have also been introduced into microfluidic device manufacturing. Zhou et al. [58] made a 3D channel structure through two-photon exposure of BSB-S2 (2002); He et al. developed a method to build a 3D ‘overpass’ at the junction of crossed microchannels as a microchannel switch [84] . Lindenmann et al. demonstrated a photonic chip-to-chip interconnection technique, named photonic wire bond (PWB), capable of linking nanophotonic SOI waveguides [85] . Lim et al. fabricated a crossing manifold micromixer (CMM) inside a microfluidic channel (Fig. 1(d)), enabling fluidic mixed with a high efficiency and mixing ration (> 90%) [86] . Some other micro-components and micro-devices applied in microfluidics are even commercially available at present, such as micro-turbines and 3D micro-filters that

5 capable of deployment in microfluidic channels were made by Nanoscribe GmbH. [87,88] .

Fig. 1. Microstructures fabricated with multiphoton polymerization, (a) a ‘Micro bull’ sculpture with fine 3D features [24] , (b) micro lens array with smooth surface [53] , (c) photonic crystal woodpile structures [74] , and (d) a crossing manifold micromixer fabricated in a microfluidic channel for fast fluid mixing [86] . Reproduced with permission from Ref. 24, 53, 74 and 86. ©2001 Nature Publishing Group, ©2006 Optical Society of America, © 2007 Elsevier B.V., and ©2011 the Royal Society of Chemistry.

In summary, multiphoton photopolymerization is an important method of micro- or nano- additive fabrication which is playing an essential role in producing polymer-based optoelectronic, microfluidic, and MEMS devices. Numerous products such as periodical structures, micro-lens arrays, micro-machines and microfluidic devices can be fabricated with sub-diffraction-limit spatial resolution.

2.2 Laser modification

The term “laser modification”, as used herein, is defined as the changes in physical and/or chemical properties of materials whereas no structural additions or losses following irradiation by focused femtosecond laser at the intensity close to the damage threshold of materials [20] . Compared with multiphoton polymerization, power densities applied in laser modification is strongly dependent on materials ranging from 1012 W/cm2 to 1014 W/cm2 [26,89] , while the power density required by multiphoton polymerization is usually at ~ 1012 W/cm2 [59] . Meanwhile, this technology is always performed within transparent solid state materials, such as glasses or polymers. The most commonly modifiable properties include the refractive index [26,38,90,91] , the chemical etching rate [39,41,92,93] , and the valence-state [94] . The mechanisms underlying different types of laser modification varies with materials. However, it is certain that the property-changes are basically due to the non-linear effect cause by multiphoton absorption and ionization when irradiated with femtosecond laser pulses. Although theoretical basis for the changes in refractive index following laser modification is still not fully understood, it is believed that the change of refractive index correspond to laser induced (i) color center formation, (ii) lattice defects and (iii) structure densification, or attribute

