Femtosecond Mathieu Beams for Rapid Controllable Fabrication of

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Femtosecond Mathieu Beams for Rapid Controllable Fabrication of Complex Microcages and Application in Trapping Microobjects Chaowei Wang, Yanlei Hu, Liang Yang, Shenglong Rao, Yulong Wang, Deng Pan, Shengyun Ji, Chenchu Zhang, Yahui Su, Wulin Zhu, Jiawen Li, Dong Wu, and Jiaru Chu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Femtosecond Mathieu Beams for Rapid Controllable Fabrication of Complex Microcages and Application in Trapping Microobjects Chaowei Wang1, Yanlei Hu1, Liang Yang1*, Shenglong Rao1, Yulong Wang1, Deng Pan1, Shengyun Ji1, Chenchu Zhang2, Yahui Su3, Wulin Zhu1, Jiawen Li1, Dong Wu1*, and Jiaru Chu1 1Hefei

National Laboratory for Physical Sciences at the Microscale and CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, China 2Institute of Industry and Equipment Technology, Hefei University of Technology, Hefei 230009, China 3School of Electronics and Information Engineering, Anhui University, Hefei 230601, China. Corresponding author E-mail: [email protected] and [email protected] ABSTRACT Structured laser beams based microfabrication technology provides a rapid and flexible way to create some special microstructures. As an important member in the propagation invariant structured optical fields, Mathieu beams (MBs) exhibit regular intensity distribution and diverse controllable parameters, which makes it extremely suitable for flexible fabrication of functional microstructures. In this study, MBs are generated by phase-only spatial light modulator (SLM) and used for femtosecond laser two-photon polymerization (TPP) fabrication. Based on structured beams, a dynamic holographic processing method for controllable three-dimensional (3D) microcages fabrication has been presented. MBs with diverse intensity distributions, are generated by controlling the phase factors imprinted on MBs with SLM, including feature parity, ellipticity parameter q, and integer m. The focusing properties of MBs in a high numerical aperture laser microfabrication system are theoretically and experimentally investigated. On this basis, complex two-dimensional (2D) microstructures and functional 3D 1

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microcages are rapidly and flexibly fabricated by the controllable patterned focus, which enhances the fabrication speed by 2 orders of magnitude compared with conventional single-point TPP. The fabricated microcages act as a nontrivial tool for trapping and sorting microparticles with different sizes. Finally, culturing of budding yeasts is investigated with these microcages, which demonstrates its application as 3D cell culture scaffolds. Keywords: two-photon polymerization, Mathieu beams, polymer-microcages, dynamic holographic processing, microobject trapping Trapping and holding micro-objects has been a hot research focus in recent years, which has important applications in biomedical research and fundamental studies of cell behavior. Optical tweezer,1-3 a simple and robust implementation for microobjects manipulation, has been successfully used to manipulate micro-objects from several micrometers to tens of nanometers. However, optical tweezer is usually generated by a tightly focused laser beam, which is harmful to living samples or chemical reactions. Therefore, to address these issues, some specifically designed microstructures were fabricated for micro-objects manipulation.4, 5 As an example, hierarchical helical arrays realized by capillary force assembly of micropillar arrays,5 which was fabricated by the two-step soft transfer of a silicon template prepared by UV lithography and plasma etching, were used to trap lots of particles rapidly and simultaneously. Miniaturized self-folding grippers which are actuated by differential residual stress,6 temperature,7,8 magnetic force,9 and chemicals10,11 can trap microparticles inside “cage-like” spaces. Moreover, corner lithography,12 which is a wafer scale nano-patterning technique 2


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involving conformal deposition and selective isotropic thinning, can form arrays of nanowire pyramids in sharp concave corners for micro-beads and cell trapping. Although a variety of complex microstructures have been achieved, many of them have distinct drawbacks of complicated fabrication process or dependence on expensive apparatus and low controllability. Therefore, it is still a highly demanded to develop a simple, rapid and controllable method to fabricate desired micro/nanostructures for manipulating microobjects. Two-photon polymerization (TPP) technique,

13-16

is extensively studied for

fabricating complex functional 3D micro/nanodevices, owing to its simple operation, sub-100 nm resolution, and intrinsic three-dimensional (3D) processing ability. However, the wide application of TPP is restricted by the low fabrication speed due to the single-point scanning strategy.17 In order to improve the efficiency, typical methods e.g., multi-foci parallel fabrication by microlens arrays,18 diffractive optical elements,19 or multibeam interference 20 are developed. Nevertheless, the disadvantage of these methods is that the positions of the foci are fixed by the optical elements. Moreover, these methods are only suitable for fabricating microstructure arrays with the same morphology. It is a much more flexible way to fabricate complicated microstructures by using a SLM.21-24 Till now, multifocal fabrication of functional microstructures have been performed by adopting SLM, such as microoptics25 and microfluidic structures.26 Besides multi-foci array, structured beams are capable of fabricating specific microstructures with high efficiency.27,

