Conceptional Pure-Tungsten Metasurfaces Based on Femtosecond Laser Nanomanufacturing

Jianing Liao; Dongshi Zhang; Zhuguo Li

doi:10.1016/j.eng.2024.06.018

Engineering ›› 2025, Vol. 49 ›› Issue (6) :65 -85. DOI: 10.1016/j.eng.2024.06.018

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Conceptional Pure-Tungsten Metasurfaces Based on Femtosecond Laser Nanomanufacturing

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Abstract

A laser-induced periodic surface structure (LIPSS), which can be easily produced by femtosecond laser ablation, is a unique nanostructure with a visible refractive color that can be controlled by altering its orientation and uniformity, making it suitable for use in colorful marking, camouflage, and anti-counterfeiting measures. However, single-mode information camouflage can no longer meet increasingly higher-level security requirements. Therefore, metasurfaces offer revolutionary solutions. In this study, conceptual metasurfaces of pure tungsten are theoretically proposed and verified using hierarchical LIPSS/nanoparticle (NP) nanostructures as meta-atoms. The anisotropy of the LIPSS nanostructure enables polarization-sensitive optical modulation, whereas the spatial configuration, NPs size, and period of LIPSS in the LIPSS/NP meta-atoms provide flexibility for tailoring broadband optical responses. In x-polarization, the LIPSS/NP meta-atom system provides more visible colors and divergent infrared absorption (emission) than in y-polarized and unpolarized modes, paving the way for vividly colorful polarization-sensitive displays and information camouflage in infrared bands. A simplified rendition of the world-famous painting “The Starry Night” by Van Gogh is used as a proof-of-concept. Preliminary experimental results are presented, based on which the feasibility and challenges for laser nanomanufacturing of the proposed conceptual metasurfaces are discussed, aiming to provide inspiration for the development of novel metasurfaces through interdisciplinary studies.

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Keywords

LIPSS / Polarization-sensitive / Coloration / Camouflage / Metasurface

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Jianing Liao, Dongshi Zhang, Zhuguo Li. Conceptional Pure-Tungsten Metasurfaces Based on Femtosecond Laser Nanomanufacturing. Engineering, 2025, 49(6): 65-85 DOI:10.1016/j.eng.2024.06.018

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1. Introduction

Metasurfaces, which consist of subwavelength arrays of meta-atoms, have ushered in a new era of optics owing to their unprecedented capability to modulate the properties of light fields such as amplitude, phase, and polarization [1]. The development of metasurfaces has driven the invention of a new generation of optically variable devices [2], including structural color printing and polarization encoding. Structural colors are more advantageous than conventional pigments and dyes because of their high spatial resolution, operational stability, environmental friendliness, and multiple functionalities [3]. Polarization-sensitive metasurfaces provide multiple sets of switchable color palettes for colorful visualization in the visible (Vis) band [4] and can be used as polarimeters [5]. Polarization-sensitive encryption metasurfaces can be flexibly switched to an “on/off” mode by controlling the light polarization, where the “off-mode” hides information by reflecting near zero in black and the “on-mode” allows encoding of an all-color image [6], [7]. Thermal management via mid-infrared (MIR) emission (mid-wave infrared (MWIR; 3–5 μm); long-wave infrared (LWIR; 8–14 μm)) provides an additional channel for Vis–infrared dual-band camouflage [8] and infrared (IR) stealth [9]. Short-wave IR (SWIR; 1.4–3.0 μm [10]) band offers an additional encoding channel for detecting objects by monitoring reflected light [11]. Vis–IR dual-band encryption or camouflage is not limited to black interfaces; however, it can also be applied to other colors and simultaneous polarization modulation, as represented by a slivery-color polarization-switched thermal-emission regulator [12]. Noble metals, such as Ag and Au [13], are common construction materials for metasurfaces. However, they are expensive and susceptible to damage from high temperatures and oxidation. Therefore, increasing attention is being paid to robust and thermally stable functional metasurfaces made from high-melting-point refractory metals [14].

Tungsten, which has the highest melting point of all metals at 3422 °C [15], is advantageous for use in harsh high-temperature environments. Tungsten is also a high-loss metal with a high imaginary permittivity [16], making it suitable for optical modulation. Once structured, either isotropically or anisotropically, tungsten can be used directly as a selective solar absorber [17] or as a functional metasurface [18], [19], [20], [21], [22]. However, compared to the extensively researched plasmonic metasurfaces, the development of pure tungsten metasurfaces lags far behind. Tungsten metasurfaces can be used for polarization-sensitive camouflage and colorful displays [23] as well as for optical sensors [24] such as surface-enhanced infrared absorption (SEIRA), and surface-enhanced thermal emission spectroscopy (SETES), as such surfaces can detect even the smallest amounts of adsorbed analytes by analyzing their “fingerprints” in the MIR region [24], [25], [26], [27]. The necessity and importance of developing novel types of metasurfaces made of pure tungsten lies in the ability of these materials to: ① reduce the risks of corrosion or oxidation in the environment in sensor applications where liquid-assisted deposition of analytes is often used; ② prevent potential damage or deformation when exposed to light or heat in various environments by acting as solar absorbers or emitters [28], or used for adaptive camouflage [29] and SETES [26] applications; ③ greatly reduce the cost of metasurface fabrication by replacing expensive plasmonic noble metals; and ④ offer diverse polarization-sensitive optical responses and the potential for colorful multimodal or multiband display or camouflage, along with their enormous optimization potential in this study.

