1. The electromagnetic excitation of resonant oscillations of the electron density at the metal–dielectric interface, the so-called surface plasmon-polariton waves, is widely used in modern optoelectronic and sensorics devices to enhance the interaction of optical radiation with matter at the nanoscale [1–3]. Such oscillations both can be maintained in single nanostructures usually made of noble metals (localized plasmon resonances) and can be excited in specially designed arrays of such nanostructures often called metasurfaces. The coherent matching of electromagnetic waves scattered on nanostructures with plasmon-polariton waves traveling and localized in nanostructures provides the interference suppression of radiative losses, which makes it possible to partially compensate high ohmic losses in the used metals, thus ensuring the excitation of collective plasmon modes with a high Q-factor [4]. These modes are, in particular, quasi-bound states in continuum; the possibility of both the excitation of these states in plasmon nanostructures and their practical use in some relevant problems in nonlinear optics, nanoplasmonics, and sensorics has been actively studied in recent years [5–9]. At the same time, the practical use of plasmon metasurfaces allowing high-Q-factor modes will obviously require in the near future efficient scalable methods for the fabrication and replication, as an alternative to expensive multistage lithographic methods.

In this work, we report on the fabrication of plasmon metasurfaces allowing high-Q-factor collective modes in the near infrared spectral band using ablation-free femtosecond laser printing. The existence of these modes is confirmed by the results of comparative study of the optical (infrared Fourier-transform spectroscopy) and nonlinear optical (third-harmonic generation) properties of fabricated nanostructures.

2. Ordered arrays of nanobumps were fabricated using the direct femtosecond laser printing on the surface of 50-nm-thick gold films deposited on glass substrates by thermal deposition in vacuum at a rate of 0.7 nm/s. The second harmonic radiation (515 nm) of an Yb:KGW femtosecond laser source (Pharos, Light Conversion; 200 fs, 1 kHz) was focused on the surface of the gold film by an objective with a numerical aperture NA = 0.42, ensuring the possibility of controlled one pulse–one structure printing of arrays of nanobumps with a minimum period down to d = 0.9 μm. The laser beam was scanned over the surface of the film by means of a system of linear nanopositioners (Aerotech), which ensured the possibility of the displacement of the target with the gold film with an accuracy up to 0.12 μm (see Fig. 1a). The pulse energy E was controlled by a photodetector and was varied from 0.7 to 2 nJ, ensuring the variation of the geometric shape of nanobumps.

Fig. 1. 
figure 1

(Color online) (a, left) Sketch of the fabrication of plasmon metasurfaces by ablation-free femtosecond laser printing on the surface of thin gold films and (a, right) a series of scanning electron microscopy images illustrating the evolution of the shape of a single nanobump with increasing pulse energy E from 0.85 to 1.22 nJ; the scale bar corresponds to 200 nm. (b) Scanning electron microscopy image of the ordered array of nanobumps fabricated at a pulse energy of E = 1.22 nJ and an interstructural period of d = 1 μm. (c, left) Optical and (c, middle and right) scanning electron microscopy images of the surface of counter-slit gold electrodes on the silicon substrate with arrays of plasmon nanobumps written on them.

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A typical scanning electron microscopy (scanning electron microscopy) image illustrating the morphology of the surface of the gold film with the \(300 \times \) 300‑μm array of nanobumps printed at d = 1 μm is shown in Fig. 1b, demonstrating the reproducibility of the shape of individual structures in the array. This occurs primarily because the ablation removal of the material in the form of melted nanoparticles and their subsequent redeposition on the surface of the film are absent in the process of the formation of nanobumps. At a used pulse repetition frequency of 1 kHz, the fabrication took no more than 2 min and can be significantly accelerated with higher-frequency laser sources in combination with methods of multiplexing of laser beams. nanobumps are hollow, which was confirmed by the scanning electron microscopy visualization of the cross sections of individual structures fabricated using the focused ion beam (see the inset of Fig. 1b). Such hollow nanostructures are formed due to the acoustic relaxation of a local laser-melted region of the gold film and its subsequent recrystallization [10–12]. The energy of the laser pulse introduced into the film makes it possible to control the shape of nanostructures, which varies from small parabolic nanobumps (at the pulse E slightly above the film modification threshold Eth = 0.7 nJ) to high-aspect-ratio nanoneedles (at E > 1.2 nJ), as shown in a series of scanning electron microscopy images in the inset of Fig. 1а. Femtosecond laser printing ensures the possibility the precise point-by-point fabrication of arrays of nanobumps with controlled shape and period with a high accuracy and repeatability (the stability of the pulse energy for the used laser source was 0.5%) even on targets with a nontrivial shape. As an example, Fig. 1с presents various arrays of nanobumps formed on the surface of counter-slit gold electrodes deposited through a mask on the surface of a bulk silicon single-crystal substrate with a 0.1-μm-thick silicon dioxide layer.

