Scalable Light-Printing of Substrate-Engraved Free-Form Metasurfaces

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Functional Nanostructured Materials (including low-D carbon)

Scalable light-printing of substrate-engraved free-form meta-surfaces Jae-Hyuck Yoo, Hoang Nguyen, Nathan James Ray, Michael Johnson, Rusty Steele, John Chesser, Salmaan Baxamusa, Selim Elhadj, Joseph T McKeown, Manyalibo Matthews, and Eyal Feigenbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07135 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

Scalable light-printing of substrate-engraved freeform meta-surfaces Jae-Hyuck Yoo, Hoang Nguyen, Nathan Ray, Michael Johnson, Rusty Steele, John Chesser, Salmaan Baxamusa, Selim Elhadj, Joseph McKeown, Manyalibo Matthews, and Eyal Feigenbaum* Lawrence Livermore National Laboratory, Livermore, CA 94551, USA *[email protected]

ABSTRACT

A key challenge of meta-surface research is locally controlling at-will nano-scale geometric features on meter-scale apertures. Such a technology is expected to enable large aperture metaoptics, and revolutionize fields such as long-range imaging, lasers, LADARs, and optical communications. Furthermore, these applications are often more sensitive to light-induced and environmental degradation, which constrains the possible materials and fabrication process. Here, we present a relatively simple and scalable method to fabricate a substrate-engraved meta-surface, with the locally-printed index determined by the induced illumination, and therefore addresses both the challenges of scalability and durability. In this process, a thin metal film is deposited onto a substrate and transformed into a mask via local laser-induced dewetting into nanoparticles. The substrate is then dry-etched through this mask and selective mask removal finally reveals the meta-

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surface. We show that the masking by the local nano-particle distribution, and therefore the local index, is dependent on the local light-induced dewetting temperature. We demonstrate a printing of a free-form pattern engraved into a fused silica glass substrate using a laser raster scan. Large scale spatially controlled engraving of metasurfaces has implications on other technological fields beyond optics, such as surface fluidics, acoustics, and thermo-mechanics.

Keywords: Nano-particles, meta-surface, meta-optics, dewetting, thermal annealing, reactive ion beam etching, plasmonics

INTRODUCTION Meta-surface technology is a rapidly growing field, enabling the tailoring of interfacial properties by texturing the surface with different geometries of nano-scale features1-12. This concept of redesigning the material properties by forming artificially designed meta-atoms, taking the role of the basic units that give the material its properties, is a defining trait among meta-materials. These meta-atoms are larger than the actual material constituents (e.g., atoms) but smaller than the characteristic length scale of physical phenomena (e.g., optical wavelength to modify the refractive index); accordingly with homogenization by effective index theory13 this becomes a powerful tool to enable properties that are inaccessible to the materials in their native state. The concept of metasurfaces is broad and could apply to different material properties, e.g., optical refractive index1415,

optical chirality16, sound wave propagation constant17, thermo-mechanical18, and fluid

dynamic19-20. The ability to spatially modify and tailor the meta-surface properties opens new design possibilities and applications. Spatially shaped refractive index, implemented as a meta-surface9-10, has been used to achieve more compact optical designs with superior optical properties, such as a tunable


