Double-Pulse Femtosecond Laser Fabrication of Highly Ordered Periodic Structures on Au Thin Films Enabling Low-Cost Plasmonic Applications

Abstract

Periodic plasmonic arrays, making possible excitations of surface lattice resonances (SLRs) or quasi-resonant features, are of great importance for biosensing and other applications. Fabrication of such arrays over a large area is typically very costly and time-consuming when performed using conventional electron beam lithography and other methods, which reduce application prospects. Here, we propose a technique of double femtosecond pulse (∼170 fs) laser-assisted structuring of thin (∼32 nm) Au films deposited on a glass substrate and report a single-step fabrication of homogeneous and highly ordered Au-based laser-induced periodic surface structures (LIPSS) over a large area. Our experimental results unveil the key importance of the interpulse delay as the determining factor rendering possible the homogeneity of laser-induced structures and confirm that highly ordered, functional LIPSS occurs solely upon double pulse irradiation under a specific interpulse delay range. A theoretical investigation complements experimental results, providing significant insights into the structure formation mechanism. Ellipsometric measurements show that such LIPSS structures can exhibit highly valuable plasmonic features in light reflection. In particular, we observed ultranarrow resonances associated with diffraction-coupled SLRs, which are of paramount importance for biosensing and other applications. The presented data suggest that femtosecond double pulse structuring of thin metal films can serve as a valuable and low-cost tool for large-scale fabrication of highly ordered functional elements and structures.

Keywords: LIPSS; femtosecond laser processing; functional nanomaterials; multiscale modeling; plasmonic biosensing; surface modification modeling; thin-films.

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Parametric results obtained upon irradiation by a 170 fs, 1030 nm pristine pulse laser (i) SEM images showing the different morphologies obtained by SPS and DPS structuring at different fluences, Φ, and interpulse delays, Δτ. Homogeneous LIPSS marked by ‘*’. An overlap Ov = 150 pulses per spot (pps) is used. Red arrows on the top row indicate the polarization direction.

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Process window graphs indicating the resulting morphology upon systematic variation of two process parameters (Φ and Ov) for six interpulse delay (Δτ) values. The color of the circle fillings corresponds to the different morphologies attained for each set of parameters as indicated in the legend shown at the bottom. The yellow marks in each graph indicate the process parameters that lead to the homogeneous (H)-LIPSS formation. The shaded yellow areas between the graphs are the estimated process windows for different delays. The number of cases of H-LIPSS formation observed within the process window is indicated at the top right of each graph.

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Comparison of structures for SPI and DPI (Δτ = 1 ns). (i) Unprocessed surface. Turquoise arrows indicate the surface defects. (ii) Comparison of structure evolution for SPS and DPS, respectively. Ov is indicated for each SEM image Φ = 140 mJ/cm² for SPI and Φ = 160 mJ/cm² for DPI. Turquoise arrows indicate the “hot spots” present on the surface, and green dashed ellipses indicate the inhomogeneous morphology between two craters. Symbol “*” indicates the surface subjected to EDX measurement provided in Supporting Information. (iii) a and a′: SEM of optimized surfaces for SPS and DPS. For SPI, Φ = 180 mJ/cm² and Ov = 100 pps, while for DPI, Φ = 170 mJ/cm² and Ov = 150 pps. b and b′: Fast Fourier transform (FFT) maps are shown for SPI and DPI, respectively. c and c′: FT maps showing the signal points with intensity I > I _max/e ² of b and b′, respectively. Red arrows indicate the polarization direction. SEM images tagged “SM” can be found in high magnification in the Supporting Information.

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(i) (a, b) SEM image of Au surface showing the particular formation of dual-period LIPSS after irradiation by double pulses having Δτ = 2 ns, N = 100 pps, and Φ = 200 μJ/cm². (ii) Image of pinching instability of convection flow in silicon oil (reproduced with permission from ref , Copyright 2025, Cambridge University Press). The red arrow indicates the polarization direction.

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(a) Intensity distribution in the X–Y (transverse) plane just below the top air/Au interface of a 32 nm Au/SiO₂ thin film for λ = 1026 nm, (b) Fourier spectrum of the intensity patterns in the XY-plane showing quasi-periodic features of two distinct periodicities, Λ_top ≈ λ = 1026 nm (inner circle k _x /k ₀ ≈ 1) and Λ_bottom ≈ 690 nm (outer circle k _x /k ₀ ≈ λ/Λ). (c) Lattice temperature spatial profile on the surface of Au for SPS at t = 0.25 ns (white regions show ablated part). Lattice temperature spatial profile on the surface of Au at t = 2 ns (d) and t ∼ 2.04 ns (e) (the white double-headed arrow indicates the laser polarization). Lastly, simulation results of surface relief resulting from irradiation with Φ = 280 mJ/cm² for a single pulse for SPS (f) and a single pair of pulses for DPS (g). Orange-colored areas indicate ablation in (f). (h) Lattice temperature upon irradiation with single and double pulse with variable interpulse delay as indicated. Horizontal lines mark the ablation temperature (red dotted line, for T _a = 0.9 × T _cr = 5625 K) and the resolidification temperature (blue dotted line at T _melt = 1337 K). (i) Comparison between the resolidification time (simulation data) and number of H-LIPSS within the process window versus the interpulse delay. Yellow and red dotted lines are added as guides to the eye.

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SEM (a) and AFM (b) images of a sample prepared at a fluence of Φ = 170 mJ/cm² and time delay Δτ = 500 ps. (c) Representative height profile obtained from the AFM image shown in panel (b). (d) Ellipsometric parameter Ψ (amplitude) as a function of the incident wavelength and angle for the sample shown in panels (a) and (b). SLR denotes the positions of the plasmonic surface lattice resonances. (e) Ellipsometric parameter Δ (phase) as a function of the incident wavelength and angle for the same sample. (f) Comparison of the measured resonance position of SLR (magenta circles) and theory (dotted line).

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Setup. Abbreviations: beam splitter (BS), linear polarizer (LPC), half waveplate (λ/2), attenuation part (AT), and polarization control part (PC). Linear displacement (Δx), time delay (Δτ).

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Schematic diagram of ellipsometric measurement for gold-patterned nanostructures and geometry of orientation of ablated samples with respect to the direction of incident polarized light.