3. Ordered arrays of nanobumps allow the excitation and destructive interference of coherent oscillations of free electrons (or plasmon-polariton waves), which can be identified by characteristic dips observed in the reflection spectra of these structures in the near- and mid-infrared bands [13–15]. The numerical calculations of the local structure of electromagnetic fields near the surface of the array of nanobumps under the resonant excitation in [16], as well as the simulation of the composition of such structures, indicate that the observed resonant plasmonic response can be attributed to the excitation of high-Q-factor modes such as quasi-bound states in continuum. This statement is partially confirmed by the nontrivial behavior of the resonant response observed experimentally under the simultaneous variation of the period d of the arrangement of nanobumps in the array and of their shape. In particular, the position of the resonance observed at the wavelength λR for the fixed shape of nanobumps is expectedly scaled linearly with the period d of the arrangement of nanostructures in the array, which is typical under the excitation of collective plasmon modes in periodic structures [4]. At the same time, the evolution of the shape of nanostructures at the fixed period d described above, which is reached due to the increase in the pulse energy E introduced into the film (as shown in Fig. 1a), also results in a significant redshift of the observed resonant response. This indicates a significant effect of the shape of single structures, which allow localized plasmon eigenmodes, both on the efficiency of the excitation/interference of plasmon-polariton waves and on the spectral position of the observed resonance. Two series of Fourier-transform infrared reflection spectra (Bruker, Vertex 70v, and Hyperion 1000) of 300 × 300-μm arrays of gold nanobumps (see Figs. 2a and 2b) fabricated at E = 0.85 nJ and d = 0.9–1.2 μm, as well as at d = 0.9 μm and E = 0.85–1.22 nJ illustrate the described trends, indicating the necessity of matching of the spectral positions of local (geometric) plasmon resonances, which are determined by the shape of single nanobumps, and the characteristic distance d between them to ensure the detection of the intensity resonant response. It is noteworthy that the characteristic amplitude of the resonance observed in the Fourier-transform infrared reflection at such matching reaches 40% at a Q-factor of ~λR/Δλ ≈ 10–13, where Δλ is the spectral half-width of the dip. The systematic studies of the optical properties of the metasurfaces fabricated at the same parameters show the reproducibility of the spectral position of the collective resonance with an accuracy of \( \pm 25{\kern 1pt} \) nm under the variation of its amplitude within \( \pm 3{\kern 1pt} \% \).

Fig. 2.
figure 2

(Color online) Series of Fourier-transform infrared reflection spectra from a series of 300 × 300-μm plasmon metasurfaces fabricated (top panel) at a fixed pulse energy of E = 0.85 nJ and the period of nanobump arrangement in the array varying in the range d = 0.9–1.2 μm and (bottom panel) at a fixed period of d = 0.9 μm and the energy varying in the range E = 0.85–1.22 nJ.

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4. The resonant excitation of plasmon-polaritons is accompanied by the localization and enhancement of the amplitude of electromagnetic fields near the surface of plasmon nanobumps. The presence of such fields is confirmed by the previous numerical simulation [16], as well as by the experimentally demonstrated enhancement of the spontaneous emission of a mercury telluride quantum dot nanolayer deposited on the array of structures at the spectral matching of the emission spectrum and the collective resonance of the structure [17]. In this work, the plasmon response of arrays of nanobumps fabricated by ablation-free laser printing was studied for the first time by measuring the third-harmonic intensity excited by the parametric femtosecond laser generator (TOPOL, Avesta project), which ensured the possibility of the variation of the pump wavelength in the spectral range of the excitation of the collective plasmon resonance. In these experiments, pump radiation (220 fs, 80 MHz) at the wavelength of the fundamental harmonic (λω = 1230–1500 nm) was focused by the objective with the numerical aperture NA = 0.5, ensuring the excitation of the region of the sample surface with plasmon nanobumps 15 μm in diameter at the appropriate tuning of the size of the input laser beam to the objective by means of a system of two lenses (see Fig. 3a). The third-harmonic signal was collected by this objective and was guided by a spectrally selective beam splitter cube to the CCD camera of an optical microscope or to a confocally coupled spectrometer joining a monochromator and a sensitive cooled CCD camera (Shamrock 303i/Newton, Andor). The typical third-harmonic generation spectrum obtained at the excitation of the array of plasmon nanobumps (d = 0.9 μm, E = 0.95 nJ) at the pump wavelength λω = λR = 1370 nm is shown in Fig. 3b, demonstrating nonlinear emission at the wavelength λ3ω = λω/3 = 456.6 nm and the signal intensity that is almost two orders of magnitude higher than that from the surface of the flat gold film. The optical visualization of the plasmon metasurface in the third-harmonic generation regime shows that the surface of nanobumps is the most intense source of the detected signal due to the expected increase in the local amplitude of the electromagnetic field at the pump wavelength matched with λR (see the inset of Fig. 3b). It is worth noting that features of the transmission of spectrally selective elements in the used optical setup prevent the detection of the second harmonic generation, which is also observed in plasmon nanostructures due to the symmetry breaking at the interface [18].