28

Mathieu Beams,29-31 as a kind of special

structured beams, have two interesting features: “diffraction-free”-i.e., preserving their 3


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shape while propagate, and “self-healing”-i.e., recovering to its original shape even if part of the beam is destroyed by an small obstacle. Due to these specific features, MBs have been already used for 3D optical trapping and micromanipulation.32 However, adopting MBs in femtosecond laser TPP processing for rapid fabrication of functional 3D structures has not been reported so far. The focusing properties of MBs under high numerical aperture (NA) and the impact of laser beam parameters on fabricated micro/nano structures have not been quantitatively investigated. In this work, we present a dynamic holographic processing method to fabricate 3D microcages. Firstly, MBs are generated by using a liquid crystal phase-only SLM without any other optical elements. The focusing properties of MBs in a high NA laser microfabrication system are investigated by Debye vectorial diffraction theory. Then, the relationship between intensity distribution of focused MBs and phase factors imprinted on MBs by SLM, including feature parity, ellipticity parameter q, and integer m, is theoretically and experimentally studied. On this basis, a simple method to fabricate 3D microcages by precisely scanning the MBs and Bessel beams (BBs) along Z direction in the photoresist is proposed and experimentally verified. This strategy exhibits great flexibility because the patterned focus can be dynamically controlled. The fabrication speed can be increased by two orders of magnitudes compared with conventional single-point direct laser writing. Finally, effective trapping/sorting (SiO2 particles) and trapping/culturing (in vivo culturing of yeast) using the resultant 3D microcages are demonstrated, which has promising applications in microfluidic and biomedical study. 4


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RESULT AND DISCUSSION Mathieu Beams Generation The generation of MBs is accomplished by displaying the phase of MBs on the SLM, behind which non-diffraction MBs are reconstructed on-axis. The phase masks we imprinted on SLM for Even, Odd, and Helical MBs generation can be expressed respectively as:33 𝜑𝑒𝑚 = 𝐼𝑚(𝑙𝑜𝑔𝑀𝑒𝑚(𝜂,𝜉,𝑞)) = 𝐼𝑚(log (𝐶𝑚𝐽𝑒𝑚(𝜉,𝑞)𝑐𝑒𝑚(𝜂,𝑞))) 𝑚 = 0,1,2,3…(1) 𝜑𝑜𝑚 = 𝐼𝑚(𝑙𝑜𝑔𝑀𝑜𝑚(𝜂,𝜉,𝑞)) = 𝐼𝑚(log (𝑆𝑚𝐽𝑜𝑚(𝜉,𝑞)𝑠𝑒𝑚(𝜂,𝑞))) 𝑚 = 1,2,3…(2) 𝐻𝑀 𝜑𝐻𝑀 𝑚 = 𝐼𝑚(𝑙𝑜𝑔𝑀𝑚 (𝜂,𝜉,𝑞))

= 𝐼𝑚(log (𝑀𝑒𝑚(𝜂,𝜉,𝑞) ± 𝑀𝑜𝑚(𝜂,𝜉,𝑞)))𝑚 = 1,2,3…

(3)

Here, 𝐽𝑒𝑚(𝜉,𝑞) and 𝐽𝑜𝑚(𝜉,𝑞) are the even and odd modified Mathieu functions of order m, and 𝑐𝑒𝑚(𝜂,𝑞) and 𝑠𝑒𝑚(𝜂,𝑞) are the even and odd ordinary Mathieu functions of order m, and 𝐶𝑚 and 𝑆𝑚 are constant. The ellipticity parameter is defined as 𝑞 = 𝑓2𝑘2𝑡 /4. 𝑘𝑡 is the transverse spatial frequency and f is the semi-focal distance. Elliptical coordinates (ξ ,η) are given by Cartesian coordinates (x, y) of SLM pixels through the transformation 𝑥 = 𝑓𝑐𝑜𝑠ℎ𝜉𝑐𝑜𝑠𝜂;𝑦 = 𝑓𝑠𝑖𝑛ℎ𝜉𝑠𝑖𝑛𝜂. Figure 1(a) illustrates the experimental setup of TPP fabrication using MBs. The incident Gaussian beam is modulated by a computer-generated hologram (CGH) displayed on the SLM. All the CGHs used in the experiment need to be pretreated, as shown in Fig 1(b). Handing CGHs in this way serves two purposes. First, due to the pixilation effect of the SLM, blazed grating (BG) phase can separate the zero-order light away from the other modulated light. Thus, with an iris diaphragm, the effect of 5