6 to a combination of these factors [21,91,95] . While the change of chemical etching rate is due to photochemical reactions [41] and/or the modification of internal stresses [92] induced by the focused femtosecond laser radiation. In addition, the valence-state change is demonstrated to be a photoreduction process caused by femtosecond laser multiphoton ionization [94] . Internal manufacturing by laser modification are widely applied in fabricating photonic devices inside transparent dielectric by utilizing fs-laser-radiation-induced refractive index change or valence-state change, such as waveguides [25,55,90,96] , gratings [25,27,95,97] and optical data storage [94,98] . However, the change of chemical etching rate are more often employed in fabricating continuous hollow structures in photosensitive glass, fused silica, or other glass material [39,41,92,93] , which we will detailedly discuss later in Section 2.4 when introducing methods for the CHISM technology. The early work on femtosecond laser modification was reported by Davis et al. [99] in 1996 using 810 nm focused femtosecond laser beam with an intensity of 1012/cm2 for direct writing inside transparent bulk glass (such as high-silica, borate, soda lime silicate, and fluorozirconate (ZBLAN)). Stable, visible laser damage and photo-induced refractive index changes were observed. After comparing with the optical damages (changes in refractive index; defects in Si E’ or Ge E’ centers, non-bridging oxygen hole centers; peroxy radicals) in different materials, the possibility of fabricating waveguides inside these bulk glasses was thus prospected. Building on this knowledge, Schaffer et al. [26] successfully wrote a waveguide inside bulk Corning 0211 glass with 5 nJ, with sub-100 fs pulse duration incident fs laser pulses at a repetition rate of 25 MHz and focused by a 1.4 NA objective in 2001. The output profile of the fabricated waveguides for 633 nm laser radiation perfectly matched a single-mode, near-Gaussian output profile. Sequently, Hirao et al. also fabricated waveguides inside silica and related materials by fs laser, [25] and demonstrated that the core diameters of the waveguides generally will increase with the increasing average laser power. However, the core diameters remained unchanged despite the waveguides were repeatedly scanned. Thus, the refractive index of the core can be increased either by increasing the laser scanning number, or by decreasing the pulse duration; the latter situation is equal to increase the peak power of the femtosecond laser. Following this principle, 15 mm long waveguides were written and tested. The results showed that the loss of fabricated waveguides is approximate the same as commercial Ge-doped waveguides (0.1 dB/cm). Meanwhile, the single mode waveguide was convinced can be obtained with this technique. Similar investigations regarding waveguides fabricated by fs laser direct writing were carried out in the early 2000s, as reported by ref. [100] and ref. [101] that the change in refractive index was realized and controlled in both Foturan glass and PMMA by fs laser irradiation (Fig. 2(c)). It demonstrated that almost every transparent dielectric material can be modified by laser irradiation. 3D stacked waveguides fabricated inside a glass plate (Corning 0215) [55,102] or splitters [103] fabricated in pure fused silica based on the refractive index change by laser modification were also reported by some other groups (Fig. 2(a)). Exploiting the same principle as for internal waveguides fabrication, Si and Qiu et al. in 2002 produced permanent holographic gratings inside bulk azodye-doped PMMA by the coherent superposition of femtosecond laser [104] . Likewise, Li et al. fabricated multiple layers of grating inside soda-lime glass with femtosecond laser pulses [105] , Scully et al. wrote a refractive index grating inside a Perspex slab [89] . Similar advanced techniques surrounding waveguide or grating fabrication are also introduced in ref. [27,95-97,106,107] .

Fig. 2. Typical photonic applications built on the femtosecond laser modification. (a) A 3D splitter fabricated in fused silica and the Near-field intensity distribution the guided light [103] . (b) Rewriteable optical data storage based on fs laser induced valence-state change [94] . (c) Diffraction pattern of the grating fabricated with fs laser induced refractive index change [100] . Reproduced with permission from Ref. 103, 94 and 100. © 2003 Springer-Verlag, ©2002 American Institute of Physics, and ©2003 Optical Society of America.

The preceding discussion has focused exclusively on long channel-shaped structures with different refractive indices to the substrates. However, another important application of laser modification is applying the generation of dot-shaped modified region for three-dimensional optical storage inside transparent materials, as reported in ref. [94,98] . Qiu et al. demonstrated that refractive-index-changed micro-dots induced by fs laser modification can be fabricated inside

SiO2 and Ge-SiO2 glass [98] . If a single dot is seemed as a binary bit, laser modification in glass thus can achieve a bit density larger than 10 GB/cm3, which illustrated is capability as a proven method for optical storage. Later, the same group developed a bit-rewriteable method for ultra-high density 3D optical memory fabrication based on the valence-state change of samarium ions (Fig. 2(b)) [94] . This work differs from the previous one since the binary data is represented by the valence-state which can be read out as fluorescence information and erased by the radiation with a CW laser. Unlike traditional photolithography which limited the fabricated structure in-plane, 3D laser modification presents a flexible means for inducing a ‘refractive index changed region’ with arbitrary three dimension shape inside bulky, transparent materials [59,95] . Thus, waveguides, gratings and micro-dots can be created inside transparent dielectrics in a three dimensional shape or distribution, enables advanced applications in photonics and optical 3D storage.