Despite the rapid progress in recent years, most tungsten-based metasurfaces suffer from time-consuming fabrication and procedural complexity. For example, the fabrication of thermally stable, highly emissive, and ultra-thin tungsten radiators requires six steps: nanoimprint lithography, reactive ion etching, first alumina deposition, annealing, tungsten deposition, and second alumina deposition [30]. Tungsten-based multilayer absorber [31], [32] and emitter [32], [33] metasurfaces require tungsten film sputtering, dielectric material deposition, etching, or lithography for structural patterning. These manufacturing disadvantages are the real “pain points” of metasurfaces. Therefore, there is a trend toward simplifying the design of metasurfaces using pure metals or dielectrics (e.g., Bi [34], Au [35], and Si [36]) and developing large-scale, rapid-processing lithography or lithography-free techniques for metasurface fabrication. In this context, the femtosecond (fs) laser nanomanufacturing technique [37], [38], laser-induced periodic surface structure (LIPSS) [15], [39], and nanoparticles (NPs) [40] have provided potential solutions to promote the development of novel metasurfaces. LIPSS is a unique anisotropic nanostructure, usually created by laser manufacturing but much more accessible using fs lasers [15], [41], [42], [43]. LIPSSs are used for colorful markings [44], anti-counterfeiting or camouflage [45], selective absorption [46], and nanosensing [47]. Additionally, fs lasers are advantageous for the fabrication of difficult-to-machine materials, including high-melting-point refractory metals such as tungsten, and the resulting tungsten-LIPSS has been shown to be an excellent selective solar absorber [46], [48]. LIPSS can be flexibly transformed into broadband absorbers after complete coverage with dense NPs [46]. The NPs formed in-situ during LIPSS nanostructuring can be deposited both tightly and loosely on tungsten LIPSS, where the density can be modulated by changing the processing parameters [46]. This opens up new possibilities for ultrabroadband optical manipulation using hierarchical LIPSS/NP nanostructures. However, the application of tungsten LIPSS as a metasurface has not yet been investigated.

In this study, based on COMSOL Multiphysics theoretical simulations (COMSOL Inc., Sweden), pure tungsten hierarchical LIPSS/NP nanostructures were demonstrated as good candidates for use as metasurfaces in multimodal optical coloration (Fig. 1). Laser fabrication of meta-gratings, such as the proposed hierarchical LIPSS/NP nanostructures, once sputtered with a gold (Au) film, can be applied for SEIRA applications [49]. The proposed polarization- and angle-sensitive hierarchical LIPSS/NP nanostructures can be used not only as absorbers or emitters for solar energy harvesting [46], [48], but also as multiplexed metasurfaces [26], [27], [50], [51], [52] for sensing applications; however, the main objectives are polarization-sensitive optical modulation and colorful displays. Geometrical parameters such as LIPSS period, NPs size, and LIPSS/NP order/disorder layouts, were systematically investigated to evaluate their influence on the polarization-sensitive absorbance or emission in different wavelength bands. Color palettes and chromaticity diagrams were calculated for different polarizations. After parameter screening, suitable LIPSS/NP meta-unit combinations were selected. Van Gogh’s painting “The Starry Night” is showcased in the Vis, SWIR, MWIR, and LWIR bands as a proof-of-concept. Initial experiments were conducted to demonstrate the feasibility and challenges of fabricating the proposed metasurface. Finally, perspectives on the fabrication of pure tungsten metasurfaces and the optimization of their functionalities are presented.

2. Principle of multimodal coloration

2.1. Multimodal coloration design

The full spectrum is briefly divided into ultraviolet (UV < 0.35 μm), Vis (0.35–0.75 μm), and IR (0.75–1000 μm) bands. The IR band [53] can be further subdivided into near-infrared (NIR; 0.75–1.4 μm), SWIR (1.4–3 μm), MIR (3–25 μm), and far infrared (FIR; 25–1000 μm) bands. MWIR (3–5 μm) and LWIR (8–14 μm) are “thermal IR” regimes (Fig. 1) that are often used for applications related to thermal regulation [54]. According to Kirchhoff’s law, the MWIR or LWIR emissivity of a structural absorber is equal to its absorptivity [55]. According to the Stephan–Boltzmann law (

E = σ ε T^{4}

), the emitted IR radiation energy (E) is positively correlated with the IR emissivity (ε), absolute temperature (T), and Stephan–Boltzmann constant σ. This means that in the sample-heating mode (e.g., tens of degree centigrades), parts with lower emissivity radiate less energy, resulting in lower detected temperatures and cooler colors under an MWIR or LWIR camera, whereas parts with high emissivity have warm colors [56]. When an object is indistinguishable in the Vis band, the discrepant absorption (emission) in the IR band enables the identification of encoded or camouflaged information [56]. SWIR imaging is based on reflected light, where a highly reflected area displays a high-brightness grayscale image, and a weakly reflective area displays a low-brightness grayscale image. SWIR imaging requires an external light source, which is not necessary for MWIR and LWIR imaging [57], [58].

2.2. Theoretical modeling

The metasurface model geometry of the hierarchical LIPSS/NP nanostructures, which allows the manipulation of the optical properties in the x- and y-polarizations, is shown in Fig. 2. For the model simulations, Floquet periodic boundary conditions were used along the y–z and x–z planes (oxyz: three-dimensional rectangular coordinate system, including the horizontal x-axis, vertical y-axis along the direction of the LIPSS groove extension, and z-axis perpendicular to the x–y plane) to enforce the periodic arrangement of the unit cells. The outermost domain (purple) is a perfectly matched layer (PML), a truncated electromagnetic domain that reduces reflections from artificial boundaries. The middle domain (purple) represents the air. The incident light (power: 1 W) propagated vertically downwards along the negative z-axis. A single layer of the pure tungsten NP array was placed on the pure tungsten LIPSS, with the center-to-center distance between adjacent NPs set to 5 nm. The LIPSS was modeled with a sinusoidal function located in the x–z plane, that is,

f (x, z) = M \cdot \sin (\frac{2 π x}{L_{P}})

, where L_P is the LIPSS period along the x-axis, and M is the LIPSS amplitude along the z-axis. In our previous study, the LIPSS amplitude M was set to 225 nm [46]. Following Guay et al. [59], it was assumed that the NPs are tangential to the LIPSS surface, and 30% of the NPs radius (r) was embedded in the LIPSS. To screen for a large-scale parameter window, the NPs diameter (size) was increased from 20 to 340 nm and the LIPSS periods were 350, 500, 650, 800, and 1000 nm. The depth of the tungsten substrate to which the hierarchical LIPSS/NP nanostructure was anchored was 500 nm. An inhomogeneous model of a hierarchical LIPSS/NP nanostructured meta-unit consisting of NPs randomly distributed on LIPSS (NPs size, NP distance, and embedding depths within the LIPSS were all random) was also constructed for comparative analysis.

