1 Introduction. Keywords: double bowtie nanoantenna, ring grating, plasmonic, field enhancement, plasmon-emitter coupling

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1 Nanospectroscopy 2015; 1: Research Article Open Access N. Rahbany, W. Geng, S. Blaize, R. Salas-Montiel, R. Bachelot, C. Couteau* Integrated plasmonic double bowtie / ring grating structure for enhanced electric field confinement DOI /nansp Received June 22, 2014; accepted May 20, 2015 Abstract: Metallic nanoparticles and nanoantennas have been extensively studied due to their capability to increase electromagnetic field confinement which is essential in numerous applications ranging from optoelectronics to telecommunication and sensing devices. We show that a double bowtie nanoantenna has a higher electric field confinement in its gap compared to a single bowtie nanoantenna, which is expected to give better fluorescence enhancement of a single emitter placed in the gap. We show that the electric field intensity can be further increased by placing the double bowtie inside a ring grating structure where the excitation of surface plasmon-polaritons (SPPs) is achieved. We perform FDTD simulations to characterise the double bowtie nanoantenna and study the effect of its dimensions on the electric field enhancement in the gap. Our proposed integrated structure with gratings is shown to increase the electric field by a factor of 77 due to a double cavity effect. Next steps would be to study the fluorescence enhancement of emitters placed inside our double bowtie / ring grating nanocavity to see if the strong coupling regime can be attained. Keywords: double bowtie nanoantenna, ring grating, plasmonic, field enhancement, plasmon-emitter coupling 1 Introduction Over the past decade, metallic nanoparticles and nanoantennas have captured the interest of researchers *Corresponding author: C. Couteau, Laboratory of Nanotechnology, Instrumentation and Optics, Charles Delaunay Institute - UMR CNRS 6281, University of Technology of Troyes, CINTRA CNRS-Thales- NTU, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Singapore, christophe.couteau@utt.fr N. Rahbany, W. Geng, S. Blaize, R. Salas-Montiel, R. Bachelot, Laboratory of Nanotechnology, Instrumentation and Optics, Charles Delaunay Institute - UMR CNRS 6281, University of Technology of Troyes in the optical and plasmonic fields. Depending on their geometry and characteristics, their use can lead to dramatic electromagnetic field confinement. Emitters placed in the vicinity of nanoantennas witness a remarkable fluorescence rate and quantum yield enhancement despite plasmonic non-radiative losses in the system [1 3]. Bowtie nanoantennas are known to highly localise incident light in the gap when resonantly excited. However, double bowtie nanoantennas have shown to be more efficient in local field enhancement as compared to single bowties. Due to their symmetrical geometry, they are polarisation independent and can preserve the far-field polarisation state of emitted waves [4 6]. Nevertheless, due to the small size of nanoantennas, a significant amount of the incident light will be lost and not focused entirely on the nanoantenna or will be lost by scattering or reflection. Plasmonic gratings act as sources for launching and orienting surface plasmon-polaritons (SPPs) in a specific desired direction. This has been used in the literature to study the quantum nature of single waveguided SPP modes [7]. Integrating a nanoantenna in the centre of a concentric plasmonic ring grating leads to a highly focused electromagnetic field at its centre as well as the collimated radiation of a dipole emitter placed inside [8 10]. If the coupling strength between the emitter and the cavity overcomes the losses in the system, the strong coupling regime can be attained. Strong coupling between surface plasmons and emitters has been previously studied in the literature [11 13]. In this work, we propose to improve the localisation and intensity of the electromagnetic field using an integrated double bowtie / ring grating structure, which can lead to fluorescent enhancement of emitters placed in its centre [14]. We first study the effect of varying gap size, thickness, side length, type of material, and tip angle of curvature on the electric field confinement of a double bowtie nanoantenna. We then compare the electric field enhancement in the gap of a single bowtie antenna to that of the double bowtie. Finally, we show how our proposed integrated ring-bowtie system increases the local field enhancement in the gap by as much as 77 times, due to a double cavity effect N. Rahbany et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