2.3 Microexplosion

Microexplosions occur when the deposited power exceeds the optical damage threshold of the material (1014 ~1015 W/cm2 [28,108] ) at the focus of the femtosecond laser. A possible explanation for fs-laser-induced microexplosion is provided as this [109] : due to the multiphoton absorption, free carrier plasma will firstly generated when the radiation intensity exceeds the threshold of optical break down. It further absorbs the energy from the incoming pulses and resulting in the secondary ionization of material. Consequently, a void with a central damaged

8 region surrounded by a shell of denser material is formed. Notably, the shell is generated as a result of thermal quenching after melting, which was observed in several experiments [109,110] . In addition, the fact that the pressure in the void reaches 10 TPa was demonstrated which is believed contribute to the densification of the shell [28,30] . The micro spherical or elliptical voids created by femtosecond laser induced microexplosions were applied in 3D optical storage [28,109,111,112] , periotic structure [29,113] and microlens [110] fabrications, among which 3D optical storage is major studying direction that attracted many scientific attentions. The original studies to apply fs laser irradiation for 3D storage started by Glezer et al. in 1996 [28,112] . They successfully created sub-micrometer sized bits with a large contrast that can be read out by detecting the transmitted or scattered light under a standard microscope. The density of bits reported in their work was 17 GB/cm3 (Fig. 3(a)). Watanabe and Sun et al. [109] then further developed this method by using a new vitreous silica material and optimized its unique advantages for 3D optical data storage, thus obtained a recording density of 72.9 GB/cm3. Now the record of the recording density has been refreshed to 500 GB/cm3 with an in-plane separation of 1 µm and layer separation of 2 µm [111] . It is believed dozens of TB/cm3 recording density would be realized by improving the focusing system to create even smaller micro voids [111] . Notably, the voids formed by microexplosions can be also used to fabricate 3D periodic structures such as void-based photonic crystals. As reported by Zhou et al. [29] , a 3D diamond void photonic crystals with microexplosion-induced voids stacked in the [100] direction at different lattice constants as 4.12, 4.44, and 4.72 µm were fabricated in NOA63 polymer resin. Three gaps were observed by measuring the transmission spectra and the suppression rate of the second gap in a 32-layer structure is up to 75%. The angle dependence of band properties were also investigated and is believed to find its application in a photonic crystal superprism.

Fig.3. Micro-voids fabricated by femtosecond laser induced microexplosion applied in (a) 3D optical data storage [112] , (b) embedded cavity microball lens [110] , and (c) Fresnel zone plate [114] . Reproduced with permission from Ref. 112, 100 and 114. ©1996 Optical Society of America, ©2015 John Wiley and Sons, and ©2002 Optical Society of America.

Whereas, a recent research on the void formation inside PMMA with a high repetition rate

9 femtosecond fiber laser by Zheng et al. [110] illustrated that the microvoid generated by fs-induced micro-explosion and micro-dissociation can be utilized as a concave microball lens (CMBL). Due to the heat accumulation effect caused by successive irradiation of laser pulses, a melting zone was observed during the fabrication process, and the cracks generated at the initial stage caused by extremely high thermal stresses and the pressure of the disassociated gas was thus filled by the melted material [115] . As a result, a void-shell structure as introduced in ref. [109] was also observed and the cross-section view of the generated CMBL showed that it is a highly transparent and crack-free structure (it is superior than the voids with cracks as reported in ref. [112] ) that can be utilized in many optical applications as a microlens (Fig. 3(b)). With this lens, a super-wide field-of-view can be achieved with a micro-telescope consisting a CMBL and microscope objective. Another interesting application in micro-optics, as reported by W. Watanabe et al. [114] , was the fabrication of a 400 µm×400 µm Fresnel zone plate in silica glass produced by imbedding multiple aligned voids. Rather than focusing on the voids region induced by microexplosions as in ref. [110] , in this work the authors paid close attention to the surrounded areas where the material was densified by the microexplosions. By investigating the properties of the Fresnel zone plate when the spot size of the primary focal point was 7.0 µm and the diffraction efficiency was 2.0 % (Fig. 3(c)). This technique thus also prove a capability for lenses fabrication in bulk materials. In conclusion, permanent micrometer-scaled void-structures appeared after microexplosion, making it a reliable method for binary optical storage, periotic photonic structures and micro-optical lenses. If the heat accumulation effect of successive pulses is well utilized, microvoids can be fabricated with smooth internal surface and without combined cracks in polymer materials, providing a broader application capability.