The Lorentz–Drude model for the permittivity ε_r(ω) [60] of bulk tungsten was used to simulate the structural absorption at room temperature:

(1)$\varepsilon_{r}(\omega)_{\text {bulk }}=1-\frac{f_{0} \omega_{\mathrm{p}}^{2}}{\omega\left(\omega-\mathrm{i} \Gamma_{0}\right)}+\sum_{j=1}^{k} \frac{f_{j} \omega_{\mathrm{p}}^{2}}{\left(\omega_{j}^{2}-\omega^{2}\right)+\mathrm{i} \omega \Gamma_{j}}=\varepsilon_{1}+\mathrm{i} \varepsilon_{2}$

where ω is the angular frequency of the external electromagnetic radiation (incident light), f₀ and Γ₀ are the strength and bulk damping constant of the Drude oscillator, respectively, k is the number of Lorentz oscillators with frequency ω_j, strength f_j, and damping Γ_j; ω_p is the plasma frequency. ε₁ is the real part of permittivity and ε₂ is the imaginary part. These parameters for tungsten were collected from Rakić et al. [60] and cover a broad band from the UV band to the IR band with a wavelength range of 0.2–12 μm. Note that LWIR band covers the wavelength range of 8–14 μm. In this study, simulation and analysis can only be performed up to 12 μm due to the parameter confinement.

For tungsten NPs, a size-dependent modification of the effective permittivity is required. Therefore, all NP sizes were corrected for size. The corrected permittivity becomes [61]

(2)$\varepsilon_{r}(\omega)_{\mathrm{NP}}=1-\frac{f_{0} \omega_{\mathrm{p}}^{2}}{\omega\left(\omega-\mathrm{i} \Gamma_{(r)}\right)}+\sum_{j=1}^{k} \frac{f_{j} \omega_{\mathrm{p}}^{2}}{\left(\omega_{j}^{2}-\omega^{2}\right)+\mathrm{i} \omega \Gamma_{j}}$

(3)$\Gamma_{(r)}=\Gamma_{0}+\frac{3 A v_{\mathrm{f}}}{4 r}$

(4)$v_{\mathrm{f}}=\left(\frac{2 E_{\mathrm{f}}}{m_{\mathrm{e}}}\right)^{1 / 2}$

where A is the dimensionless parameter for the surface scattering, which correlates with the modeling physics and the shape of the NPs, and was set to 0.8 in this study [62]; υ_f and E_f are the Fermi velocity and energy (9.2 eV) [63] of tungsten, respectively; r is the radius of the NPs, and

m_{e}

is the mass of the electron.

Γ_{(r)}

is the damping constant as a function of the size of NPs.

The simulation model was based on electromagnetic waves frequency domain (EWFD) of the wave optics module. When incident light of a certain wavelength (λ) strikes a sample, the light generally undergoes absorption A(λ), reflection R(λ), and transmission T(λ) according to the equation:

A (λ) + R (λ) + T (λ) = 1

. As the transmission T(λ) is zero for a bulky tungsten plate, the absorbance can be simplified as

A (λ) = 1 - R (λ)

. The x- and y-polarized light had electric field components in the x- and y-directions, respectively. The reflectance within the Vis band was used to simulate the colors in the color palette according to the color-matching functions of the human eye defined by the International Commission on Illumination (CIE)–1931 standard [64] using Origin software (OrginLab, USA). This data was converted to standard red green blue (sRGB) colors using the CIE standard illuminant D65. For a comprehensive comparative analysis of the polarization-dependent optical responses of samples with different layouts, both sRGB and CIE-1931 color chromaticity diagrams are presented.

2.3. Experimental fs laser ablation

An fs laser system (Trumpf, Trumicro 2030, Germany) with a pulse duration, wavelength, and repetition rate of 400 fs, 1030 nm, and 400 kHz, respectively, was used for laser ablation of pure tungsten plates (dimensions: 30 mm × 30 mm × 1 mm) under air and argon (Ar) atmospheres. In the latter case, the tungsten target was placed in a glass jar filled with Ar to create an oxygen-free working environment. Laser powers of 5 and 15 W were used for laser ablation. The scanning speed of the galvanometer was set to 400 mm∙s⁻¹. A line-by-line scanning method was used for fs laser ablation. The scan interval between adjacent lines was set to 3 μm. Immediately after laser ablation, all samples were ultrasonically cleaned in ethanol for 15 min. To simplify the description of the samples, an environment–power–speed notation is used hereafter; for example, Ar-5 W-400 mm∙s⁻¹, indicates that the tungsten structure was produced using fs laser ablation in an Ar atmosphere at a laser power of 5 W and scanning speed of 400 mm∙s⁻¹.

The reflectance in the wavelength bands of 250–2500 nm and 2500–12000 nm was measured by UV–Vis–NIR spectrophotometers (Lamda 950, PerkinElmer, Inc., USA) and Fourier transform IR spectroscopy (FTIR; Nicolet 6700, Thermo Fisher, USA), respectively. The resulting nanostructures and their elemental distributions were characterized by scanning electron microscopy (SEM; Rise-magna, TESCAN, Czech Republic) and energy-dispersive X-ray spectroscopy (EDS; Rise-magna, TESCAN). The depths and cross-sectional profiles of the nanostructures were measured by atomic force microscopy (AFM; Dimension XR, Bruker, USA). The structural compositions were analyzed by X-ray diffraction (XRD; Mini Flex 600, Rigaku, Japan; using Cu Kα radiation, λ = 0.15406 nm). The colloids formed during ultrasonic cleaning were subjected to transmission electron microscopy (TEM, Talos F200X G2, Thermo Fisher) to determine the NPs morphology and quantify the NPs size. The Ar-assisted synthesized colloids were stored in ethanol for seven days for TEM characterization to demonstrate the high activity of the laser-synthesized colloids.

