Nanoscale broadband deep-ultraviolet light source from plasmonic

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Letter

Nanoscale broadband deep-ultraviolet light source from plasmonic nanoholes Liping Shi, José R. C. Andrade, Jue-Min Yi, Marius Marinskas, Carsten Reinhardt, Euclides Almeida, Uwe Morgner, and Milutin Kovacev ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00127 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Nanoscale broadband deep-ultraviolet light source from plasmonic nanoholes Liping Shi,∗,†,‡ José R. C. Andrade,† Juemin Yi,¶ Marius Marinskas,† Carsten Reinhardt,§ Euclides Almeida,∗,k,⊥ Uwe Morgner,†,‡ and Milutin Kovacev†,‡ Institute of Quantum Optics, Leibniz University Hannover, Welfengarten 1, 30167, Hannover, Germany, Cluster of Excellence PhoenixD (Photonics, Optics, and Engineering-Innovation Across Disciplines), Hannover, Germany, Institute of Physics and Center of Interface Science, Carl von Ossietzky University Oldenburg, 26129, Oldenburg, Germany, Hochschule Bremen City University of Applied Sciences, Neustadtswall 30, 28199 Bremen, Germany, Department of Physics, Queens College of the City University of New York, Flushing, NY 11367, USA, and The Graduate Center of the City University of New York, New York, NY 10016, USA E-mail: [email protected]; [email protected]

∗ To

whom correspondence should be addressed of Quantum Optics, Leibniz University Hannover, Welfengarten 1, 30167, Hannover, Germany ‡ Cluster of Excellence PhoenixD (Photonics, Optics, and Engineering-Innovation Across Disciplines), Hannover, Germany ¶ Institute of Physics and Center of Interface Science, Carl von Ossietzky University Oldenburg, 26129, Oldenburg, Germany § Hochschule Bremen City University of Applied Sciences, Neustadtswall 30, 28199 Bremen, Germany k Department of Physics, Queens College of the City University of New York, Flushing, NY 11367, USA ⊥ The Graduate Center of the City University of New York, New York, NY 10016, USA † Institute

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Abstract We employ a broadband Ti:sapphire femtosecond oscillator to simultaneously launch two localized surface plasmon modes in rectangular plasmonic nanoholes. The resonant frequencies of these two modes match well with our laser spectrum. As a result, the nanoholes do not only efficiently boost the third harmonic radiation intensity, but also significantly broaden the harmonic’s bandwidth, producing a nanoscale deep-ultraviolet light source in the range of 240 to 300 nm. Due to the involvement of two modes, the third harmonic beam becomes elliptically polarized and reaches its maximum intensity when laser polarization direction is 60° with respect to the long edges, rather than the commonly used 90°. Keywords: Nonlinear Plasmonics, Broadband deep-ultraviolet, Babinet-inverted metasurfaces, Substrate-free plasmonic nanoapertures, Third harmonic generation

Fundamental research and several applications such as background-free bio-sensing or imaging, nanomedicine, photocatalysis, as well as from a fundamental research standpoint, benefit immensely from a boosted third harmonic generation (THG) within a nanoscale medium. 1–4 However, such a small volume strongly limits the nonlinear frequency conversion efficiency. Plasmonics, a field treating the subwavelength interaction between light and noble metals like gold (Au) provides a platform to overcome this limit. Plasmonic nanoantennas, which usually consist of metallic nanoparticles and a supporting dielectric substrate, can concentrate impinging electromagnetic waves into nanoscale volumes, 5 resulting in strongly enhanced light interaction cross sections and electric near-field strength. 6,7 Moreover, thanks to the extremely high intrinsic nonlinearity of Au, efficient enhancement of the THG radiation by Au antennas has been widely observed. 8–18 Indeed, discernible harmonic radiation from Au nanostructures is crucial in some studies such as nonlinear beam shaping, nonlinear phase control and anomalous phase matching. 4,19 However, for plasmonic metasurfaces using Au antennas as their meta-atoms, THG emission from the bulky substrate may be much stronger than that from the thin Au antennas, hindering the signal arising from plasmonic effects. 20,21 As a solution, Babinet-inverted plasmonic metasurfaces based on nanoapertures in Au film as nonlinear meta-atoms can efficiently suppress the background THG radiation from the thick substrate and thus improve signal-to-noise ratio. 22,23 2