2.4 Continuous hollow internal structure manufacturing (CHISM)

With the development of microfluidic devices, the demand for 3D hollow structures is increasing, but most of the so-called 3D microfluidic chips are made of several layers of planar structures which are subsequently bonded together [116,117] . The size of such structures is always in the range of several centimeters in length and width, and several millimeters thick. In that case, a consequence of the multilayer bonding process is somehow lack of 3D flexibility hampered by improper bonding processes (imprecise alignment; inappropriate bonding temperatures or pressures) which may cause the products to ultimately be discarded. As a result, continuous hollow structure manufacturing has a great application potential in fabricating real three-dimensional micro-structures fabrication in bulk material, in another word, microstructures are directly fabricated in a 3D pattern inside a bulk material based on the focused femtosecond laser direct writing. To date, many methods have already been developed to employ CHISM fabrications for diverse applications, especially in microfluidics [32-37] . Based on the formation mechanisms for CHISM, the methods can be simply classified as (1) Liquid (etchant, such as acid/alkali aqueous solutions; or non-etchant, such as water) assisted femtosecond laser selective etching and (2) femtosecond laser direct-write fabrication, which are applied largely depend on the materials. We will discuss the CHISM technology according to these two types of methods in the following section.

10 2.4.1. Liquid assisted femtosecond laser selective etching

As discussed above in Section 2.2, when femtosecond laser radiation is applied in transparent bulk dielectric material with relatively lower intensity, the etching rate for several materials would be modified due to laser-radiation-induced photochemical reactions or radiation-induced structure changes. Thus, internal microstructures are able to be fabricated inside photosensitive glasses [41,118-120] , fused silica glass [34,52,121-124] , sapphire crystal [30,125] , semiconductors [126] , porous glass [35,127,128] , and polymer materials [129] by a selective etching method. In this subsection, all the methods introduced here are related with a liquid assisted fabrication procedure, i.e. either acid/alkali aqueous solutions are used for selective chemical etching, or water is introduced for enhancing the fs laser ablation efficiency as well as excludes the debris generated during the fabrication process. Typical methods for the 3D continuous structure fabrication in different materials are introduced as following paragraphs. For femtosecond laser 3D manufacturing, optical nonlinearities of the processing materials should be taken into consideration for obtaining a better processing efficiency and quality. Some noble metal nanoparticle-doped glasses, however, turn out to exhibit large third-order non-linear susceptibilities and ultrafast non-linear responses. Pioneering attempts to fabricate hollow structures were thus first undertaken inside so-called photosensitive glass (Foturan glass, Schott Glass Corporation) [39] . Take Foturan glass for an example, this photosensitive glass is composed of a lithium aluminosilicate doped with trace amounts of silver, cerium and antimony, where cerium acts as the photo-sensitizer. When Foturan glass is exposed to femtosecond laser radiation, Ce3+ gives up an electron to become Ce4+ and the free electrons will then reduce the silver ions Ag+ to Ag0 [39,41] . Following successive heat treatment protocols [118] , metasilicate crystallites (Li2SiO3) are formed which are preferentially soluble in dilute solutions of hydrofluoric (HF) acid, thus allowing 3D structures to be fabricated inside Foturan glass. It is remarkable that the irradiated region can be etched with a velocity 20 times faster than those regions not irradiated. Metastable Li2SiO3 etched by HF acid forms LiF and H2SiF6. LiF is soluble in dilute solutions of HF acid and H2SiF6 exists in aqueous solution. Based on this mechanism, a bridge-like microchannel was fabricated inside Foturan glass by Cheng et al. with a fs-laser at a pulse energy of 2200 nJ with 775 fs pulse duration and 800 nm wavelength focused by 0.46 NA objective [33] . The novelty of the experiment setup is that a slit of 0.5 mm width was used as a simple experimental setup to obtain an elliptical Gaussian beam. By calculating the energy distribution near the laser focus, the beam waist would be significantly expanded when a circular Gaussian input beam was modified to an elliptical Gaussian beam. Additionally, investigations into the critical fluence for modification was under taken as this drives the optimal parameters for microstructuring photosensitive glass [39] . Dependence of critical fluence on the numbers of laser pulses suggested that the modification occurs in Foturan -5 glass is a six-photon process (m=6), and the photoreaction threshold would be Dc=1.3×10 J6/cm12. The m value was calculated based on the regression line of ,