3. Results and discussion

3.1. Pure tungsten NPs and LIPSS nanostructures

3.1.1. Pure tungsten NP nanostructures

To better compare the differences arising from hierarchical LIPSS/NP nanostructures, pure NP and LIPSS nanostructures with different dimensions were modeled and simulated. A pure tungsten plate was also modeled for comparison. The absorbance of the NP arrays was not polarization-sensitive because of the isotropic arrangement of the NP arrays along the x- and y-directions; therefore, no polarization-sensitive absorption spectra could be detected.

For the pure NPs meta-unit, the featured absorption spectra varied distinctly with NPs size (Fig. 3(a)). At a NPs size of 20 nm, a slight increase in UV-to-NIR absorbance was observed compared to that of the pure tungsten plate. As the size of the NPs increases, the UV-to-SWIR broadband absorbance increases significantly. The absorption peak becomes increasingly clear and is red-shifted to longer wavelengths in the SWIR band (Fig. 3(b)). For NPs with a size of 60 nm, a distinct absorption peak was observed at 392 nm. When the diameter of the NPs reaches 220 nm, two prominent absorption peaks appear: one in the UV band at λ = 306 nm, the other in the SWIR band at λ = 1820 nm, where the absorbance reaches as high as 98.0% and 99.6%, respectively. Increasing the NPs size to 340 nm resulted in a red-shift of the peaks to 468 and 2350 nm, with absorbance values of 98.1% and 86.4%, respectively. Quantification of the average absorbance in different bands (Fig. 3(c)) indicates that increasing the NPs size can increase the average absorbance in the UV–NIR and SWIR bands but has little effect on the absorbance in the MWIR/LWIR bands. This trend suggests that large NP arrays enable highly selective, broad UV-to-SWIR absorbance coupled with narrow-band emission, making them potential solar absorbers and thermal-emitter structures [65]. The sRGB colors of the pure NP nanostructures are depicted in Fig. 3(a). They show that the size modulation of the pure tungsten NPs can alter the structural colors, which paves the way for tailoring the color gamut of metasurfaces.

Excitation by localized surface plasmon resonance (LSPR) can induce magnetic and electric resonance, resulting in localized “hotspots” that enhance structural light absorption [66]. To this end, the electric field (E-field), magnetic field (H-field), and absorbed power (heat power density, Q) distributions of the structures at wavelengths of 392 and 1140 nm, corresponding to the absorption peaks of the nanostructures composed of 60 and 140 nm sized NPs were simulated, along with the E-/H-field and Q distributions at wavelengths of 358 and 1990 nm, corresponding to the absorption peaks of the NP-260 nm nanostructure. The results shown in Fig. 4 indicate that the H-field enhancements are mainly distributed in the “triangular” region between the tungsten substrate and its upper anchored two adjacent NPs, whereas E-field enhancements are distributed within the gaps between two adjacent NPs. The interplay between the enhanced E- and H-fields leads to localized hotspots, and thus to increased broadband light absorbance. The absorption peak at 358 nm for the NP-260 nm nanostructure was attributed to enhancement of the H-field at the outermost surfaces of the NPs and the upper space directly above the gaps between two adjacent NPs.

3.1.2. Pure tungsten LIPSS nanostructures

Fig. 5 shows the optical properties of the pure LIPSS nanostructures, calculated average absorbance in different bands, E- and H-field, and Q distributions on LIPSS at wavelengths of 406, 1340, 820, and 1350 nm in y- and x-polarization, corresponding to the absorption peaks of the LIPSS-350 and LIPSS-800 nm nanostructures, respectively. The polarization-dependent absorbance is notable, with the x-polarized absorbance much higher than y-polarized absorbance in the UV-to-SWIR band. This is evident from distinct peaks at approximately λ = 1350 nm, which mainly originate from the localized hotspots induced by H-field enhancement in the valley and summit of the LIPSS. As for the y-polarized absorbance enhancement, H-field enhancement occurs at the upper part of the LIPSS ridges. For both y- and x-polarized and unpolarized absorbances, the larger the LIPSS period, the lower the broadband absorbance. This trend was more evident after quantifying the average absorbance in the UV-to-NIR, SWIR, MWIR, and LWIR bands (Figs. 5(a)–(c) and Fig. S2 in Appendix A).

The LIPSS nanostructures exhibit more sets of colors in x-/y-polarization than in the unpolarized colors (inset images in Figs. 5(a)–(c)), indicating the suitability of LIPSS for polarized coloration in the Vis band. Structural color analysis shows that ① structural color is more sensitive to x-polarization, but less noticeable for y-polarization; ② LIPSS in small periods exhibits distinct structural color differences in the y-/x-polarization; ③ and the polarized colors of large periods of LIPSS (e.g., 1000 nm) tend to become similar. Given the low absorbance in the 250–2500 nm band, pure LIPSS is not suitable as a selective solar absorber but requires hierarchical nanostructuring with embedded NPs to increase absorbance.