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zero-order light on the fabricated microstructures can be effectively reduced. Second, since the cross section of Gaussian light incidents on the SLM is circular, the CGHs are also tailored to a circle to better match the incident light. The phase distribution of BG is expressed as 2𝜋𝑥 𝛥, in which Δ represents the period of the BG (15 pixels), and the pixel pitch of SLM is 8 m. So the final phase distribution of the CGH loaded on SLM can be expressed as: 𝑃ℎ(𝑥,𝑦) = 𝑚𝑜𝑑(𝜑𝑚(𝑥,𝑦) + 𝜑𝐵𝐺(𝑥,𝑦), 2𝜋), where mod is remainder operation, 𝜑𝑚(𝑥,𝑦) is the Mathieu beam phase and 𝜑𝐵𝐺(𝑥,𝑦) is the BG phase. The intensity distribution of MBs along the propagation axis is calculated by the Fresnel diffraction theory since the paraxial approximation condition is satisfied. The intensity profiles in the transverse direction, at distances of 0.4, 0.7, 1.0, 1.3 and 1.6 m from the SLM are calculated and experimentally measured [Fig. S1]. It can be seen that the intensity profiles change little both in theoretical prediction and experimental measurement, which demonstrates the propagation invariant property of MBs. Further, simulation of the transverse intensity distribution of reconstructed optical field at different planes are shown in Fig. 1(c). The intensity distribution from plane 1 to plane 4 corresponds to the intensity distribution at the front plane of Lens1 (600 mm), focus of Lens1, back of Lens2 (200 mm), and the entrance pupil of objective, respectively. It is found that the intensity distributions exhibit a main multi-foci ring containing most of the energy and additional surrounding pattern containing much lower energy. An iris was placed at the Fourier plane of Lens1 to filter out unwanted orders and let MBs pass though, as shown in Fig. S2. 6


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Focusing Properties under High Numerical Aperture Objective Lens with Different Focal Length Ratio Lens Group in 4f System It is well known that the diffraction behavior of light tightly focused by a high NA objective is different from the scalar diffraction.34 In order to facilitate the visualization of the field distribution at the focal region, we used Debye vectorial diffraction theory for optical field simulation.35 The intensity distribution in the focal region can be rewritten as:36 𝐸1(𝑥1,𝑦1,𝑧1) =

𝑖𝐶 𝜆

𝛼 2𝜋 ∫0 ∫0 𝑠𝑖𝑛𝜃1 𝑐𝑜𝑠𝜃1𝑃(𝜃1,𝜑1) × exp [𝑖𝑘𝑛0(𝑧1𝑐𝑜𝑠𝜃1 + 𝑥1𝑠𝑖𝑛𝜃1𝑐𝑜𝑠𝜑1 + 𝑦1𝑠𝑖𝑛𝜃1𝑠𝑖𝑛𝜑1)]𝑑𝜃1𝑑𝜑1

(4) where C is a constant, 𝜆 is the wavelength of incident light; 𝑛0 is the refractive index of the immersion medium; 𝛼 is the maximum focusing angle of the objective lens and can be calculated by 𝛼 = 𝑎𝑟𝑐𝑠𝑖𝑛 (𝑟𝑁𝐴/𝑅𝑛0); r is the off-axis coordinate of the incident wave, R is the object lens pupil radius. 𝜃1 is the focusing angle of objective lens, and 𝜑1 is the azimuthal angle of object plane. 𝑃(𝜃1,𝜑1) is the polarization state of the EM field in the focal region, which can be expressed as: 𝑷(𝜃1,𝜑1) = [1 + (cos𝜃1 ― 1)𝑐𝑜𝑠2𝜑1]𝒊 + [(cos𝜃1 ― 1)cos𝜑1sin𝜑1]𝒋 ―(sin𝜃1cos𝜑1)𝒌

(5)

for incidence with linear polarization in the X direction. In the TPP fabrication system, many factors will affect the light field distribution, such as the focal length ratio of the lens in the 4f system. To show the effect of focal length ratio on the focused light under high NA objective lens, theoretical simulations 7


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and experiments are conducted. According to equations (4) and (5), taking the EvenMBs for example, simulated optical intensity distribution with the focal length ratio of lenses L1 and L2 changed from 1:1 to 5:1 are shown in Figure 2. As the focal length ratio of the lenses L1 and the L2 becomes larger, the diameter of the light field entering the pupil is reduced [Fig. 2(a)], and the diameter of the focused light field [Fig. 2(b)] under the objective lens becomes larger (marked in a white line) and the Z direction is stretched [Fig. 2(c)]. The reason is that the numerical aperture of the objective lens is not fully utilized due to the smaller diameter of the incident beam. The ratio of 1:1 and 3:1 were experimentally measured [shown in Figs. S3 and S4], which is in agreement with the theoretical prediction. It is found from the experimental light field in X-Y plane along different Z positions that a discrete intensity profile becomes a single-ring structure gradually. With this factor, it is possible to obtain focused light with different diameters and different Z-direction lengths by changing the ratio of the lens groups. Optical Intensity Distributions and Corresponding Microstructures Fabricated by Mathieu beams with Different Feature Parity and Integer m. Mathieu Beams are characterized by three parameters: the feature parity, the ellipicity parameter q, and the integer m, which is termed topological charge. We systematically studied the effects of these parameters on the intensity profile of MBs and the final fabricated microstructures. On the basis of equations (1) to (3), it can be seen that only in the case of Even-MBs, the integer m is beginning from 0 and the rest two cases are from 1. Figure 3a is a series of CGHs with different parameters for EvenMBs, in which the bright and dark grey values correspond to a phase of 0 and π, 8