m Dc = Fc N (10) where Dc represents the critical dose, Fc the critical fluence, m the power dependence (multiphoton index), and N the numbers of pulses. Such a high order of the six-photon process is beneficial to obtain high spatial resolution in fabrication since the absorption and reaction region is confined to a volume much smaller than the laser spot size if the pulse energy is precisely controlled very

11 close to the critical dose. Since the microstructuring of Foturan glass by femtosecond laser radiation is essentially a non-laser-ablative process, the created structures after chemical etching is demonstrated to have a debris-free internal surfaces. This provides for the possibility of fabricating optical components such as micro-mirrors inside photosensitive glass. Based on this property, a dual-color microfluidic laser array (Fig. 4(a)), which simultaneously emits light at 568 nm and 618 nm by integrating micromirrors and two microfluidic chambers for an optical microcavity, has been manufactured in Foturan [41,118,130] . Laser action occurs when the micro chamber is filled with a gain medium laser dye Rhodamine 6G (Rh6G), and pumped by a frequency-doubled Nd:YAG laser radiation. In addition to the microfluidic laser, micromixers [118] , microlenses [120,131] , microorganism nano aquariums [132,133] , and other microfluidic devices [134,135] were developed inside photosensitive glass. Although photosensitive glass can be structured to the components with excellent optical properties, this material is less economical to be used in industrial application. A more frequently used material is fused silica glass, a glass consisting of silica in amorphous (non-crystalline) form. As the optical and thermal properties of fused silica are superior to other types of glass due to its purity, it is widely used as a substrate for microfluidic devices. Attempts to fabricate 3D structures inside fused silica started by a two-step process involving: (i) recording 3D patterns inside silica by femtosecond laser irradiation and (ii) chemical acid/alkali etching the recorded patterns [136,137] .The physical process occurs when the deposited laser radiation power exceeds the optical damage threshold, and micro-explosion of the SiO2 sample takes place at the focal point of femtosecond laser, resulting in the focal region being surrounded by a shell of densified material of a higher refractive index. However, there is a second plausible explanation regarding the formation of this densified region, that is, the laser-induced pressure (shock) wave may cause this densification. Consequently, both bond strain and material densification are greater after irradiation, which is related to the decrease in the average bridging angle leading to greater exposure of the silica to the acid [137] . Bellouard et al. [92] demonstrated that etching is faster in the center of the laser-irradiated track, and that etching selectivity can extend over a distance several times larger than the track depending on the pulse energy level. Two suspected mechanisms were proposed as (1) the increased etching rate is driven by the presence of internal stress, or (2) an average ring size reduction is indicative of higher SiO2 polymorphs in the laser track. Additional techniques to improve fabrication properties and expand potential applications have been developed since the two-step process was first proposed. Hnatovsky et al. demonstrated polarization direction of the writing beam has great influence on the etch rate [124] , which could be modified by rotating the polarization of the writing beam under a specified writing condition. However, if chemical etching an embedded straight-line-shaped channel with the two-step, it is always combined with some drawbacks, such as, because the etching process always begins at the surface of the fused silica. It always results in microstructures broader at the entrance and narrower or discontinuous in the center. Recent progress for the shape control of the microchannel fabrication includes conical shape compensation, refraction compensation, and spatiotemporally focusing of fs-laser pulses [93,138] which greatly enhanced the shape control capability in microstructure fabrication. Thus, embedded microchannels for microfluidic applications fabricated by two-step method are continuously updating [122,139] . However, the length of the embedded

12 channel usually cannot be fabricated over long distances (always less a few centimeters) due to the low etching efficiency (e.g. 4 hours etching for a 2 cm microchannel [139] ) and difficulties in shape control as talked above. An improved method to solve these problems was proposed by He et al. [140] that a segmented chemical etching method followed by polymer sealing allowing for a fabrication of a helical channel of more than 1 cm in about 1 hour [140] .