3.2. Homogeneous hierarchical LIPSS/NP nanostructures

3.2.1. NPs size influence

Once LIPSS and NP are integrated into hierarchical LIPSS/NP nanostructures, polarization-sensitive responses such as the color palette and absorbance in specific bands can be more freely designed by changing the structural parameters. Fig. 6 and Fig. S3 in Appendix A show how the optical properties of the hierarchical LIPSS/NP nanostructures change when setting the LIPSS period is set to 800 nm and the NPs diameters vary from 20 to 340 nm. In the y-polarized mode, increasing the NPs size resulted in much broader UV-to-SWIR absorption (Fig. 6(a)). For the 180 nm sized NPs, a new absorption peak appears at λ = 1700 nm, which is due to the appearance of hotspots at the outermost NPs and the corresponding LIPSS–NP interfaces (Fig. 7). This peak undergoes a gradual red-shift to a longer wavelength in the SWIR range (from 1700 to 2480 nm) with increasing NPs diameter, accompanied by a weakening of the peak intensity from 99.9% to 82.9% (Fig. S4(a) in Appendix A). The red-shift of the y-polarized absorbance peaks caused by the increase in NPs size was mainly attributed to the variation in the positions, densities, and intensities of the localized hotspots induced by the enhancement of the H-field (Fig. 7). For the 60 nm NPs, the average absorbance in the UV–NIR band increased sharply and reached its maximum at a NPs size of 100 nm. With a further increase in the NPs size, the UV–NIR absorbance gradually decreased (Fig. 6(e)). In comparison, the average absorbance in the SWIR band gradually increases with increasing NPs size, reaching a maximum value of 64.7% at a NPs size of 300 nm, followed by a slight decrease with a further increase in NPs size to 340 nm. The absorbance in the MWIR band showed a steadily increasing trend of up to 17.6% at 340 nm, whereas the absorbance in the LWIR-band gradually increased and reached a saturation point when the NPs sizes were 300 and 340 nm (Fig. 6(e)).

In the x-polarized and unpolarized modes, a new absorption peak emerges at λ = 2920 nm when the NPs size is 140 nm. Further increasing the NPs size to 220 and 260 nm led to the appearance of three absorption peaks (Figs. 6(b) and S4(b) and (c)) due to the appearance of E- and H-field enhancement at different positions of NPs, and correlated LIPSS–NP interfaces (Figs. 8 and S4(d)), highlighting the importance of hierarchical NP integration in LIPSS. For the LIPSS/NP-800/260 nm nanostructure, the high x-polarized absorbance at λ = 1050, 2100, 2540, and 3450 nm is due to the localized hotspots that occur in different regions of NPs on the LIPSS and are caused by synergistic electric and magnetic resonances (Fig. 8). More precisely, the positions of the hotspots shift from the LIPSS summit to the valley as the wavelength of the absorption peak increases. When the NPs size increases from 140 to 340 nm, the x-polarized absorption peak (shown by the red color in Fig. S4(b)) was red-shifted from 2920 to 3990 nm because of the variation in E- and H-field resonances [34] between the NPs in the LIPSS trough (Fig. S4(d)). In agreement with the y-polarization trend, the average absorbance in the UV-to-NIR band at x-polarization reaches the highest value of 96.8% for the LIPSS/NP-800/100 nm sample (Figs. 6(b) and (f)). This sample is considered one of the best polarized tungsten-based metasurface absorbers (Table S1 in Appendix A), and is also comparable with many metal–insulator–metal absorbers (e.g., W/SiO₂/W [55], W/Al₂O₃/W [19], and W/MgF₂/W [67]). Owing to the compromise in y-polarized absorbance (Figs. 6(a) and (e)), some newly observed x-polarized absorbance peaks became indistinguishable in the unpolarized mode (Figs. 6(c) and S4(c)). Unpolarized color similarity, distinctly polarized color differences in the Vis band (Fig. 6(d)), and distinctly polarized average absorbance differences in the SWIR/MWIR bands (Figs. 6(e) and (f)) provide the possibility of polarized coloration in the Vis band and camouflaging of the information in the IR band.

Compared to the pure LIPSS nanostructures (Figs. 5(a)–(c)), all the y-/x-polarized and unpolarized absorbances in the UV–NIR and SWIR bands were significantly increased for the hierarchical LIPSS/NP nanostructures. x-polarized absorption peaks of the pure LIPSS were mainly located at approximately 1350 nm. In this context, the E-/H-field and Q distributions of the hierarchical LIPSS/NP-800/140 nm, LIPSS/NP-800/260 nm, and LIPSS/NP-800/340 nm nanostructures were comparatively analyzed at 1350 nm, as shown in Fig. 9. Compared to the limited hotspots on the ridges of pure LIPSS, the hotspots in the LIPSS/NP nanostructures are located in almost all regions of the NPs-related interfaces, including the gaps and outermost surfaces of the NPs as well as the “triangular” regions between adjacent NPs and the underlying LIPSS. This means that after the dramatic enlargement of the specific surface area via NP embedment on the LIPSS, the number, the intensity, and the region of “hotspots” induced by the enhancement of the E-/H-field are all increased, leading to a noticeable increase in x-polarized broadband absorbance, and also y-polarized absorbance enhancement (Fig. 7 vs Fig. 5(d)).

3.2.2. Influence of angle of incidence

For multimodal coloration/camouflage applications, the angle of incidence-sensitive optical response is an important factor to evaluate because light can irradiate the sample at an oblique angle [68]. LIPSS/NP-800/260 nm was chosen as an example to investigate the effects of the angle-of-incidence (0°–80°) on the structural color and absorbance, as shown in Fig. 10 and Fig. S5 in Appendix A. When the incidence angle was no more than 60°, y-polarized average absorbance in the UV–NIR, SWIR, MWIR, non-atmospheric window, and LWIR bands decreased slightly with increasing incidence angle; however, the dominant peak located at approximately 2070 nm gradually increased until it reached 98.6%. In the x-polarized mode, three absorption peaks were slightly red-shifted at an incidence angle of 20° at first, and then one peak disappeared in the SWIR band peak at incidence angles of 40° and 60°. When the angle of incidence was not greater than 60°, the average absorbance of specific bands did not change distinctly; therefore, only a slight color alteration was induced, which makes the proposed LIPSS/NP nanostructures more suitable for encryption, camouflage, and coloration [69] when the angle of incidence changed. At an incidence angle of 80°, the absorbance decreased significantly in both x- and y-polarizations, resulting in a distinct color change toward white/gray, thus transforming the colored display into a grayscale display in the Vis band, reaching the threshold for a vividly colorful display.