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respectively. These CGHs have a similar feature parity (Even) and ellipicity parameter q of 5 without the loss of generality. The integer m changes from 0 to 6 successively. From the simulated results in Fig. 3(b), it can be seen Even-MBs show multi-foci arranged in a ring manner at the focal plane. Around the focus ring, there is still a little of stray light with weak intensity. Apparently, the number of focus 𝑁𝑒𝑚 in the ring is directly related to the integer m, which can be expressed as: 𝑁𝑒𝑚 = (𝑚 + 1) × 2, (𝑚 = 0,1,2,3)

and𝑁𝑒𝑚 = 𝑚 × 2, (𝑚 ≥ 4). The diameters of the rings remain

unchanged. The measured results are consistent with the simulation [Fig. 3(c)]. In experiment, focused optical field is projected into a ~8 μm thick SZ2080 photoresist sample, and complex 2D multi-foci patterns can be fabricated by single exposure without single-point scanning. In order to ensure sufficient TPP all through the polymer and obtain complete microstructures, a longer focal length in Z direction is needed, then the focal length ratio of lenses L1 and L2 is selected as 3:1. All the microstructures are fabricated with a laser power of 110 mW, which is measured in front of the objective, and an exposure time of 100 ms [Fig. 3(d)]. Similar results for Odd-MBs are shown in Figs. 3(e) - (h). Likewise, the measured intensity profile and fabricated microstructure agree well with simulation. It is worth noting that the relationship between the number of focus 𝑁𝑜𝑚 and the integer m is different from the Even-MBs, which can be expressed as: 𝑁𝑜𝑚 = 𝑚 × 2. The results for Helical MBs are shown in Fig. S5, and the relationship between the number of focus 𝑁𝐻 𝑚 and the integer m is the same as Odd ones. We can find that the multi-foci microstructures are not perfectly homogeneously polymerized. This is caused by three aspects. First, the optical system in the experiment 9


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was not perfect. For example, all lenses in the optical system are hardly 100% completely coaxial and it is difficult to completely guarantee the laser beam focus perfectly vertically to the sample after passing through the microscope system. These non-ideal hardware conditions may bring errors to the fabrication. Second, there is a refractive index mismatch between different materials behind objective, which will bring aberrations. Finally, linear polarization incidences also leads to an inhomogeneous intensity distribution after high NA focusing. In order to enhance the resolution and uniformity of fabricated final structures, the point-spread-function of the patterned focus can be optimized by compensating the aberration of high NA fabrication systems.37-39 Optical Intensity Distributions and Corresponding Microstructures Fabricated by Mathieu beams with Different ellipicity parameter q From previous results, it is obvious that when the ellipicity parameter q is constant, the diameter of the micro-ring structures are constant at different integers, for all Even, Odd and Helical MBs. Here, we take the Even-MBs with integer 6 as an example to analyze the effect of q on the diameter of fabricated microstructure, without the loss of generality. Figure 4a is a series of CGHs for the generation of Even-MBs with the same integer m=6 and a set of different ellipticity parameters. From the simulated results in Fig. 4(b), it can be seen that the diameter of the intensity pattern at the focal plane increases with the ellipticity parameter changing from q=1 to q=20. The measured diameter change is consistent with theoretical simulation. Meanwhile, the smaller the ellipicity parameter became, the more evenly spaced the multi-foci is. In Fig. 4(d), all 10


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the microstructures are fabricated by a single exposure with the intensity profile in Fig. 4(c) in a ~8 μm thickness SZ2080 photoresist sample. When the ellipticity parameter q=1, the single exposure time is 80 ms with a laser power of 100 mW. When the ellipicity parameter q=15, the single exposure time is 400 ms and a laser power of 140 mW is needed. As the diameter of the microstructure increases, the needed single exposure time and laser power also increase. All processing parameters are displayed in the Table 1 in Supporting Information. As mentioned in the above section, the focal length ratio of lenses L1 and L2 is selected as 3:1 to ensure that TPP occurs all through the polymer. For the precise control of microstructures diameter, a quantitative study between the diameter and ellipicity parameter q is conducted. The red line in Fig. 4(e) represents the simulated results. When ellipticity parameter q changes from 1 to 20, the diameter of microstructures varies from 4.3 μm to 21 μm. The black line represents the average of measured five microsturctures under the same experimental conditions. Correspondingly, the average diameters of microstructures vary from 4.1 μm to 20.2 μm. In the simulation, the diameters are calculated according to the half-height full width of the intensity distribution. It should be noted that a wider range of diameters of the microstructures could be obtained when an appropriate beam reducer and low magnification objective are used. The deviation between measured and calculated diameters may come from two aspects. The first and the most important reason is the shrinkage of photoresist, especially when the laser exposure dosage is close to the photoresist polymerization threshold. The second reason is that the simulation is based on a series of ideal 11