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In particular, rectangular nanoholes attract considerable interest for the investigation of nonlinear plasmonic effects, because one can easily tune the resonant wavelength by changing their aspect ratios. 24–27 According to the Babinet’s principle, nanoholes in metallic film differ from their complementary geometries, i.e., isolated metallic nanoparticles, in light polarization. Equivalent behaviour is seen for polarizations rotated by 90°. Thus, in order to obtain the highest nonlinear frequency conversion efficiency, laser is intuitively polarized along the short axis of the rectangles 24–28 to launch the localized surface plasmons (LSP) at the long edges, i.e., longitudinal mode. However, a rectangular hole is typically in presence of several LSP modes that are launched between parallel or orthogonal edges. 29,30 These modes also exhibit a broad bandwidth of resonance, considerably wider than the laser spectra that are considered in Refs. 24–28. Therefore, the nonlinear responses of rectangular nanoholes are not yet fully understood over the entire broadband LSP resonance. The use of wideband light sources is required to better understand LSP’s linear 31 and nonlinear properties. Here we excite the LSP resonances of rectangular nanoholes on a gold film by a broadband Ti:sapphire oscillator. Moreover, we investigate the impact of different incident polarizations on the plasmonic metasurface and uncover the interplay between multi-mode resonances. The observed response effectively broadens the resulting deep-UV THG radiation from 20 nm (in the absence of plasmon resonances) to 60 nm. We note that due to a strong interband absorption of Au in the deep-ultraviolet (200-300 nm), 32 generation of coherent light sources in this spectral region by Au nanostructures, especially broadband radiation, remains largely unexplored. Our high repetition rate, wideband and nanoscale coherent deep-ultraviolet light source holds potential applications in ultrafast spectroscopy and photon photoemission electron microscopy. Using a focused ion beam, the investigated Au nanoaperture array with an area of 50 × 50 µm2 is milled through a high quality 250 nm thick Au film sputtering on a 15 nm thick silicon nitride (Si3 N4 ) membrane. We also completely remove the 15-nm-substrate material under the Au apertures, leaving substrate-free plasmonic nanoholes to effectively reduce the background radiation. As shown in the upper inset of Figure 1, the periodicity of the holes within an array is

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set to 420 nm. Previous works have shown that a rectangular nanohole in Au film with an aspect ratio of around 2.1 is resonant with Ti:sapphire lasers. 24,25 Therefore, here we tailor each hole with long- and short-ridges to be 290 and 140 nm, respectively. Well-compressed few-cycle pulses from a Ti:sapphire oscillator centered at 825 nm are tightly focused onto the sample (see the inset in Fig.1). The beam diameter at the focus spot is estimated to be around 7 um, giving rise to a peak intensity of 0.2 TW/cm2 on the Au film. A broadband half-wave plate is employed to rotate the laser polarization angle θ , which is defined as angle between the electric field direction and the xaxis. A deep-ultraviolet polarizer is used to examine the polarization states of the generated THG. The transmitted harmonic photons are collected by a confocal monochromatic setup (modified McPherson 234/302, 1200 lines/mm grating for 110 nm to 310 nm, where the generation volume acts as an entrance slit. Finally, a photo multiplier capable of single photon counting (Hamamatsu H8259-09) in combination with a photon counter (Scientific Research SR400) is utilized to measure the far-field THG photon counts. We first measure THG spectra at various laser polarization states (Figure 2a). When the laser is polarized along the long axis of the nanoholes, namely θ =0° or 180°, we observe a narrowband THG ranging from 270 to 290 nm (θ =0°, gray area, Figure 2b). Using the broadband laser as a white light source, we measure in situ linear transmission spectra (curves, Figure 2b). The absence of a plasmonic resonance at θ =0° indicates a weak field enhancement. Hence the THG spectrum in this case reflects the intrinsic spectral amplitude of the incident laser. In case of θ =90°, a pronounced extraordinary optical transmission 33 is observed around 758 nm. This resonance is described as ω1 -mode, where ω1 denotes the LSP resonance frequency. The propagating surface plasmon resonance (SPR) can be easily excluded, as the period of the nanoholes does not match the SPR wavelength neither at the Au-Si3 N4 nor at the Au-air interfaces. 34–36 The Fabry-Perot resonance of our nanoholes peaks at around 650 nm, 24 which is not within the spectral range of our interest. Therefore, the ω1 -mode mode is ascribed to a dipole resonance between the long ridges (longitudinal mode), which is launched by the Ey component of the incident field. 29 This localized plasmon mode gives rise to strong enhancement of THG around 255 nm. Because its