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lens6-7, a flat achromat lens11-12, a chiral flat lens4, and a multi-wavelength flat lens8, 21. Beyond implementing more traditional optics like lenses, the ability to print the local refractive index without the limitation of standard surface finishing offers the flexibility required for the emerging field of freeform optics22. Patterned meta-surfaces could be used to tune several surface properties simultaneously such as refractive index and hydrophobicity to obtain self-cleaning optical surfaces23. However, current meta-surface technology is limited with respect to large aperture optics, which requires both area-scalability and durability. The principal challenge arises from the necessity of patterning nanoscale features (to control the local effective properties) while being able to modify their geometry at-will on a large optics scale, for potential application of long-range imaging, lasers, LADARs, optical communications, and lasers (e.g., about 40 cm aperture for Megajoule class lasers24-25). Frequently, these applications are also more sensitive to laser-induced damage and environmental degradation, which constrains the possible materials that could be deposited on the substrate and the fabrication process itself. An ideal pathway to achieve these criteria altogether would be a method for engraving the required nano-features onto a damage resistant substrate, in a spatially controlled way, to avoid adding more materials or interfaces which are known to reduce stability. Currently, potential technologies available for such arbitrary spatial nanopatterning (i.e., to allow for freeform optics) are deep UV and E-beam lithography or focused ion beam (FIB) milling, which are limited with respect to scaling up of the final product size and the fabrication complexity. Here, we present a relatively simple and scalable process for the printing of substrate-engraved free-form meta-surfaces (i.e., apply arbitrary programable spatial patterns). The proposed process is a four-step process, illustrated in Figure 1 and outlined as follows: (1) deposition of an ultrathin


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metal film on the substrate; (2) light-induced dewetting of this film into metal nanoparticles, with a local spatial distribution controlled by a tailored light pattern, resulting in an etching mask; (3) dry etching through the mask to engrave its pattern into the surface; and (4) selective removal of the mask. The directional dry etching through the nanoparticle mask results in a pillar-like structures under the masking nanoparticles, and thus the masking fraction of the surface (i.e., fillfactor) translates into a volumetric fraction filled with substrate material, with the void space occupied by air. Therefore, based on effective index theory (EIT)26, at optical wavelengths much larger than the meta-surface features, the local effective index of the etched layer is proportional to the volumetric average of the substrate and surrounding air indices. That is, as we show here, the local illumination during the dewetting step determines the fill-factor of the mask to locally control the resulting meta-surface index. All the steps in this process are area-scalable: dry etching is being used for processing “meter-scale” optics toward high-power lasers27-28 and large-area surface illumination with programable arbitrary shape could be achieved by either small beam raster scan (e.g., like in traditional printing) or by reshaping a large beam using a spatial light modulator (for printing the entire pattern at once). The end result of this process is a monolithic geometry of the meta-surface engraved into the substrate and thus is likely to be more robust compared to processes achieved by depositing layers of the same or varying materials. This is justified by the fact that the meta-surface is a monolithic part of the substrate, and therefore does not introduce new weaker components, mismatch (e.g., thermal), or interfaces (e.g., mechanical weakness), and thus is more stable than deposited layers. Furthermore, using an etching method for the fabrication (as opposed to, for example, chemicalmechanical finishing) is known to produce high laser damage resistivity by avoiding the formation of absorption precursors27-30. Moreover, since thermal dewetting of thin films and dry etching both


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have broad applicability to materials, this method could be applied to a wide variety of optical materials. Here we demonstrate this proposed process on a fused silica glass, a ubiquitous material for optics. Two noteworthy, related previous demonstrations are spatially invariant dewetting of metal films31-33 to be used as an etching mask and laser-induced metal re-texturing to generate colors using plasmonic effect34-37. We note that other strategies to obtain spatially invariant substrate engraved meta-surfaces were also proposed38-42, however reviewing all of them is outside the scope of this paper. The advancement from a spatially invariant to a programable printed mask substantially expands the application space, from replacement of coating methods (e.g., antireflective coating) to replacement of thin freeform optical elements (e.g., lens, aberration correction, grating). For this advancement in the technology to become viable, it requires the demonstration of a predictable relationship between the local illumination and the resulting fillfactor of the etching mask, which is a major finding presented here. The previous demonstrations aimed at the different application of metal film laser-texturing for plasmonic coloring are focused at controlling a different virtue of nano-particles, being the spectral transmission or reflection, rather than the fill-factor that is the relevant one for the etching mask application of the present study.