3.2.3. Effect of LIPSS period

The effect of the LIPSS period on the absorbance of the hierarchical LIPSS/NP nanostructures was investigated by setting the LIPSS period to 350, 500, 650, 800, and 1000 nm and fixing the NPs diameter to 260 nm, as shown in Fig. 11 and Fig. S6 in Appendix A. Figs. 11(a)–(c) show that the structural colors and absorption spectra are more sensitive to x-polarization, especially in the SWIR and MWIR bands. In contrast, the structural colors and absorption spectra changed less distinctly in the y-polarized and unpolarized modes (Figs. 11(a) and (c)). In the y-polarized mode, the positions and intensities of the absorption peaks fluctuated slightly when the LIPSS period was changed (Fig. 11(d)); therefore, the average absorbance in specific bands changed only very slightly (right part of Fig. 11(a)). In the x-polarized mode, the embedment of 260 nm NPs on LIPSS with different periods led to the appearance of new SWIR-to-MWIR absorption peaks (Fig. 11(e)). Taking LIPSS/NP-800/260 nm as an example, three absorption peaks with intensities of 99.7%, 98.4%, and 99.2% appear at λ = 2100, 2540, and 3450 nm (Figs. 11(b) and (e)) owing to the E-/H-field enhancement and the corresponding emergence of hotspots in different positions of hierarchical LIPSS/NP nanostructures (Fig. S7 in Appendix A). Increasing the LIPSS period to 1000 nm lowers the intensity of absorption peaks at longer wavelengths, which is owing to the variation in hotspots for E-/H-field enhancement caused by the change in geometrical arrangement (Fig. S7). The Q at λ = 2100 and 3450 nm is weak for LIPSS/NP-500/260 nm sample; therefore, the absorbance is weak in this section. The distinct E-/H-field enhancement and the corresponding hotspots increased the intensity of the x-polarized absorption peak at 2600 nm to 98.8% for the LIPSS/NP-500/260 nm sample. When the LIPSS period was 350 nm, no x-polarized absorption peaks appeared in the SWIR, MWIR, or LWIR bands because there was no E-/H-field enhancement and hotspots.

The x-polarized average absorbance of the hierarchical LIPSS/NP nanostructures in the MWIR band followed the sequence LIPSS/NP-650/260 nm > LIPSS/NP-800/260 nm > LIPSS/NP-1000/260 nm > LIPSS/NP-500/260 nm > LIPSS/NP-350/260 nm. In contrast, the average absorbance values of the LIPSS/NP-800/260 nm and LIPSS/NP-1000/260 nm nanostructures are almost the same in the SWIR band, but both are higher than those of the LIPSS/NP-500/260 nm nanostructure. All samples possessed distinct polarization-sensitive SWIR/MWIR-emission (absorbance) properties, while comparing y-/x-polarized absorbance simultaneously. The polarization-sensitive coloration of these LIPSS/NP nanostructures in the Vis-band was also evident (Figs. 11(a) and (b)), indicating that hierarchical LIPSS/NP nanostructures are good candidates for multimodal coloration and camouflage [7], [70], [71]. In this regard, they were selected for the conceptual polarization-sensitive coloration/camouflage described in Section 3.4. As the unpolarized absorbance is the average value of the y-/x-polarized absorbances (Fig. 11(c)), the IR emission difference is a slightly affected (Figs. 11(c) and (f)). Most hierarchical LIPSS/NP nanostructures in unpolarized mode are good absorber/emitter mixers with potential utility in solar thermophotovoltaic systems [65].

3.3. Inhomogeneous hierarchical LIPSS/NP nanostructures

Laser-synthesized NPs generally exhibit broad size distributions [46], [72]. Therefore, to get much closer to the possible experimental results, an inhomogeneous LIPSS/NP meta-unit model containing NPs of different sizes randomly located on a LIPSS of 800 nm in the period was created, as shown in Fig. 12(a). The specific NPs sizes and numbers are shown in Fig. 2(d). The absorption spectra in y-/x-polarized and unpolarized modes are shown in Fig. 12(b), where the polarized/unpolarized colors and the average absorbance in different bands are also included. The colors of the Vis-band of the inhomogeneous LIPSS/NP nanostructures were similar to those of the LIPSS/NP-800/260 nm sample (Fig. 10), whereas the absorption spectra were different (Fig. 12(b) vs Figs. 10(a) and (b)). The y-polarized absorption spectrum is similar to that of LIPSS/NP-800/140 nm, although the average absorbance of the NIR band is slightly lower. In the x-polarized absorption spectrum, only two small peaks appeared at 1980 and 2490 nm, in contrast to the homogeneous LIPSS/NP nanostructures, where distinct peaks with high intensity were located in the MWIR band (3000–5000 nm) (Fig. 6(b) and Fig. S4(b)). The x-polarized absorbance of the inhomogeneous LIPSS/NP nanostructures in the MWIR band is lower than that of the homogeneous LIPSS/NP nanostructures with NPs sizes larger than 180 nm (Fig. 6(b)). The unpolarized spectrum indicates that the inhomogeneous LIPSS/NP structures can act as a selective solar absorber/emitter with emission at λ = 2450 nm. As shown in Fig. 12, the different sizes of NPs in the inhomogeneous LIPSS/NP nanostructures can affect the characteristic peaks induced by the specific size of the NPs on LIPSS (Figs. 6(a)–(c)), synergistically influencing the y- and x-polarized absorbance values. The spatial distribution of the electric field, magnetic field, and absorbed power of the inhomogeneous LIPSS/NP nanostructures are shown in Figs. 12(c)–(e) and Figs. S9–S11 in Appendix A. Large and small NPs synergistically govern the broadband absorbance; therefore, the absorption spectra with NPs of different sizes are affected because they lack distinct peaks in the MWIR band.