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resonance frequency is not located at the laser central frequency, the THG spectrum is efficiently widened, forming a broadband deep-ultraviolet light source (θ =90°, Figure 2b), spanning from 240 to 300 nm. As mentioned above, the longitudinal mode is launched by the electric components that are orthogonal to the long rims. One expects that THG peaks at θ =90°, and its intensity versus θ follows the classical Malus’s law for polarization. 24–27 Indeed, the peak at 255 nm (red line, Figure 2a) is in agreement with this law, i.e., IT HG ∝sin6 (θ )(see Figure 2c). However, the signal around 275 nm (blue line, Figure 2a) reaches its maximum intensity at θ =60° and thus represents a quadrupolelike distribution versus θ (see Figure 2d). The emitted intensity has a third-power dependence on the excitation intensity (Figure 2f), showing clear signatures of THG. The total photon counts over the entire THG spectrum is measured to be 2 × 108 photons/s (Figure 2b). The photon energy at this spectral range is 4.6 eV, therefore, the average power of THG is estimated to be 0.15 nW. The incident laser power at the sample surface is 80 mW. The THG conversion efficiency is of the order of 10−9 . The corresponding linear transmission spectrum (Figure 2b) indicates that this nonintuitively enhanced harmonic signal is due to the appearance of a second localized mode centered at around 820 nm (ω2 -mode). Nevertheless, the total linear transmission intensity at 820 nm versus θ still follows the Malus’s law (Figure 2e). At a first glance in Figure 2 (d) and Figure 2(e), there is the impression that the nonlinear response of a rectangular nanohole does not fully match its linear properties. To obtain a deeper insight into this unexpected THG enhancement, a deep-ultraviolet polarizer is employed to analyze the polarization state of the generated THG in the far-field. Figure 3 depicts the THG spectra versus the polarizer orientation φ . Here φ =0° and 90° are maximal transmission for y- and x-polarized light, respectively. As it can be seen from Figure 3(a), when the excitation laser is polarized along the y-axis, the produced harmonics at both peaks are also y-polarized (φ =0°). When θ =60° [Figure 3(b)], THG around 255 nm strongly enhanced by the ω1 -mode still consists of a dominant y-polarized field (φ =20°). However, for the harmonic around 275 nm, comparably enhanced by both modes, the x-polarized THG is evidently stronger than its

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orthogonal counterpart (φ =60°). This result suggests that the ω2 -mode also launches LSP at the short edges by the Ex component of incident field. Moreover, considering that plasmons, i.e., collective oscillations of free electrons exhibit a certain phase response, 37,38 we find that at θ =60°, the THG around 275 nm nm is elliptically polarized, having an ellipticity of 0.51 with respect to the electric field. Surface plasmon enhances the near-field strength, causing locally boosted THG radiation. In order to understand this unexpected behaviour between linear and nonlinear effects in a microscopic picture, one needs to integrate coherently over all sources. 39 However, for plasmon-induced THG enhancement, the much simpler macroscopic nonlinear oscillator model is generally sufficient to understand the underlying mechanisms. 39 In this picture, one can model the LSP as harmonic oscillators with resonance frequencies ωi , damping constant γi , mass m, and charge q. 14,40,41 Here i = 1, 2 denote the localized modes at ω1 and ω2 , respectively. Such a harmonic oscillator can be described by a linear polarizability with number density Ni (1)

