Deformation Behavior of Foam Laser Targets Fabricated by Two-Photon Polymerization

Abstract

Two-photon polymerization (2PP), which is a three-dimensional micro/nano-scale additive manufacturing process, is used to fabricate component for small custom experimental packages (“targets”) to support laser-driven, high-energy-density physics research. Of particular interest is the use of 2PP to deterministically print millimeter-scale, low-density, and low atomic number (CHO) polymer matrices (“foams”). Deformation during development and drying of the foam structures remains a challenge when using certain commercial acrylic photo-resins. Acrylic resins were chosen in order to meet the low atomic number requirement for the foam; that requirement precludes the use of low-shrinkage organic/inorganic hybrid resins. Here, we compare the use of acrylic resins IP-S and IP-Dip. Infrared and Raman spectroscopy are used to quantify the extent of the polymerization during 2PP vs. UV curing. The mechanical strength of beam and foam structures is examined, particularly the degree of deformation that occurs during the development and drying processes. The magnitude of the shrinkage is quantified, and finite element analysis is used in order to simulate the resulting deformation. Capillary drying forces during development are shown to be small and are likely below the elastic limit of the foam log-pile structures. In contrast, the substantial shrinkage in IP-Dip (~5–10%) causes large shear stresses and associated plastic deformation, particularly near constrained boundaries and locations with sharp density transitions. Use of IP-S with an improved writing procedure results in a marked reduction in deformation with a minor loss of resolution.

Scaling‐Up of Nano‐Architected Microstructures: A Mechanical Assessment

“…Other researchers, such as Liu et al, [30] observed similar axial shear stress in analogous regions along the interface of a printed log-pile structure and in a fully dense "top cap," which is comparable to the grip pieces printed in the present work. In addition, we also observe non-uniform shrinkage at the stitch line (marked by yellow arrows) as evidenced by the sudden noticeable change in gage width (Figure 2e,f ).…”

“…Figure 1 shows the gage sections of both low-and high-density samples, as well as the as-printed feature dimensions. [30,31,33] The effects of this shrinkage on the materials' functionality, however, can be widespread, and its impact on the mechanical integrity of the architected materials is still unknown. The 1) width and 2) length of the elliptical voxels for both sets of samples were determined to be 250 and 850 nm, respectively (Figure 1c,d).…”

“…The 1) width and 2) length of the elliptical voxels for both sets of samples were determined to be 250 and 850 nm, respectively (Figure 1c,d). [30][31][32][33] In general, the root cause of these defects is the non-uniform material shrinkage, which is typically concentrated at interfaces, such as at the substrate-sample interface and at the grip/gage www.advancedsciencenews.com www.aem-journal.com transition, where the material properties, like density or modulus, change abruptly. A comparison of the as-designed to the as-printed feature dimensions indicates %12-16% material shrinkage, consistent with the previously reported values for Nanoscribe's photoresist, IP-Dip.…”

“…We hypothesize that non-uniform shrinkage at the stitch line is due to proximity effects, [32,37,38] which leads to reduced oxygen inhibitor concentration within the photoresist at the writing front, which, in turn, increases the degree of conversion [39] and ultimately reduces the extent of shrinkage observed for a given laser power. [30,31,33,41] Micro-tensile tests were performed to assess the impact of these defects on the mechanical performance. Upon continuing the writing process, this results in a lower degree of conversion and consequently greater degree of shrinkage.…”

Optically-Thin Broadband Graphene-Membrane Photodetector

“…The E-field distribution of light in these structures can be calculated and examined indirectly, however, there are no tools to probe the field internally. Ideally, an optically thin detector has to be integrated inside 3D photonic lattices, e.g., made by state-of-the-art direct laser writing 3D nanolithography [22,23]. A photodetector which is weakly absorbing and has low dispersion (non-pertubing 3D photonic crystal) could be made using a graphene-on-membrane approach.…”