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experimental conditions. For example, the incident light we supposed is a uniform standard Gaussian beam and the optical system is idealized. Actually, errors will be caused by the pixelization of holograms, the high NA objective system and the nonideality of the optical system. With this MBs based TPP processing, a complex 2D microstructure can be easily fabricated combined with the movement of sample anchored to the piezoelectric platform. Figures 4(f)-(g) and Fig. S5 are the SEM and fluorescence images of patterned micro-ring array. The processing parameters for each micro-ring are the same as in Fig. 4(d). Dynamic Holographic Processing of Various Controllable 3D Microcages Based on systematic study on TPP with MBs, a method for rapid fabrication of 3D microcages by scanning patterned focus along the Z direction is proposed. Different from the conventional TPP process by single-point scanning, this method can greatly reduce the processing time. Furthermore, by changing the tailored holograms during the scanning of patterned focus with evenly spaced, microcages with variable cross section could be successfully produced. With this dynamic holographic processing method (DHP), we can easily and rapidly fabricate different kinds of microcages. Figure 5(a) is the schematic illustration of DHP method. Different layers are processed by MBs and BBs. Taking single-layer microcages as example, the stretching speed of the platform is 50 μm/s, and the designed height of bottom pillars in the microcages is 15 μm. So the fabrication time is 0.3 s, which means the hologram (Mathieu beam) is displayed for 0.3 s. Meanwhile, the designed height of junction in the microcages is 3 12


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μm, making the fabrication time to be 0.06 s, which means the hologram (Bessel beam) is displayed for 0.06 s. Therefore, we designed a Gif figure in advance which contains a 0.3 s Mathieu beam hologram and a 0.06 s Bessel beam hologram. In this way, with the integrated control software, the dynamic holographic processing has been realized. In experiment, the thickness of photoresist is ~500 μm thick, which ensures that there is enough space to process the 3D microcages. For precise 3D microfabrication, a higher Z-direction resolution is required, then the focal length ratio of lenses L1 and L2 is selected as 1:1. Figure 5(b) is the SEM image of single-layer microcages with eight pillars at the bottom. The microcages have ~18 μm height and ~10.5 μm diameter. The fabrication time for one single-layer microcage is 0.36 s, while it increases to 54 s for conventional single-point scanning [Fig. 5(c)], which means that the fabrication time can be reduced by 2 orders of magnitudes. Figure 5(d) is the SEM image of two-layers microcages with eight pillars at the bottom layer and six pillars at the top layer. The microcages have ~36 μm height and ~10.5 μm diameter. The stretching speed for fabrication is 50 μm/s, while the used power is 90 mW. Similarly, the fabrication time can reduce from 102 s to 1.12 s [Fig. 5(e)] compared with single-point TPP processing in theory. Figure 5(f) is the SEM image of three-layers microcages with eight pillars at the bottom, six pillars at the middle and four pillars at top. The microcages are ~54 μm in height and the diameters are the same as in Figs. 5(b) and (d). Compared to singlepoint scanning method, the fabrication time-consuming is 100 times less. Since the scanning strategy is moving the sample along the z direction, the height of the microcages could be fabricated as long as the working distance of the oil13


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immersion lens. Therefore, this processing method can fabricate high aspect ratio microcages [Fig. 5(h)]. These microcages have ~90 μm height and the aspect ratio is 9: 1 approximately. Further, the location of microcages can be well designed, for example, a “USTC” pattern was designed by microcages with different heights [Fig. 5(i)]. It is challenging for other techniques to fabricate these 3D microcages, and thus our approach shows the distinct advantages in terms of 3D and customization capabilities. 3D Microcages for Trapping Microobjects The ability to manipulate microobjects is highly desirable in biological devices and chemical analysis and have drawn great attention. Although entirely new perspectives are provided by the complex patterning of light fields in advanced optical tweezer micromanipulation,40 substantial 3D microstructures are still desired to be fabricated so that energy beam do not cause harm to living samples or chemical reactions. In this work, we demonstrate the feasibility of 3D microcages fabricated by DHP for trapping particles, as illustrated in Figure 6(a). A deionized water solution containing 5 μm diameter SiO2 particles is dropped on the sample which is fabricated on glass slide. A part of particles are trapped in the microcages by gravity and hydrodynamic forces. Then, the sample is cleaned by deionized water and shaken slightly to flush the non-trapped particles away. The trapped particles are confined in the microcages [Figs. 6(b)-(d)]. Trapping efficiency is an important factor to evaluate the trapping capability. We define the trapping efficiency as the ratio of the microcages that trap SiO2 particles with success to the total number of microcages. In our experiments, the trapping efficiency can reach more than ~90% [Fig. 6(a) and Fig. S7] 14