χi (ω) =

1 Ni q2 , 2 ε0 m ωi − ω 2 − iωγi

(1)

The linear optical transmission enhancement is due to re-emission of plasmons back into free space. Therefore, the measured linear transmission spectrum can be fitted by T (ω),

T (ω) = ∑ Ti (ω) = ∑ i

i

ω (1) Im[χi (ω)]. c

(2)

Figure 4 (a) shows the measured (black curve) and fitted (green curve) transmission spectrum when θ =60°. The transmission spectra of both modes (red curve: ω1 ; blue curve: ω2 ) are extracted from the fitting. The damping constants are fitted to be γ1 =2.6 × 1014 s−1 , γ2 =2.2 × 1014 s−1 , respectively. The smaller damping constant and lower resonant frequency of the ω2 -mode confirms that a higher field enhancement factor inside the Au film is present, since it is inversely proportional to both terms. This explains why the ω2 -mode shows a lower linear transmission while exhibiting a higher THG enhancement. Figure 4 (b) shows the extracted spectra of transmission enhancement 6


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by the localized mode centered at ω2 at various incident polarization states. It is found that the maximal linear transmission actually occurs at around θ =50°. We further extract the THG signal at 275 nm enhanced by the ω2 -mode. Taking advantage of the aforementioned fact that the ω1 -mode-enhanced THG follows sin6 (θ ) (black curve), Figure 4 (c) depicts the measured intensity at 275 nm versus laser polarization (blue curve), and extracted component caused by the ω2 -mode (red curve). Indeed, we observe that the ω2 -mode results in highest harmonic generation also at around θ =50°. This is in agreement with the evaluation of the linear transmission spectra (Figure 4b). We also carry out numerical simulations by using the finite element method (JCMwave) to map the near-field distribution at frequencies ω1 and ω2 . The optical constants of Au at each frequency are taken from Ref. 42. Figure 4(d) and (e) display the near-field maps when θ =90° and 45°, respectively. The exciting source is a monochromatic plane wave at ω1 . As expected, the field enhancement factor is halved when the laser polarization angle is rotated from 90° to 45°. Figure 4(f) and (g) depict the near-field distribution with an incident source frequency of ω2 , which shows a lower field enhancement within the nanohole when compared to the ω1 -mode. This is because the dipole resonance between the long edges is resonant at ω1 . However, as shown in Figure 4(g), when laser polarization (white arrow) crosses the corners of the rectangular hole, an additional dipole forms between the short and long ridges, leading to a strong field confinement at the corresponding corners. The smaller radius of curvature at the corners gives rise to a weaker restoring force and thus a stronger near-field enhancement factor inside the Au film, 39,43,44 which is in agreement with the nonlinear anharmonic model mentioned above. Furthermore, the localized field at the corners contains a considerable Ex component, because in this case a portion of charges also accumulate at the short ridges. This corroborates the observed high amount of x-polarized THG photons when the ω2 -mode is efficiently launched (Figure 3b). In summary, we have utilized a broadband Ti:sapphire oscillator to investigate coherent nonlinear properties of the wideband LSP resonances, and demonstrated the generation of LSP-enhanced wideband THG in the deep-ultraviolet spectral region from substrate-free rectangular nanoholes

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in Au film. Such a nanoscale source is highly promising for transient spectroscopy, microscopy and phototherapy. We would like to point out that, in the literature, the optimal polarization for rectangular plasmonic nanoholes is normal to the long edges. However, in this paper, we have shown that due to the existence of multi-modes, a portion of the harmonic intensity reaches its peak value when laser polarization direction is 60° with respect to the long edges. Our findings offer an additional degree of freedom to tailor and manipulate the nonlinear response of plasmonic nanoapertures.