Fig. 3.
figure 3

(Color online) (a) Sketch of the pumping and detection of the third-harmonic signal from plasmon metasurfaces consisting of arrays of hollow nanobumps fabricated on the surface of the gold film at the period d = 0.9 μm and the pulse energy E = 0.95 nJ. (b) Typical third-harmonic generation spectrum of these nanostructures in comparison with a similar spectrum measured from the flat surface of the gold film. The wavelength and average power of pump radiation of nanostructures at the fundamental harmonic are 1370 nm and 40 mW, respectively. The inset shows the micro-image of the plasmon metasurface in the third-harmonic generation regime. (c) (Blue circles) Intensity of the third-harmonic signal versus the pump wavelength at the fundamental harmonic and (gray line) the Fourier-transform infrared reflection spectrum of the corresponding metasurface.

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The third-harmonic intensity depends on the pump intensity according to a cubic power law, confirming the three-photon character of absorption in this nonlinear process. At the same time, even an insignificant spectral detuning of the pump wavelength λω from the position of the collective plasmon resonance by ±30 nm dramatically reduces the intensity I3ω by an almost an order of magnitude (see blue line, Fig. 3c). Systematic measurements of the third-harmonic signal as a function of the pump wavelength λω allowed the reconstruction of the spectral shape of the resonance, whose width is almost halved (ΔλNL ≈ 50 nm) compared to the half-width of the dip (Δλ ≈ 110 nm) observed in the Fourier-transform infrared reflection spectrum. This discrepancy can be due to the use of the reflection objective with a quite high numerical aperture (NA = 0.5) to measure Fourier-transform infrared reflection spectra of 300 × 300-μm arrays of nanobumps. This leads to the excitation of nanostructures at angles different from normal incidence, increasing the half-width Δλ of the detected resonant peak. It is noteworthy that the authors of [19, 20] reported on a decrease in the Q-factor ~λR/Δλ of the collective resonances such as quasi-bound states in continuum in the array of plasmon nanostructures with an increase in the numerical aperture of the objective used to collect the optical signal. Furthermore, measurements of the plasmonic response by the third-harmonic generation method are of a more local character, minimizing the possible variation of the shape of nanobumps from structure to structure. A small discrepancy between the maxima of plasmonic response in linear and nonlinear optical measurements can be due both to a small deviation of the angle of incident of the fundamental harmonic pump radiation from the normal to the surface of the gold film and to the angular dispersion of the collective plasmon mode. At the same time, the Q-factor of the collective plasmon resonance in the array of plasmon nanobumps, which were fabricated by direct femtosecond laser printing, measured from the variation of the nonlinear third-harmonic generation response reaches ~λR/ΔλNL ≈ 27, which is at the level of best experimental demonstrations of plasmon metasurfaces that allow quasi-bound states in continuum and are implemented using expensive multistage lithographic methods [19–21].

5. To summarize, plasmon metasurfaces consisting of ordered hollow gold nanobumps have been fabricated by direct femtosecond laser printing, and their optical and nonlinear optical properties have been studied. It has been shown that the fabricated metasurface demonstrates high-Q-factor collective plasmon resonances detected in the Fourier-transform infrared reflection spectrum, which ensure the enhancement of the third-harmonic generation by two orders of magnitude compared to the flat gold film at the spectral matching of the resonance of the structure with the wavelength of the fundamental harmonic pump. The analysis of the third-harmonic intensity as a function of the pump wavelength shows that the Q‑factor of the collective plasmon resonance reaches the value for advanced metasurfaces allowing quasi-bound states in continuum, indicating that scalable femtosecond laser printing methods are promising in application to the fabrication of resonant nanostructures for various problems of the implementation of advanced sensorics devices, as well as of nano-optics and nonlinear optics.