The optical features of LIPSS [41], [46], [73], [74] have been exploited in anti-counterfeiting applications [45] and can be realized by tuning the iridescent colors of LIPSS [75] and their composition [76], [77]. The present study theoretically proves that the polarization-display colors of LIPSS can be modulated by embedding NPs and changing the layout of the hierarchical LIPSS/NP nanostructures, providing a solid foundation for polarization-sensitive multimodal coloration in different bands. Through comparative studies with pure tungsten LIPSS, it was found that embedding NPs in LIPSS can selectively increase the UV-to-SWIR and MWIR absorbance but cannot increase MIR absorbance at wavelengths above 5 μm. Moreover, the absorbance values of the homogeneous and inhomogeneous LIPSS/NP nanostructures were comparable to those of the selective W/SiO₂/W and W/AlN/W metamaterials containing periodic tungsten gratings [78], [79], indicating the competence of the proposed metasurfaces.

3.4. Conceptional polarization-sensitive coloration/camouflage

Fig. 13 shows the proof-of-concept of pure tungsten LIPSS/NP metasurfaces with different sets of LIPSSs and NPs (Fig. S12 in Appendix A) for “The Starry Night” (Vincent van Gogh) polarization-sensitive multimodal coloration/camouflage (Figs. 13(a) and (b)). The LIPSS and LIPSS/NP nanostructures shown in Fig. S12, except for the pure tungsten plate, were all utilized. Illumination with y-/x-polarized and unpolarized light produces the vivid coloration of “The Starry Night” as shown in (Fig. 13(a)), similar to the effect of plasmonic nano gratings [71]. Five sets of hierarchical LIPSS/NP nanostructures are shown in Fig. S13 in Appendix A, which are used for multimodal camouflaging. In the Vis-band, the simplified “The Starry Night” image is nearly indecipherable in both unpolarized and y-polarized modes but is recognizable in x-polarized mode (Fig. 13(b)). SWIR imaging allows the gray-color identification of image details because it is based on the light reflectance of an object [57]. The areas with high reflectance had high brightness in the identified grayscale image, whereas the areas with weak reflectance had low brightness. Consequently, some image details were much clearer in the x-polarized mode than in the other modes. In MWIR/LWIR imaging, according to Stephan–Boltzmann’s law [80], the lower-emissivity regions radiate less energy so the low- and high-emissivity regions have cool and warm colors, respectively. MWIR imaging allows colorful imaging in x-polarized mode, but is less distinguishable in unpolarized mode owing to the tradeoff of invisibility in the y-polarized mode. LWIR-imaging cannot provide information because of the low emissions of all sets of nanostructures in this band.

3.5. Feasibility/challenges for fs laser nanomanufacturing of metasurfaces

To demonstrate the feasibility of fabricating the proposed conceptual pure tungsten metasurfaces, a preliminary investigation of fs laser nanomanufacturing in Ar and air atmospheres was conducted using the experimental setup shown in Fig. 14(a), and the results of some challenges to be overcome are listed. Laser powers of 5 and 15 W were set as the variable for structural modulation using the line-by-line scanning method, and the scanning interval and speed were kept constant at 3 μm and 400 mm∙s⁻¹, respectively. The samples prepared by fs laser ablation in Ar and air showed gray and black colors, respectively. The XRD results (Fig. 14(b)) indicate that fs ablation in air induces the formation of W₃O crystals (PDF#41-1230) due to laser-ablation-induced oxidation [72]. No oxide phase dominated by a pure tungsten phase (PDF#04-0806) could be identified in the XRD pattern of the Ar-supported prepared sample. EDS analysis shows that the LIPSS/NP nanostructures prepared in an Ar environment still have a 6%–7% atomic percentage of oxygen, which is lower than the 13%–15% of the structures prepared in an air environment, but still higher than the approximately 3% of the non-ablated tungsten target (Fig. 14(c)). This means that LIPSS and NPs can adsorb water molecules, organic molecules, or other impurities in air, which have been reported to facilitate the transition of surface wettability [81], [82], [83], [84].

The AFM/SEM, and TEM results (Figs. 14(d) and (e)) indicate that LIPSS regularity was influenced by the ablation environment. Air-assisted ablation results in a LIPSS with higher regularity. The LIPSS prepared in Ar is enriched with bifurcations, and the size of the LIPSS ridges varies greatly, which could be caused by the scattering effect of the generated NPs floating in the laser beam path [85]. The chamber is an enclosed space where the shock waves [86] generated during fs laser ablation can push the NPs far away [87]. Therefore, after the ablation, the sealed chamber became black owing to the attachment of numerous NPs to its sidewalls (Fig. 14(a)). In air, the NPs are quickly removed by an equipped exhaust fan near the ablated target, so that the negative effect of the NPs can be partially prevented. In terms of the optical properties (Fig. 14(f)), the structures prepared in an air atmosphere had a larger window for modulating the UV-to-LWIR absorbance than the structures prepared in an Ar atmosphere. This is due to the presence of deposited NPs on the LIPSS, which becomes clearer after quantifying the average absorbance in the UV–NIR, SWIR, MWIR, and LWIR bands, as shown in Fig. 14(g).

In-situ fs laser embedded fabrication can embed ejected NPs into the LIPSS instantaneously [46]. However, considering that laser ablation is difficult to control [88], controlling the NPs state (deposition rate, homogeneous embedment, deposition thickness, and oxidation rates of NPs) is the major difficulty ahead. Unlike ex-situ fabrication routes, in which the NPs size can be post-screened by centrifugation [89], the sizes of in-situ embedded NPs vary greatly and the arrangement of NPs is significantly disordered; therefore, it is necessary to use ordered or disordered NPs for metasurface fabrication. The experimental sample color and absorbance in the UV–Vis–NIR and SWIR bands (Figs. 14(a) and (f)) were very similar to the theoretical trends shown in Fig. 12(b), demonstrating the feasibility of fabricating inhomogeneous LIPSS/NP meta-units for the proposed metasurfaces. In our previous study [46], it was demonstrated that it is possible to modulate the density of NPs deposited in-situ on a LIPSS by changing the scan interval. This suggests that the disordered LIPSS/NP meta-units can be modulated by controlling the laser process parameters.