RESULTS AND DISCUSSION Thin solid films deposited on substrates are typically thermodynamically metastable and they evolve into islands at elevated temperatures, explained by energy minimization considerations. By utilizing short laser pulse rapid melting techniques43-44, thin-film hydrodynamic instabilities such


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as spinodal dewetting45 of metal films can be achieved. Under the idealized spinodal dewetting model, a uniform liquid thin layer breaks into nano-islands with a characteristic length scale that depends on the initial layer thickness and material properties (e.g., Hamaker coefficient)43. However, for the continuous wave (CW) laser illumination levels we use here, the nanoparticle formation occurs at temperatures well below the bulk melting temperature, dominated by solidphase dewetting driven by surface diffusion rather than liquid phase dewetting46. We selected gold (Au) as the thin metal film material, since it has a relatively low dewetting temperature, a smaller predicted particle size due to its high Hamaker coefficient, and a relatively good dry etching selectivity relative to a fused silica substrate. Furthermore, gold is resistant to oxidation and therefore handling and dewetting can be performed under ambient conditions. When a gold film with target thickness of about 10nm or below is deposited on a fused silica substrate, the film is not uniform, since these thicknesses are close to the percolation threshold value. These sub-10nm target thicknesses were previously observed to reach nano-scale diameters after dewetting, making them ideal for fabricating meta-surfaces in the optical wavelength range. We investigated the dewetting behavior of ultrathin 10nm thick Au film by a continuous-wave (CW, 532nm) laser line-scan, as illustrated in Figure 2a. As the laser beam with a Gaussian intensity profile is scanned over the sample (1/e2 beam radius, ro=272m), the gold film is dewet along the scan line track. By varying laser parameters such as peak laser intensity (I) and scanning speed (v), dewetting under various illumination conditions was studied (as indicated in Figure 2). The change in the scanning speed was used to modify the effective time of exposure at a given location (essentially being used as a time shutter), and thus the energy deposited was locally controlled (Iro/v). We used a section (black dashed box) across the scan line for dewetting analysis for each of the different illumination conditions.


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Decoloration in the optical micrographs of the dewetted sections at the different conditions indicate that the resulting nanoparticle distributions were determined by the illumination conditions (see Figure 2b). The spectral transmission of metal nanoparticles is known to be sensitive to the particle size and shape distribution due to a localized plasmonic resonance effect47. Therefore, the local change in color is a good indication for the formation of nanoparticles, with the spatial change in color indicative of a change in the spatial distribution characteristics. For example, regions that were exposed to less energy deposition (increasing v, reducing I), show less decoloration from the as-deposited “greenish” transmission color. However, the process parameter that is of consequence for the use as a dry etching mask is the mask fill-factor, rather than the particle size and shape, which predominantly control the decoloration. Because of the nanoparticles size, sub-optical wavelength imaging techniques are required, and as such SEM images were utilized to measure the fill-factor. The laser-induced nano-particles formation motivates further exploration for applications of large area plasmonic arrays, beyond the scope of this paper. SEM characterization was conducted for the case where I=8.6kW/cm2 and v=2.5mm/s (indicated by yellow star in Figure 2b), with images acquired every 25µm. A prototypical morphology at different locations across the laser induced track are provided in Figure 2c (for only 6 out of the entire set of SEM images taken along the laser induced track). In these images, the masked regions are composed of randomly distributed metal film network (lighter gray regions) separated by unmasked regions (darker gray regions). The different stages of dewetting are evident and correlate to the different locations within the beam footprint, such that higher local deposited energies result in more dewetting. The masking fill-factor measured from SEM images across this laser induced track is depicted as a function of position in Figure 2d. The mapping of location-dependent intensity to mask fill-factor is evident. Assuming the mask fill-factor translates into the same


