Full Length ArticleDirect laser printing of tunable IR resonant nanoantenna arrays
Graphical abstract
Introduction
Resonant plasmonic structures exhibiting strong enhancement of electromagnetic near-fields under excitation with infrared (IR) radiation are of special interest for bio- and environmental sensing applications based on detection of molecular vibrational fingerprints via surface-enhanced infrared absorption (SEIRA) effect [1], [2], [3], [4], [5], [6], [7]. This effect is based on multi-fold enhancement of the characteristic signals from the vibrational modes of various molecular species in the vicinity of the plasmon-mediated hot spot driven by resonant free carrier oscillations of the IR-pumped plasmonic nanoantennas.
Typically, direct electron- or ion-beam lithography techniques are well adopted to fabricate an isolated IR resonant nanoantennas or their lab-scale arrays in well controlled manner with nanoscale resolution [8]. However, these methods are rather time-consuming and extremely expensive, especially when multiple nanoantenna arrays are required for serial biosensing measurements. Photolithography being a commercially available and scalable technology provides common way to produce the IR-resonant plasmonic structures over a large surface area [9]. However, the related mask-based procedures make such approach rather expensive and inflexible for routine applications. In this respect, direct mask-free scalable methods allowing to produce the surface textures possessing strong plasmonic IR response are of great demand.
In recent years, direct laser printing using short (nanosecond) and ultrashort (femtosecond, fs) laser pulses has emerged as a promising flexible technology allowing to produce various functional plasmonic structures [10], [11], [12]. In particular, ultrashort fs-pulse when focused into a diffraction-limited spot provides precise energy deposition to a irradiated material with a minimized heat-affected zone resulting in its ultrafast melting and further irreversible modification. Specifically, for chemically-pure plasmonic-active thin films (as Au, Ag or Cu) the direct impact of the fs pulse above the ablation threshold fluence launches - through a sequence of thermal, acoustic and hydrodynamic processes - formation of unique surface morphologies, e.g., nanobumps (or nanovoids, [13]) or high-aspect ratio upright-standing protrusions (nanojets, [14], [15]), demonstrating pronounced size- and shape-dependent electromagnetic response in the visible and near(mid)-IR spectral regions [16], [17], [18]. Specifically, for IR resonant rod-like nanojets their response was explained in terms of excitation of the dipolar-like plasmon mode with the corresponding resonant wavelength linearly scaled with the nanojets height [18].
In this paper, we report on a range of experimental results for laser-printed nanoantenna surfaces, produced via direct fs-laser printing over thermally thin Au films [19]. We have fabricated a set of surfaces with different nanojet geometries and array periods, and analyzed the origin of the measured IR resonances in these structures. We conclude that the origin of such resonances cannot be explained in terms of a simplistic λ/4-antenna model, reported previously, and requires a more detailed consideration of how the excited plasmons propagate through the structure. We have observed that the resonant wavelength scales linearly with the effective plasmon running length, defined by both the array period and the nanoantenna geometry. To this end, a good agreement to the theoretically proposed model was obtained. Finally, for the first time to our knowledge, we have performed direct fs-printing on the surface of several two-component plasmonic alloys, and found that the proposed technology preserves the initial chemical composition of the metal film throughout the produced surface structures, thus allowing to produce fully alloyed plasmonic nanoantennas with a desired composition. Overall, our results demonstrate a capability to fabricate plasmonic surfaces having on-demand chemical composition, and featuring high-Q IR resonances with a spectral position easily tailored within at least 2–6 μm by varying the array period and the shape of the nanoantennas. This, in particular, makes a clear promise towards realisation of molecular and gas SEIRA sensors.
Section snippets
Metal film fabrication
Thin noble metal film having the thickness of 50 nm was coated onto a silica glass substrate without adhesion sublayer using a magnetron evaporation procedure performed at a chamber pressure of 10−5 atm and a constant sputtering rate of ∼1 nm s−1, while rotating the sample holder at a speed of 30 rpm to ensure uniform deposition over the entire sample surface. The two-component alloyed films were co-sputtered using two corresponding magnetron sources. The sample holder was tilted to fix the
Pulse-energy dependent types of surface structures
Ultra-short laser pulse spatially confined onto a diffraction-limited focal spot is known to produce the fast solid-liquid-solid phase transitions of the irradiated section of the thin noble-metal film covering heat insulating substrate. By varying the only experimental parameter, an applied (absorbed) pulse energy E, several specific structures can be produced on the surface of such film (see Fig. 1(a)). For a relatively thin film, which can be completely melted by an absorbed fs-laser pulse
Conclusion
In conclusion, using direct femtosecond-laser modification of thermally thin Au film we produced IR-resonant surfaces made of square-shaped periodically arranged structures, nanobumps and nanojets. We provided the convincing experimental results revealing the origin of the IR resonances of the produced laser-printed surface textures, and suggested a simple model explaining the observed resonant behavior in terms of propagation and interference of the excited surface plasmons, whereby the
Acknowledgments
Authors acknowledge the support of the Russian Science Foundation (grant no. 17-19-01325).
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