Acknowledgement The authors thank funding supports from Deutsche Forschungsgemeinschaft (DFG) (KO 3798/41) and from German Research Foundation under Germany’s Excellence Strategy-EXC-2123 and Germany’s Excellence Strategy within the Cluster of Excellence PhoenixD (EXC 2122, Project ID 390833453); Lower Saxony through "Quanten und Nanometrologie" (QUANOMET, Project Nanophotonik).Carsten Reinhardt is grateful funding supports from "Hochpraezises Laserdrucken von Nanopartikel Metaoberflaechen for die Kontrolle von Licht, Sensorik und Nanolaser" - "HighPrecision Laser Printing of Nanoparticle Metasurfaces for Controle of Light, Sensors,and Nanolaser" (RE3012/4-1). "Silber-Nanodraht-Hyperlinsen" - "Silver Nanowire Hyperlenses" (RE3012/2-1).

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Figure 1: Schematic illustration of the experimental concept. A broadband half-wave plate (HWP) is used to rotate the electric field direction of the incident pulses. A deep-ultraviolet polarizer is rotated to analyze the polarization states of the produced third harmonic generation. Here φ =0° and 90° are maximal transmission for y- and x-polarized light, respectively. Inset (left upper): scanning electron microscopy image of a unit of nanohole array; The crossing angle between the electric field direction and long-rim (x-axis) of the nanohole is defined as polarization angle θ . Inset (right bottom): Laser spectrum. Please note that the peaks at around 700 nm and 900 nm are out of phase, which do not contribute to third harmonic generation.

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ACS Photonics

Figure 2: (a) Spectra of third harmonic emission from the nanoholes at various laser polarization states. (b) Representative spectra of third harmonic (gray areas) and linear transmission (curves) at θ =0°, 60°, 90°, respectively. Please note that the spectrum intensity at θ =0° is multiplied by 5 for a better visualization. (c) Dependence of third harmonic intensity on laser polarization at 255 nm [indicated by the red line in (a)], and (d) at 275 nm [indicated by the blue line in (a)]. (e) Intensity of linear transmission at 820 nm (red squares), following Malus’s low (black curve). (f) Power dependence of the third harmonic emission at θ =60°.

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Figure 3: Polarization analysis of the transmitted third harmonic generation in the far-field. Polarization angle φ is measured with respect to the short-axis, i.e., φ = 0° corresponds to maximal transmission for y-polarized light. (a) Incident laser is y-polarized; (b) laser polarization angle is 30° with respect to the y-axis, i.e., θ =60°. 16


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ACS Photonics

Figure 4: Fitting (green curve) of experimentally measured linear transmission spectrum at θ =60° (black curve) by a nonlinear anharmonic model. Red and blue curves are corresponding to the localized mode with resonant frequency at ω1 and ω2 , respectively. (b) Extracted spectra of linear transmission enhancement by the ω2 -mode at various laser polarization states. (c) Measured intensity of third harmonic at 275 nm emission from the nanoholes in the far-field versus laser polarization (blue curve), and extracted components induced by ω1 -(black curve) and ω2 -mode (red curve). Finite element method-based numerical simulation of near field distribution at frequency of ω1 (d, e), and ω2 (f, g). The monitor plane locates at half the nanohole’s height to avoid perturbations from the edges. The polarization directions (indicated by white arrows) are 90° (d, f), and 45° (e, g) with respect to the long edges (x-axis) of the rectangular nanohole. The dashed curve in (g) shows an isoline of electric field strength. 17


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For Table of Contents Use Only Title: Nanoscale broadband deep-ultraviolet light source from plasmonic nanoholes Authors:Liping Shi, Jose R. C. Andrade, Juemin Yi, Marius Marinskas, Carsten Reinhardt, Euclides Almeida, Uwe Morgner, and Milutin Kovacev The authors fabricate rectangular nanoholes in gold film and subsequently remove the dielectric substrate within the holes. Such substrate-free nanoholes effectively reduce background radiation. The plasmonic resonance frequency is intentionally blue-shifted with respect to the laser central frequency. Consequently, the bandwidth of third harmonic generation is effectively broadened. Third harmonic signal reaches its maximum when laser polarization direction is 60°, rather than commonly used 90° with respect to the long edges, radiating elliptically polarized light in deep-ultraviolet.

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Nanoscale broadband deep-ultraviolet light source from plasmonic

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