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volumetric removal ratio (i.e., straight side walls after dry etching), the local effective index translates approximately to: 𝑛𝑒𝑓𝑓 ≈ 𝑛𝑎𝑖𝑟 +𝑓𝑖𝑙𝑙 𝑓𝑎𝑐𝑡𝑜𝑟 ∙ (𝑛𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 ― 𝑛𝑎𝑖𝑟) (see supplementary, Fig. S1). Therefore, a controlled masking fill-factor variation between 65% to 15% based on the local illumination levels enables substantial induced local effective index-change in the metaoptics layer. Noteworthy is the abrupt fill factor drop from 20% to 15% near 100µm from the beam center, which coincides with the abrupt color change in the optical micrograph (indicated by yellow star). The difference observed in the nanoparticle geometry between the two regions is faceted edge (lower intensities) vs rounded particles (higher intensities). Beyond that transition point (at higher intensities, or alternatively closer to the beam center) no substantial change in particle-size distribution was detected. We attribute this change to a transition from a sub-melting point solid-state diffusion dominated dewetting regime to above melting point liquid dewetting dominated regime (see SEM image of the transition, and extended discussion in supplementary Fig. S3). We developed an alternative to SEM for estimating the fill-factor, more adequate for the large area analysis required, using optical transmission at ultraviolet wavelengths, away from the plasmonic resonance. An alternative to SEM imaging for resolving the fill-factor of the mask, which has a limited applicability for characterization of large area patterns, is finding an optical wavelength where the average light transmission is proportional to the fill-factor. To find such a wavelength of potential characterization, the transmission spectrum was measured at the multiple locations characterized previously with SEM, as presented in Figure 3a. As the film transitions from “as deposited” (red curve) to nanoparticles (purple curve), the expected plasmonic resonance absorption footprint around 550nm becomes more apparent48-49. The manifestation of a clear mapping between fill-factor and transmission at a given wavelength is a monotonic increase /


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decrease as a function of location (as location is also monotonically changing with fill-factor). It is evident that a monotonic change is observed at wavelengths away from the plasmonic resonance, at the two ends of the measured spectrum. At wavelengths smaller than ~475nm, there is an evident monotonic relation between the location related fill-factor and the transmission, and thus we select 450nm as the wavelength for this measurement. A conversion table was developed from the correlation between SEM-based fill-factor and the 450nm-based transmission (Figure 3b). The transmission at 450nm inversely correlates monotonically with the masking fill-factor. A conversion table obtained for an additional illumination case (5.2kW/cm2 and 2.5mm/s) is also shown in Figure 3b, showing a good agreement between the two cases. Applying this conversion table to all the illumination conditions shows that the masking fill-factor is determined by the local temperature (Figure 3c). When the fill-factor was calculated for all the illumination conditions as a function of local intensity (see supplementary), the curves even for same scan speed do not align with each other, indicating that heat diffusion contributions should be considered. Under shaped illumination, the gradients in the locally generated heat leads to diffusion, which results in the local temperature profile as the source term to dewetting. The local temperature could be described using a simplified linear heat diffusion model with the two parameters ro and v (see supplementary). When the fill-factor is plotted over local temperature (Figure 3c), the curves do align each other with reasonable agreement and this result suggests that local temperature drives the dewetting process. We utilized a reactive ion-beam etching (RIBE) technique to imprint the mask pattern onto the substrate, since it offers vertical side wall profiles even for dielectric substrates due to the directionality of ion beams27-28. To study the dry etching control on the shaped meta-optics,


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separated from the dewetting previous part, we first studied dry etching with spatially invariant nanoparticle distributions. The mask was formed using furnace heating and dewetting of thin metal films50 (known to give uniform distributions), to give comparable nanoparticle size distributions. A 10nm Au film deposited on a fused silica substrate was heated at 1010ºC for 30 minutes under atmospheric conditions using a furnace. A SEM image of the dewetting mask from top view is shown in Figure 4a. The dewet sample was then dry-etched with varied etching time, and finally the remaining metal mask was selectively removed. To minimize thermal loading, the substrate was translated in a planetary system to be exposed to localized ion beam, where etching depth is controlled by the number of passes. After 250 passes (Figure 4b), the substrate was etched with nearly vertical slopes and the height of the pillar was 205±6nm. When the sample is etched for 400 passes (Figure 4c), the side walls have started sloping as the mask material is etched away, presumably from the nano-particles edge region. When the mask is over-etched (i.e., when the entire nano-particle masking material has been removed and etching continues), the etched structures turn into cones but with the particle-particle distance preserved (see Figure 4d). This introduces another level of control of the resulting structure geometry in the separate dry etching step. For example, the side-wall tilting is equivalent to gradual change in the refractive index as a function of the depth, which is known to form a broader optical bandwidth coating51-52. Nevertheless, if deeper etching with straight side profile is desired, other materials such as Cr and Al with higher etching selectivity than Au could be used53. The process was used to print an arbitrary shaped substrate-engraved meta-surface on fused silica glass. First, a spatially controlled mask (‘LLNL’) was formed by dewetting a 7.5nm gold film deposited on fused silica glass using a raster-scanned laser (Figure 5a). The entire process was applied to three different metal films thicknesses (5nm, 7.5nm, 10nm), and the optimal resulting