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when the number of repeating cycles exceeds to 5. After we repeated the trapping experiments for several times (>5), the microcages can trap microparticels in every experiment and have no obvious deformation. In recent years, microobjects sorting is important to enrich or purify bio-samples into well-defined group for plenty of applications. A variety of microfluidic sorting methods based on 2D substrates have been developed to separate microparticles based on size.41, 42 Here, we show that our 3D microcages can separate particles of different diameters by changing the number of pillars at the bottom, similar to the process in Fig. 6(a). Deionized water solution containing different SiO2 particles with various diameters (e.g., 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7μm) is dropped on the samples fabricated on the slide. There are two types of microcages. One is single-layer microcages with four pillars at the bottom [Fig. 6(e)] and the other is single-layer microcages with eight pillars at the bottom (Fig. 6(h)). Due to different number of pillars, the distance between the pillars is different. For four pillars microcages, small particles (2 μm and 3 μm) will be washed out from the gap (~4.3 μm) between two adjacent pillars while large particles (5 μm and 6 μm) remain inside the microcages [Figs. 6(f) and (g)]. In contrast, both small and large particles remain inside the microcages with eight pillars [Figs. 6(i) and (j)], due to smaller pillar gap (~1.2 μm). Therefore, particles sorting is achieved by these microcages with different shapes. In general, the growth environment of cells in vivo is mainly a 3D microenvironments, in which cell behaviors are different from that in 2D substrates. Here, the microcages can act as functional 3D cell culture scaffolds for cell tapping and 15


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culturing. As shown in Figure 7, yeast cells (ANGEL YEAST CO., LTD) are trapped and then subsequently cultured in liquid culture medium. The trapping of yeasts are realized by dropping a few drops of high concentration yeast culture medium on the microcages. A part of yeasts are trapped in the microcages by gravity and hydrodynamic forces, after which the sample is cleaned by the culture medium. Finally, the growth and dividing process of trapped yeast is observed in situ with a bright field microscope for 3 hrs in 20 mL culture medium. The 3 hrs culturing of yeast in microcage can be divided into two steps. The first step is the growth of the primary generation of yeast [Figs. 7(a) (b) and (c)]. The second step is the budding process and the growth of the first generation of yeast [Figs. 7(d) (e) and (f)]. The dependence of the diameter of the yeasts on culture time is shown in Fig. 7(g). With the increasing of culture time, the diameter of the primary generation of yeast increases obviously in the first step (blue area) but enlarges little when the budding process starts (gray area). After 0.5 hrs of culture, the first generation of yeast grows to a normal volume. The SEM images of the 3D microcage with the cultured yeasts are shown in Fig. 7(h). This method provides a valid solution for observing cellular behaviors in 3D microenvironment. CONCLUSION In summary, a rapid and flexible 3D microcages fabrication method based on dynamic holographic processing is developed. The focusing properties of MBs generated by phase modulation are theoretically and experimentally investigated. 2D circle microstructures with multi-foci are fabricated by single exposure of focused MBs. 16


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Moreover, 3D microcages with various geometry are fabricated by dynamically controlling the different patterned focuses. Compared with the traditional single-point scanning direct laser writing, the fabrication speed can be increased by 2 orders of magnitudes. These 3D microcages are demonstrated to have fine ability in trapping SiO2 particles with a high trapping ratio and sorting microparticles with different shapes. Finally, the 3D microcages are used as cell culture scaffolds for the observation of growth and budding of yeast. We believe these controllable 3D microcages can act as a capture-hold-analyze system for a broad application such as microfluidics, and biological research. METHODS SECTION Numerical simulation of the intensity distribution in the focal region under a high NA objective: It is crucial to investigate the focusing properties of the MBs. Since the paraxial approximation does not consider the vectorial nature, in this work we use Debye vectorial diffraction theory, which describes the depolarization effect of a high NA object by respectively calculating the three orthogonal field components 𝐸𝑥, 𝐸𝑦, 𝐸𝑧. Holograms Generation: In this work, we use Matlab software to generate and compute holograms (BG’s hologram and Mathieu beam’s hologram). All the holograms are designed to be matrixes with the same dimension (1080*1080 in our experiments) corresponding to the modulated liquid crystal panel pixels of SLM. Every element value in the matrixes is the phase to be modulated. As we know, the incident wave-front can be expressed as 𝐸(𝑥,𝑦) = 𝐴(𝑥,𝑦)𝑒𝑖𝜑𝑎(𝑥,𝑦). When it propagate through a pure phase plate 𝜙(𝑥,𝑦) = 𝑒𝑖𝜑𝑏(𝑥,𝑦), the modulated output beam can be expressed as 𝐸𝑜𝑢𝑡 17


