handling and assembling of low-density foam

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Handling and assembling of lowdensity foam structures fabricated by two-photon polymerization

O. Stein, Y. Liu, J. Streit, R. Cahayag, Y. Lu, et al.

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HANDLING AND ASSEMBLING OF LOW-DENSITY FOAM STRUCTURES FABRICATED BY TWO-PHOTON POLYMERIZATION a

O. Stein*a, Y. Liub, J. Streita, R. Cahayaga, Y. Lub and N. Pettaa Schafer Livermore Lab, 303 Lindbergh Ave, Livermore CA USA 94551; bDepartment of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, NE USA 68588-0511 ABSTRACT

Low-density foam targets for laser induced shock wave propagation experiments were produced via laser direct writing from polymeric materials. These targets were designed with dimensions of 2.0X0.25X0.3mm with a density of 100mg/cm3 and included a fully polymerized ablation layer, 15μm thick, written contiguously with the low density material in a single fabrication step. We developed a unique protocol in order to assist in the release and harvest of the foam targets from the glass substrate on which they were fabricated. The fabrication process, as well as the assembly and handling of these delicate brittle samples is described in this paper, demonstrating the flexibility, versatility and efficiency of two photon polymerization as fabrication technique that may complement conventional approaches for low density materials fabrication. An ongoing research effort is being conducted in order to reduce to the minimum the content of stitching defects between the target’s building blocks. Keywords: Low density material, 2PP, Laser induced shock, high energy density physics, target fabrication

1. INTRODUCTION Laser direct write via two photon polymerization (2PP) has emerged as a versatile technique for the realization of mesoscale structures, micrometer to millimeters in length with sub-micron resolution. The technique is based on focusing a laser into a polymer resin and photo-chemically initiating polymerization in the focal region. The inherent difference between 2PP and conventional photolithography is that the laser wavelength lays within the near infra-red (I.R.) spectral range, where most photo sensitive resins do not absorb light and are practically transparent. Tightly focusing the light using a high numerical aperture (typically in the range of 1.4) objective elevates the probability of two photon absorption with the sum of the two photon energies equal to a single equivalent ultra-violate photon typically required for photoinitiation.1,2. The absorption rate in that case is proportional to I2, where I is the intensity in the focal region3. The fact that this process involves relatively low absorption cross section results in the volume of photon absorption and polymerization being restricted to the focal region. This volume, dubbed a “voxel”, is the smallest written element, of order 0.1femtoliter. Translating the laser beam in 3D space over a volume of resin results in a 3D object produced with resolutions below the diffraction limit. The spatially controlled laser focus is controlled by using dimensional information taken directly from computer assisted design (CAD), leading to short times between design and fabrication compared with other fabrication techniques. 2PP is used in photonics research4,5 through bio-compatible scaffold structures for cell growth6 to high mechanical strength materials7,8. In all of these examples, micron-scale architecture samples are written directly on the substrate upon they were fabricated demonstrating the vast potential of 2PP as a fabrication tool. Low density materials (LDM’s) are extensively used in High Energy Density (HED) experiments in large laser and pulsed power facilities. For example hydrocarbon foams soaked with deuterium-tritium fuel are used in inertial confinement fusion (ICF) ignition experiments9, 10. Different production processes such as High Internal Phase Emulsion (HIPE) can produce polystyrene foams in densities ranging from 30 to 650mg/cm3 and with varying pore sizes 11. For astrophysical experiments investigating flow dynamics of super-sonic jets in the interstellar medium a recorcinol-formaldehyde foam is used due to its small (