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meta-optics was obtained for the 7.5nm film (see supplementary). The laser intensities for the four characters were increased from left to right and from top to bottom (but constant within each letter). The transmission color difference between the characters indicates that different dewetting nanostructures were developed for the different laser induced illumination conditions, as expected. The last L region (bottom-right) is expected to have the lowest fill factor since it was formed at the highest illumination conditions and is also brightest among the four characters in the mask (see supplementary Fig. S11 for SEM images of the resulting metasurface of the different letters). The substrate with the mask was etched using RIBE for 250 passes (~205nm substrate etch depth, detch). In this ‘LLNL’ arbitrarily shaped pattern, the local effective index of the meta-optics correlates to the printed raster-scan illumination. The optical wave-front phase difference locally induced by the meta-surface modified areas is measured using a phase shifting diffraction interferometer (PSDI) technique54-55, at λ=532.2nm free-space wavelength, and the resulting 2D map of the phase difference (Δϕ) is shown in Figure 5b. The meta-surface effective index change for the layer (Δn) is estimated assuming the optical path difference (OPD) is a result of a uniform index change within the layer (Δn=OPD/detch, OPD=λΔϕ/2π). Using the SEM measured etching depth value of detch=205nm for the layer thickness, the Δn from pristine fused silica (no=1.461) for the last N and L is 0.083 and 0.093, respectively. Even though shallow OPD meta-optics, such as the one demonstrated here has applications (e.g., aberration corrections, gratings, designed reflectivity), further optimization of the fill-factor and the depth (nano-particles height and material) should take place in the future. The printed meta-optics displays spatially modulated reflectivity due to the varying interface index (Figure 5c). To measure the reflectivity of a single interface (e.g., to separate the top etched surface from the back-reflection) at zero angle of incident, we utilized a laser confocal microscope


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(=658nm). In correlation with the phase difference measurement (Figure 5b), the first two ‘L’s, exhibit a small apparent reflection reduction from the reflectivity for interface without a metasurface (i.e., Fresnel reflection of 3.45%). The darker edge indicated by the arrow in the second L is attributed to the turn-point of laser beam raster-scan, where the laser beam dwells longer near the edge, resulting in higher temperature (also noticeable at the mask, see Fig. 5a and Fig. S9). At the bottom two letters, ~30% reduction in the reflectivity is observed, which could be further increased by optimizing the process parameters. A relatively high reflectivity uniformity is achieved within each of the patterned letters (the measured standard deviation reflectivity only exceeds the background noise level by 0.007, which is 0.24% of the mean reflectivity value, see supplementary Table S1).

CONCLUSION In conclusion, we present a relatively simple 4-step process for light-printing of substrate-engraved meta-surfaces. The process includes planar metal layer deposition, followed by its light-induced dewetting into a nano-particle mask to then be used for dry etching, and finally selective removal of the mask. All the steps are scalable to large aperture optics and have wide applicability to different substrate materials. The resulting meta-surface could be tailored at-will using laser beam raster-scan or at-once illumination by shaped laser beam. While we have laid down here the foundations for a relatively simple and scalable approach for spatially free-form tailoring of surface-engraved metasurfaces, there is much room left for future exploration. We have demonstrated here a control of the fill-factor between about 15% and 70%, and depth that in some cases approaching about 500nm, with Au based dewetted masks. Further