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(𝑥,𝑦) = 𝐸(𝑥,𝑦) ∗ 𝜙(𝑥,𝑦) = 𝐴(𝑥,𝑦)𝑒𝑖[𝜑𝑎(𝑥,𝑦) + 𝜑𝑏(𝑥,𝑦)]. In our experiment, we can modulate the incident Gaussian beam by the Mathieu beam’s hologram phase 𝜑𝑚(𝑥,𝑦) and the BG’s hologram phase 𝜑𝐵𝐺(𝑥,𝑦), so we just need to add two matrixes together numerically. The final phase distribution should be loaded on the SLM is 𝑃ℎ(𝑥,𝑦) = 𝑚𝑜𝑑(𝜑𝑚(𝑥,𝑦) + 𝜑𝐵𝐺(𝑥,𝑦), 2𝜋), where mod is remainder operation. Materials and Equipment: A commercially available zirconium–silicon hybrid sol-gel material doped with 4, 4’-bis (diethylamino)-benzophenone photoinitiator at 1% by weight (SZ2080, IESLFORTH) was used for photopolymerization in our experiment. Before microstructure processing, the SZ2080 was placed on a thermal platform set to be 100℃ for 1 h to evaporate the solvent. The femtosecond laser source is a modelocked Ti:sapphire ultrafast oscillator (Coherent, Chamleon Vision-S) with central wavelength at 800 nm, pulse duration of 75 fs, and repetition rate at 80 MHz. The laser power is modulated with a half-wave plate and a Glan laser beam splitter. After expansion, the laser beam illuminates a phase-only SLM (Pluto NIRII, Holoeye, 1920×1080 pixels, 256 grey levels, and pixel pitch of 8 μm). The modulated beam is relayed to a 4f system and focused by a high NA object lens (60×, NA=1.35, Olympus) into the photoresist which is mounted on a piezoelectric platform (E545, from Physik Instrumente GmbH & Co. KG, Germany) with nanometer resolution and a 200 µm × 200 µm ×200 µm moving range. After polymerization, the sample was developed in 1propanol for 1 h until all the unpolymerized parts were washed away. Manipulation of Micro-particles: SiO2 microparticles with different diameters synthesized by Wakely Scientific Corp. Inc were mixed in deionized water solution 18


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with a concentration of 10-3 g ml-1. In order to make the SiO2 particles evenly distributed in the deionized water solution, the container was shaken gently. The sample was horizontally placed on the stage of an optical microscopy (Leica DMI3000B). The solution was dropped on the sample by a pipette and most particles sank to the bottom after 10 min. In this process, a part of particles were trapped within the microcages. Then, the sample with trapped particles was transferred to a petri dish with deionized water to wash untrapped particles away. Finally, the sample is mounted on the stage for a few minutes for the evaporation of the rest deionized water. This process is repeated for several times in order to achieve a high trapping ratio. Yeast Culture: A kind of Saccharomyces cerevisiae (ANGEL YEAST CO., LTD) was used in the culture experiment. The liquid culture medium was produced by sterilizing the mixture of solid granular Granular Sabouraud’s Glucose broth medium (Peptone 10.0 g L−1, Glucose 40 g L−1, pH-value 5.6 ± 0.2 at 25 °C, QingDao Hope Bio-technology CO.,LTD) and distilled water with a concentration of 50 g L−1 at 120 °C for 20 min. The high concentration yeast medium was produced by dissolving 0.1 g dry yeast uniformly into 15 mL liquid culture medium. Imaging Characterization: The SEM images were taken with a secondary electron scanning electron microscope (ZEISS EVO18) operated at an accelerating voltage of 10 keV, after the samples were sputter coated with gold. The gold vaporization time is 80 s and the gold thickness is ~ 10 nm. ASSOCIATED CONTENT ACKNOWLEDGMENTS 19


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This work was supported by the National Natural Science Foundation of China (Nos. 61475149, 51675503, 51875544, 61805230, 11801126), the Fundamental Research Funds for the Central Universities (WK2090090012, WK2090000013, WK2480000002, 2192017bhzx0003), Youth Innovation Promotion Association CAS (2017495) and National Key R&D Program of China (2018YFB1105400).We acknowledge the Experimental Center of Engineering and Material Sciences at USTC for the fabrication and measuring of samples. This work was partly carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: More details about Numerical simulation, experimental results, and discussions (PDF) ORCID Dong Wu: 0000-0003-0623-1515 Yanlei Hu: 0000-0003-1964-0043 Jiawen Li: 0000-0003-3950-6212 Jiaru Chu: 0000-0001-6472-8103

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Figures and their Captions

Figure 1. Dynamic holographic femtosecond laser processing system and the propagation properties of generated MBs. (a) Schematic diagram of an SLM-based fabrication system. HWP, half wave plate; GTP, Glan Laser beam splitter; Lcos-SLM, liquid-crystal-on-silicon spatial light modulator; M, mirror; L, lens. Femtosecond laser is modulated to MBs by the SLM, which displays the designed holograms. b) Illustration of the computer-generated hologram. c) The simulated transverse intensity distribution of MBs at different planes along the propagation direction in the optical systems. Plane 1 to plane 5 correspond to front of Lens1, focus of Lens1, back of Lens2, front of objective and focus of objective, respectively. 25


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Figure 2. Simulated optical intensity distribution at the pupil and the focal region of a 1.35 NA oil immersion objective for Mathieu beams with different ratio between Lens 1 and Lens 2. a) The intensity distribution at the pupil of the oil immersion objective. b) and c) are the intensity distributions at XZ and XY planes of focused Mathieu beams. The hologram for the simulation is Even-MBs with integers m=6 and q=6, without the loss of generality.