Similar two-step chemical etching methods are also available in fabricating microstructures in sapphire crystal [30,125] , semiconductors [126] , and polymer materials [129] . Hörstmann-Jungemann et al. reported that a microchannel was fabricated in sapphire crystal by the following processes: (1) Scan a line pattern with the focused femtosecond fiber laser pulses at an average power of 1.5 W and pulse duration of 450 fs. Laser pulses were focused by an 0.6 NA objective and the repetition rate of laser pulses is chosen as 500 kHz (3µJ/pulse); (2) The scanned sample was immersed into 48% HF acid etching for 48 hours. The average surface roughness of the obtained microchannel was 64 nm. Besides, micro-machines, micro-holes and 3D fluidic devices were also reported successfully fabricated in sapphire with FsLDW followed with etching in aqueous solution of potassium hydroxide (KOH) [52] . Other works related to sapphire CHISM were also reported in ref. [125,141] . Semiconductor material such as single-crystal gallium nitride (GaN) was also reported by Nakashima et al. [126] applied to fabricate 3D microstructures with a modified “two-step” approach with 12 mol/l HCl as an etchant. Moreover, Wochnowski et al. [97,129,142] PMMA and PI are more suitable polymer materials for femtosecond laser CHSIM based on the results of similar two-step fabrication experiments applying etchants with different polarities, i.e., pure n-hexane (nonpolar solvent agent), pure benzene (minor polar solvent agent), and a 5% methyl isobutyl ketone (MIBK) aqueous solution (polar solvent agent). Besides, non-fluorinated PMMA is proved can be sufficiently developed in MIBK solution without any swelling effects occurring.

Apart from a two-step fabrication method, another popular method for liquid assisted femtosecond laser selective etching is water assisted femtosecond laser selective etching. Note that water is not chemical etchant, the underlying mechanism of forming the hollow structure inside bulk material with water can be understood based on the works reported by Li and Qu [34] : the ablation pressure generated by the high power density fs laser irradiation is about 105 MPa, which is much higher than the Brinell hardness of fused silica, the femtosecond laser-induced high-speed jet can thus etch the silica glass easily. This none-etchant liquid assisted channel fabrication can be initiated in three phases [123] : (1) Ensure that the rear surface of the material is in contact with water. (2) Irradiation by focused femtosecond laser should initiate from the same rear surface. By controlling the translation, a continuous hollow micro-channel can be written step by step. As soon as a cavity is formed by the laser, water will fill the microchannel, efficiently removing debris and heat. (3) Consequently, a 3D microchannel is formed. By investigating the relationship between the maximum length of the laser modified region and key variables such as average power and scanning speed, Li and Qu successfully fabricated microfluidic devices with micropools and microchannels [34,143] at a fabrication velocity of ~ 200 µm/s (Fig. 4(b)). Besides, Hwang et al. fabricated microchannels in different shapes with a high aspect ratio and compared the performance of several liquids in their research [144] . Ke et al. recorded the dynamics of low energy femtosecond laser-induced bubbles and also fabricated a spiral channel [145] . An et al. reported a double chambered structure fabricated by serial drilling in silica glass [146] while Li et al. [147] constructed a microfluidic chip with complex 3D

13 microchannels. Liu et al. developed the liquid assisted method with unique applications that three-dimensional metallic micro components was made in the pre-fabricated microchannel where liquid metal was injected in. Then, the liquid metal was solidified in the icebox (-1°C) thus formed the metallic micro-component [148] . In addition, water-assisted method was also demonstrated in the fabrication of three-dimensional microfluidic channels of arbitrary lengths and configurations inside porous glass using femtosecond laser direct writing [35] . The fabrication process includes two steps: (1) Femtosecond laser direct writing of hollow microchannels in the porous glass substrate immersed in water; (2) Post-annealing of the glass substrate at 1150oC to consolidate the porous glass, and thereby producing a square-wavelike channel 1.4 cm in length with a 64 µm diameter located 250 µm beneath the glass surface. Using this method, Liao et al. fabricated a 3D microfluidic mixer of superior performance [127] and sub-50 nm nanofluidic channels buried inside porous glass which they used for DNA analysis [128] . A distinct advantage of fabricating microstructures inside porous glass is that the porous structure is naturally discrete, making it easier to fabricate hollow structures inside the glass. It should thus be possible to fabricate microchannels of an arbitrary length as required, but the requirement for baking after irradiation extends the processing period.