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studies of the thin film deposition conditions (e.g., thickness and percolation state) and film material with higher etch ratio to the substrate, will help expanding the achievable span of OPD with this method. Materials with higher known etch ratio to glass substrate exist (such as Cr), yet potential challenges could be producing a large dewetting fill-factor span or avoid oxidization of the resulting mask. We have also demonstrated here a broad control of meta-surface features sidewall slope. This control depends on the design of over-etching the mask once depleted, and therefore in-principal should enable a broad set of side-walls slope and depth combinations, with accurate control of the mask thickness and etch rate. This picture should be further tested and explored. Furthermore, since spatially uniform random anti-reflective structures of the type shown in Fig. 4d, are known to perform over very wide wavelengths bandwidth, it would be valuable to further explore and optimize the performance of the meta-optics with the approach presented in this work, for a broad wavelengths and acceptance-angles range applications. Since the meta-surface is engraved into the substrate it presents enhanced stability with respect to structures that include additional deposited materials and interfaces, making it even more suitable for large optics applications, such as lasers, long range imaging, LADARS, and optical communications. These meta-surfaces could find numerous industrial and scientific applications beyond optics, such as for acoustics, surface-chemistry, surface-fluidics, and even multi-properties simultaneously.

EXPERIMENTAL SECTION Dewetting parametric study: 1 mm thick fused silica substrates (Valley Design) were Piranha cleaned followed by an e-beam deposition of Au film. The target film thickness was 10 nm. The


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deposition rate was 0.1 nm per second. The substrate temperature during the deposition was set to 100 °C. To generate the dewetting lines (Fig. 2), a CW 20 W fiber laser (IPG GLR-20,  = 532 nm) was used, with the 1/e2 beam radius of 270 µm, measured using a knife edge method. For the line scan, a XY motorized stage (Aerotech, PlanarDL-200XY) was used. Reactive ion beam etching study: for the etching mask, a 10 nm Au film was deposited (at room temperature) on a Piranha cleaned 1 mm fused silica substrate (Valley Design), followed by heating at 1050 ºC for 30 minutes under atmospheric conditions using a furnace. To minimize unintentional heating during reactive ion beam etching, the substrate was translated in a planetary system (rotation rate of 1.67 rpm) to be exposed to localized ion beam (16 cm, three-grid, RF ion beam source, Plasma Process Group) under a gas mixture of Ar, CHF2, and O2. Printed meta-surface demonstration: three target thicknesses of Au film were deposited (5 nm, 7.5 nm, and 10 nm) at room temperature on a Piranha cleaned 1 mm fused silica substrate (Valley Design). The data in the paper refers to the 7.5 nm film, but details for the 3 thicknesses results are brought in this supplementary. A CW 5 W solid state laser (Spectra physics) was used to produce the LLNL pattern (Fig. 5), with the 1/e2 beam radius (Mitutoyo, NA: 0.42,  = 532 nm) of 4.1 µm, measured using a reflection beam intensity image. For the LLNL patterning, a XYZ motorized stage (Newport) was used and the raster-scan speed was set to 2 mm/s with a step size of 2 µm. The laser intensities for the L, L, N, and L characters were set to 170, 341, 512, and 682 kW/cm2, respectively (increasing in intensity from left to right, and from top to bottom, Fig. 5). Characterization of dewetting-made nanoparticles: SEM images of various dewetting conditions were captured under the same magnification (×25k) and acquisition conditions (acceleration voltage: 5 kV, working distance: 6 mm). Metal coating before SEM imaging was not


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performed for the samples with Au dewetting structures. To calculate the fill factor for each SEM image, a smoothing image processing filter was applied to reduce noise level and the image is then converted to binary (e.g., black for substrate, white for Au). The fill factor is calculated as the number of white pixels over the total number of pixels. This image processing routine is automatically performed for all images.