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Figure 3. Numerical simulations, experimental measured intensity profile and fabrication results of MBs with different feature parity and integer. (a) Holograms for Even-MBs with different integers from m=0 to m=6, while fixing q=5, without the loss of generality. (b) Calculated and (c) measured intensity distribution in X-Z plane at the focus plane. d) SEM images of multi-foci microstructures by single exposure of femtosecond MBs. All the microstructures are fabricated with a laser power of 110 mW, and an exposure time of 100 ms. e) Holograms for Odd-MBs with different integers from m=1 to m=7, while fixing q=5, (f) Calculated and (g) measured intensity 27


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distribution in X-Z plane at the focal plane. h) SEM images of multi-spots microstructures by single exposure of femtosecond MBs. The processing parameters are the same as mentioned in (d). All the scale bars are 5 μm.

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Figure 4. Controlling the diameter of fabricated microstructures by tuning the ellipticity parameter of MBs. (a) Holograms for Even-MBs with different ellipticity parameter from q=1 to q=20, while fixing m=6. (b) Calculated and (c) measured intensity distribution in X-Z plane at the focus plane. d) SEM images of multi-foci microstructure fabricated by a single exposure of femtosecond MBs with different diameters from 4.1 μm to 20.2 μm. e) Dependence of the diameter of the polymerized micro-ring on ellipticity parameter q. The inset shows the measured diameter. f) SEM and g) fluorescence images of a “Micro-ring array” pattern. All the scale bars are 10 μm. 29


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Figure 5. Various kinds of 3D microcages fabrication with the dynamic holographic processing technique. (a) Illustration of the fabrication process of a three-layer microcage. Different layers are processed by MBs and BBs. The scanning speed is 50 μm/s, while the used power is 90 mW. (b), (d), and (f) are SEM images of different kinds of microcages. (c), (d) and (f) are the quantification of single structure processing time. The black column represents dynamic holographic femtosecond laser processing, and the red column represents single-point scanning. (h) The SEM image for high aspect ratio microcages (height: ~90 μm). (i) 3D microcages array with two different heights distributed in “USTC” pattern. All the scale bars are 10 μm.

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Figure 6. Microparticles trapping and sorting by microcages with different pillar numbers. (a) Schematic procedure of the SiO2 particles trapping. Deionized water solution containing 5 μm diameter SiO2 particles is dropped on the sample. Then, the sample is put in a petri dish with deionized water and shaken slightly to flush away free particles. At last, almost all microcages are filled with particles after repeating this process several times. b) SEM image of the sample after trapping SiO2 particles. c) A tilted view (45°). d) A magnified view. e) and h) are the schematic diagram for 31


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separating particles of different diameters using two different kinds of microcages. f), g), i) and j) are the SEM images of the separating results with top view and tilted view (45°). It is obvious that the microcages with four pillars (gap: ~4.3 μm) at the bottom can only trap large diameter particles (5 μm and 6 μm) and small particles (2 μm and 3 μm) are washed out from the gap (~4.3 μm) between two adjacent pillars, while the microcages with eight pillars (gap: ~1.2 μm) at the bottom can trap particles with different diameters. All the scale bars are 10 μm.

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ACS Nano

Figure 7. Microcage as 3D culture scaffold for yeast trapping and culturing. (a) A yeast is trapped by a 3D microcage for culturing. The diameter of the primary yeast is 4.2 μm. (b) After 1.0 h of culture, the diameter increases to 5.5 μm. (c) After 2.0 hrs of culture, the diameter increases to 6.5 μm. (d) The diameter of the primary yeast changed little after 2 hrs of culturing. Afterwards, it begins to bud at ~2.3 hrs, the diameter of the first generation of yeast is 0.5 μm at 2.5 hrs, 2.6 μm at 2.7 hrs (e), and 4.5 μm at 3 hrs (f). (g) Dependence of the diameter of the yeasts on culturing time. (h) SEM images of the 3D microcage with the cultured yeasts. All the scale bars are 5 μm.

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Table of Contents Entry

Mathieu beams with diverse intensity distributions in a high NA laser microfabrication system is systematically studied by Debye vectorial diffraction theory. Complex 2D microstructures are processed. Meanwhile, 3D functional microcages are rapidly and flexibly fabricated based on dynamic holographic processing. Effective trapping/sorting (SiO2 particles) and trapping/culturing (in vivo culturing of yeast) using the resultant 3D microcages are demonstrated.

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Femtosecond Mathieu Beams for Rapid Controllable Fabrication of

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