Fig. 4. Micro-devices fabricated by femtosecond laser CHISM technology. (a) A microfluidic laser fabricated in photosensitive glass by etchant-assisted femtosecond laser selective etching (top view) [41,130] , (b) microfluidic chip fabricated in silica glass by water-assisted femtosecond laser fabrication [34] , (c) Microchannel fabricated in PMMA with femtosecond laser direct-write fabrication [37] . Reproduced with permission from Ref. 41, 34 and 37. © 2005 Springer-Verlag, © 2013 Elsevier B.V., and © 2003 Springer-Verlag.

In conclusion, liquid assisted femtosecond laser selective etching are classified with two types, either etchant-assisted (acid/ alkali aqueous solutions), or water assisted. With etchant-assisted selective etching, microstructures are able to be fabricate inside photosensitive glass, fused silica glass, semiconductors, and polymers. The mechanism for the manufacturing in photosensitive glass is different from others since the fs-laser-induced photochemical reaction plays an important role. While, there are still some challenges remaining for the future study of the

14 etchant-assisted method such as the optimization of the shape-control methods, long-distanced microchannel fabrication and the enhancement of the productivity. On another hand, with the water-assisted method, long channels can be fabricated very quickly (200 µm/s) and the surface roughness can be significant reduced annealing [143] . Still, there are also drawbacks associated with the water assisted method, as a hose should be set up at the entrance of the channel to effectively exclude the debris for long channel fabrication, it requires a considerably more complex experimental setup.

2.4.2. Femtosecond laser direct-write fabrication

Compared with liquid assisted femtosecond laser selective etching, femtosecond laser direct-write fabrication indicates that microstructures are fabricated in bulk material simply by laser direct writing, or combined with simple post-annealing process, but no liquid is needed in the fabrication process. Thus, the manufactured materials requiring to have relatively lower optical breakdown threshold. Polymer materials and semiconductors are frequently applied in the past studies. Relative to inorganic materials, polymers are often more suitable for selected biomedical microfluidic applications, such as protein absorption and cell cultivation [51] . Compared with the micromachining of inorganic materials, polymer processing with ultrafast lasers is increasingly finding novel applications in microfluidics, since hollow structure is easier to be created by fs-laser-induced photochemical dissociation [149] . Yamasaki et al. [37] first reported by using 12 nJ femtosecond laser pulses with a pulse duration of 250 fs and a wavelength of 800 nm, when laser beam was focused by a high numerical aperture objective with NA 1.35, microchannels can be fabricated in a one-step direct-writing at a speed of 235 µm/s. The fabricated channels had a self-formed densified cladding and could be 3D stacked into any pre-designed pattern which extended over a 1 mm length. Kondo et al. [150] then fabricated microchannels inside PMMA using a similar experimental setup, and the channel was also examined by rhodamine photoluminescence to find out whether the channel was blocked or discontinuous. Farson et al. [151] further developed the one-step method by introduced a gas flow setup in the fabrication process to assist the debris removal and 5-10 mm long microchannels with relatively smooth surface were manufactured . It is very convenient to fabricate 3D channels in one step inside PMMA. However, the reported works are mostly restrained in fabricating the microfluidic channels with nanometer-scaled diameters. Since currently nanofluidic devices are somewhat premature and have limited applications due to strong capillary forces and low flow rates, nano-channels created in PMMA using a high NA value ( >1) objective, although technically feasible to fabricate in one-step with a perfect cross section, have limited practical application currently. Scaling up the diameter of the channel to microchannels, there are both obvious microfluidic applications, and ready manufacturing methods by combining FsLDW with assisted compressed air removal. However, assisted channels may be prudent to be generated before the fabrication of the desired channels, as the assist channel may hinder the intended use in a functionally integrated device. Further, the possibility of fabricating 3D hollow channels inside silicon (of ~300µm thickness) by one-step FsLDW was by [36] . Although the possibility of forming a hollow structure in less transparent material has been demonstrated, only thin slices (in the order of several hundred microns) of semiconducting material can currently have hollow channels fabricated, although

15 further work producing continuous hollow structures in semiconductor of greater dimensions should and will be developed.