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Figure 1. Illustration of the proposed method for meta-surface printing. a, an ultrathin metal film is deposited on the substrate and a spatially shaped illumination and thus heating is applied to it. (The rest of the process is illustrated only on a segment of the mask illuminated by a spatial Gaussian intensity profile – as indicated by a dashed-line box). b, the local heating-induced dewetting of the film into metal nanoparticles, with local distribution that depends on the local illumination. c, dry etching to engrave the nanoparticle mask pattern into the surface (followed by a selective removal of the mask) creates monolithic meta-optics.


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Figure 2. Laser dewetting of ultrathin Au film. a, Schematics of laser dewetting experiments: a Gaussian laser beam was scanned along a line at a given peak intensity and scan velocity (1/e2 beam radius, ro = 272 m). b, Micrographs of a spatial patch of the gold film across the laser scan (after illumination) at various laser peak intensities and velocities (the data for two additional peak illuminations: 6.9 and 10 kW/cm2 is given at the supplementary). The decoloration from green to purple indicates the formation of nanoparticles. The scale bar is 100 µm. The yellow star indicates on a transition point across the melting point. c, SEM images of the locations indicated by colored dots in b (for 8.6 kW/cm2 peak intensity at 2.5 mm/s scan velocity), illustrating the dependence of the nanostructure on local illumination. The distance of the SEM-imaged segment from the beam center is indicated on the top of the SEM images. The scale bar is 200 nm. d, Measured mask fill-


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factor (from SEM images) as a function of the distance from the center of the beam shows that the fill-factor could be controlled by local illumination.

Figure 3. Laser-induced fill-factor is driven by local temperature. a, Measured transmission spectrum across the previously SEM characterized film dewetting segment (8.6 kW/cm2 and 2.5 mm/s). The inset shows the corresponding micrograph from Figure 2b, and the dashed line arrow is brought to illustrate the relation between distance from the beam-center and the corresponding transmission spectrum. b, SEM-based measured mask fill-factor vs measured transmittance at 450 nm, for two illumination cases. c, Estimated empty-factor (100% - fill-factor) for different illumination conditions, as a function of local temperature.


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Figure 4. Dry etching for engraving the dewetted mask features onto the substrate. a, SEM top-view image of the Au nanoparticle mask obtained by thermal dewetting in a furnace. Cross sectional SEM of the end result meta-surface geometry on the fused silica surface after dry etching (followed by the remaining metal mask removal) of: b, 250, c, 400, and d, 550 etching passes. All images were acquired at the same magnification. The scale bar in the images is 200 nm.


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Figure 5. printed meta-surface demonstration. a, Optical micrograph of the spatially lightinduced dewet mask pattern. Each letter in the ‘LLNL’ pattern was implemented with different illumination conditions, and the change in transmission color is indicative of the difference in the nanoparticle spatial distribution. The white dashed arrows illustrate the laser beam raster scan direction. b, Measured phase modulation (Δϕ) of the etched substrate through the mask in Fig 5a. A cross section along the dashed line is given below the 2D pattern image. c, Measured reflectivity of etched substrate. A representative reflectivity value for each character is computed as the mean value on a 100 µm × 100 µm region (illustrated by dashed line square). Scale bars are 100 µm.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Simplified thermal diffusion model of laser induced dewetting process; micrographs of dewetting lines and estimated mask fill factor at various laser dewetting conditions; the relationship between dewetting fill factor and local laser input intensity; calculated temperature profiles under laser line scan; experimental parameters for the LLNL pattern, and measured optical reflectivity with standard deviation of the etched-substrate pattern; optical micrographs with dewetting masks and reflectivity 2D maps of etched-surfaced from three mask film thickness; SEM images of etchedsurfaces (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT


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This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE‐AC52‐07NA27344 within the Laboratory Directed Research and Development Program (LDRD) at LLNL. (#18-ERD-005)


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TABLE OF CONTENTS (TOC) GRAPHIC


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Scalable Light-Printing of Substrate-Engraved Free-Form Metasurfaces

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