Although the feasibility of fabricating LIPSS/NP meta-units has been demonstrated, there are numerous challenges in fabricating the proposed conceptual multifunctional metasurfaces that need to be further optimized. These include the selection of the ablation atmosphere, the choice of modes for in-situ or ex-situ embedding of NPs, the treatment of structural oxidation/impurities, the choice of an ordered/disordered structural layout, the control of structural regularity of the LIPSS/NP, the optimization of downstream processing procedures, and the verification of metasurface functions. From the perspective of achieving structural regularity (Figs. 14(d) and (e)), open-space gas assistance (e.g., air, N₂, or Ar) is preferable for the preparation of metasurfaces. High- and low-power fs laser ablation can be used for in-situ and ex-situ fabrication, respectively. In the ex-situ method, tungsten-LIPSS are prepared in open gas, pure tungsten NPs are collected in Ar, classified, and finally melted on the LIPSS in an inert gas atmosphere. Laser-synthesized NPs have a broad size distribution ranging from one to hundreds of nanometers [72]. Such an ex-situ downstream process is quite complex, and each procedure must be optimized to fabricate the desired hierarchical structures. Another possibility is to take advantage of the high activity of laser-synthesized NPs without capping agents on their surface, which can self-drive growth into network-like NP chains after one week of storage in ethanol (TEM image in Fig. 14(e)), as has been reported for laser-synthesized Au [90] and Pt [91] NPs in liquids.

The aforementioned theoretical and experimental results provide a simple guide for the development of pure tungsten metasurfaces. Regardless whether an ordered/disordered layout or an ex- or in-situ method is chosen, systematic experimental investigations, optimization of theoretical models, and verification of the polarization-sensitive function are essential for the fabrication of conceptual pure tungsten metasurfaces. Notably, the color variety of the proposed pure tungsten metasurfaces is much smaller than that of colorful plasmonic/dielectric metasurfaces [92], [93]. Fig. 15 shows the color palettes of all investigated tungsten metasurfaces in the unpolarized mode. Notably, pure tungsten NP metasurface has the broadest color distribution in the CIE-1931 chromaticity diagram, which is much wider than the color diversity of the LIPSS within the studied parameters. Once NPs of different sizes are embedded in the LIPSS with a specific period (e.g., 800 nm) to construct the hierarchical LIPSS/NP meta-unit, the color is concentrated in a narrow window. This trend was also observed for the LIPSS/NP nanostructures composed of NPs with specific diameters (e.g., 260 nm) and LIPSS with varying periods. Fig. 15 illustrates the challenge of diversifying the colors of the proposed pure tungsten metasurface composed of hierarchical LIPSS/NP nanostructures.

Color diversification and functional optimization can be achieved by modulating the structures, materials, and synergistic interactions. More layers of disordered/ordered NPs can be considered for the construction of complex three-dimensional LIPSS/NP meta-units, allowing the modulation of broadband absorption [94]. The LIPSS itself exhibits intrinsic vivid Vis-band iridescence with a full color gamut, allowing angle-dependent camouflaging of information [45]. Therefore, another possible route is to replace the hierarchical LIPSS/NP nanostructures with pure LIPSS to enrich the color and absorption modulation, albeit with a tradeoff in the local broadband absorption capacity. The layout can be used for quick screening using artificial intelligence. The use of LIPSS with different orientations/shapes [39], [95], [96] and different NP shapes [68] as building blocks in the construction of metasurfaces may enable the arrangement of LIPSS orientation from unidirectional to omnidirectional. Enrichment of the color gamut of the proposed tungsten metasurfaces can also be achieved by using hybrid materials, such as laser printing of plasmonic NPs on tungsten hierarchical LIPSS/NP nanostructures by reduction of deposited Ag salts [92] or by inducing insufficient oxidation of embedded tungsten-NPs into WO₃₋_x oxygen-vacancy rich transition metal oxide [97] or by deposition of transition metal oxide layers [98].

4. Conclusion

In this study, the development of pure tungsten metasurfaces for multimodal coloration/encryption based on hierarchical LIPSS/NP nanostructures was proposed and theoretically demonstrated. By characterizing the UV-to-MIR broadband absorption in x-/y-polarized and unpolarized modes and quantifying it in sub-bands, and the depiction of the electric field, magnetic field, and absorbed power distribution of LIPSS/NP structures with different geometrical parameters, suitable nanostructure candidates for the construction of metasurfaces for multimodal displays were screened, and the mechanism for absorbance manipulation was clarified. Using a simplified reproduction of Van Gogh’s painting “The Starry Night” as a proof-of-concept demonstration, vivid paintings in different color palettes using both x-/y-polarized and unpolarized modes and polarization-selective (mainly in x-polarized mode) identification of the encoded information in the Vis and SWIR/MWIR bands are presented. Preliminary experimental results show the possibility of fabricating inhomogeneous LIPSS/NP meta-units. Based on this, the challenges in the fabrication of optically functional metasurfaces are discussed. This study is expected to stimulate interest in the development and preparation of novel pure-tungsten-based metasurfaces based on femtosecond laser nanostructuring, thus unlocking their potential for the novel fabrication of metasurfaces. This could be helpful to meet the growing demand for robust coloration, higher security measures against counterfeiting, and information encryption in harsh environments (e.g., high-temperature). Other applications of the as-proposed metasurfaces include solar absorbers/emitters in thermophotovoltaics and SETES-sensing substrates.

CRediT authorship contribution statement

Jianing Liao: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Conceptualization. Dongshi Zhang: Writing – review & editing, Writing – original draft, Supervision, Methodology, Funding acquisition, Conceptualization. Zhuguo Li: Writing – review & editing, Supervision, Methodology.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We gratefully acknowledge financial support received from the Shanghai Pujiang Program (23PJ1406500). The computations in this paper were run on the cluster supported by the Center for High Performance Computing at Shanghai Jiao Tong University.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2024.06.018.

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