3. Summary and Outlook

In summary, four typical three-dimensional internal microstructure manufacturing technologies including multiphoton polymerization, laser modification, microexplosion and continuous hollow internal structure manufacturing are classified according to the generated characterized structures after femtosecond laser irradiation. The mechanisms and advanced applications in micro-optics, micro-machine, microfluidic devices, and photonic devices of each technology were addressed in this review, with advantages and deficiencies of each discussed. As a brief review, the applications of these three dimensional internal manufacturing methods, as well as the common used materials, the advantages and disadvantages of the technologies, are summarized in Table 1, providing a direct cognitive impression of the differences between these technologies. Since femtosecond laser 3D internal manufacturing can be employed anywhere within materials with high spatial fabrication resolution, it has virtually unlimited potential to fabricate highly functional integrated devices, especially if multiple or even all these technologies are integrated in a single device. Recent progress for a combination of two photon polymerization and CHISM in photosensitive glass set up very good examples for the future applications of 3DIM technologies [57,152] . Heading towards the bright future of 3DIM technologies, formidable challenges in the precise control of fabricating conditions (such as power density, pulse duration, pulse-shaping methods and other additional variables) and the improvement in processable materials will remain as heavy tasks. Nevertheless, the field of 3DIM is increasingly attractive to researchers developing novel commercial applications, which in turn can revolutionize manufacturing, life science, information science and other sectors employing nanotechnology.

Acknowledgements The authors gratefully acknowledge reproduction permissions provided by Optical Society of America, the Royal Society of Chemistry, Springer-Verlag, American Institute of Physics, John Wiley and Sons, and Elsevier B.V.. We also acknowledge and thank the financial support from the National Natural Science Foundation of China (Grant 50875007), the Ministry of Science and Technology of China Major Project of Scientific Instruments and Equipment Development (Grant 2011YQ030112), Key Projects of Science and Technology of Beijing Municipal Commission of Education (Grant KZ201210005009 and KZ201410005001), the Beijing Natural Science Foundation (Grant 4132017), the Beijing high level overseas talent project and the international exchange grant of the graduate school of Beijing Institute of Technology. In addition, we appreciate the research initiative funding provided by the University of Tennessee as a new hire package.

16 Table 1. Comparison of 3DIM technologies Required Peak Method Applications Materials Characteristics Power Intensities Complex 3-dimensional Microstructures, Photoresists such as: microstructures can be precisely microfluidic devices, SU-8 Multiphoton fabricated with micromechanical SCR500 1012 ~ 1013 W/cm2 [18] Polymerization sub-diffraction-limit spatial structures, periotic AZ-9260 resolution and low surface structures, cell scaffolds, IP-x series roughness . Refractive modified region with Polymer [89] : arbitrary shape and length can 1012 ~1013W/cm2 be fabricated inside most of the Waveguide, grating, Transparent bulk transparent dielectrics. Obvious Laser beam splitter, optical dielectrics Glass [26] : property contrast (refractive Modification storage, selective 1013~1014W/cm2 index, etching ratio, chemical etching valence-state) between modified area and bulk material is generated. Permanent void like structural change with micrometer scale diameter can be generated. Data Optical data storage, recording density reaches as Transparent bulk Microexplosion micro-optic devices and 1014~1015W/cm2 [28,110] high as 500 GB/cm3 and dielectrics 3D sculpture artworks. microlenses fabricated inside the bulk material can be obtained with the help of microexplosion. [33,52] Etchant-assisted : 13 14 2 Complex 3D continuous hollow Photosensitive glass, 10 ~10 W/cm structure can be fabricated Microfluidic devices, fused silica, inside various bulk materials. Water-assisted [34] : CHISM micro-optical devices, porous glass, 15 2 Fine optical properties, arbitrary micro-machines polymer, 10 W/cm length, one-step fabrication can

semiconductor. [37,151] be realized when carefully Direct-write : chosen the materials. 1013 ~1015 W/cm2

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