2 62 N. Rahbany et al. 2 Methods We propose to add a double bowtie nanoantenna in the centre of a ring grating in order to increase the local electric field in its gap. A schematic of single and double bowtie nanoantennas made of equilateral triangles is shown in figure 1-a and 1-b where d is the gap size, s is the bowtie thickness, and L is the bowtie side length. The double bowtie is then integrated inside a ring grating with period a, as shown in figure 1-c. After excitation, enhanced local fields are created in the gap between the arms of a bowtie nanoantenna. In addition, a plasmonic ring grating generates SPPs when illuminated resonantly. These SPPs propagate perpendicularly to the grating grooves, i.e. along the ring diameter, with a well-defined wave vector k SPP and θ 0 according to the conservation of momentum equation: k SPP ω 2π = sinθ 0 ± n c a (1) where ω is the frequency of the incident light wave, θ 0 is the angle of incidence, a is the grating period, n is an integer, and c is the speed of light. Therefore, a plasmonic ring grating structure acts as an efficient SPP source that directs and focuses incident light into its centre. So placing a bowtie nanoantenna inside the ring grating helps to further increase the electric field intensity in the gap due to a double cavity effect arising from both the plasmonic ring and the bowtie nanoantenna combined. We chose to place a double bowtie in the centre because it causes higher electric field confinement compared to a single bowtie (as it will be shown in figure 4-a). We performed FDTD numerical simulations using Lumerical software to study the effect of some parameters on the efficiency of the double bowtie nanoantenna. To benefit from the polarisationindependent nature of our structure, circularly polarised light is used to illuminate a gold double bowtie nanoantenna on a gold substrate, which efficiently excites both the horizontal and vertical components of the structure. The complex permittivity of gold, shown in figure 2, is obtained from experimental data collected by spectroscopic ellipsometry done on a 150 µm gold film on a Si substrate using the Drude model. A monitor was placed at a height equal to half the bowtie thickness (s/2) to calculate the electric field in the gap. To observe the effect of each parameter on the electric field, only a single parameter at a time was varied. To study the effect of our integrated structure on a) b) c) Figure 1: Schematic of a) single bowtie nanoantenna with side length L, thickness s, and gap d b) double bowtie nanoantenna with side length L, thickness s, and gap d c) integrated double bowtiering grating structure with grating period a. the electric field enhancement, we compared the electric field intensity in the gap of a single bowtie nanoantenna, a double bowtie nanoantenna, and finally the integrated double bowtie / ring grating structure.

3 Integrated plasmonic double bowtie / ring grating structure for enhanced electric field confinement 63 a) b) Figure 2: Panels (a) and (b) show the real and imaginary parts, respectively, of the complex permittivity of gold obtained by spectroscopic ellipsometry. 3 Results 3.1 Double Bowtie Nanoantenna For the FDTD simulations of the double bowtie nanoantenna, we used Lumerical software in which circularly polarised light with a wavelength range from 500 nm to 1500 nm is incident on a gold double bowtie with L = 900 nm, s = 30 nm, and d = 30 nm, on a gold substrate. For each study, one of the parameters is varied while keeping the others fixed. The results are shown in Figure 3 where the electric field intensity in the gap is recorded as a function of wavelength for the varying parameters. In figure 3-a, we observe that as the gap decreases, the electric field intensity increases and redshifts to higher wavelengths. The reason for this is that as the gap decreases, opposite charges from the bowtie arms face each other which causes an attractive force that delays their motion [15 17]. This leads to a longer period of resonance, and thus a redshift. In figures 3-b and 3-c it is shown that more electric field confinement occurs for double bowties with a higher thickness and side length. This is a wellknown phenomenon for metallic nanoparticles where the coupling strength is proportionally related to the particle size. However, we notice that the resonance does not depend on the antenna size. In our case, the bowtie on gold cannot be considered as separate nanoparticles and the coupling of plasmons through the metallic substrate underneath the nanoantenna should be considered. The gold substrate provides a suitable channel for the propagation of the evanescent field which strengthens the interaction between the components of the double bowtie nanoantenna and leads to enhanced electromagnetic field confinement in the gap. We also studied the effect of material on the electric field. In this case, we change the material for both the bowtie and the substrate. We show that aluminium has lower enhancement as compared to silver and gold as seen in figure 3-d. This is because aluminium has a lower conductivity which leads to a lower current density in the structure [18]. The radius of curvature of the bowtie tips affects the field enhancement only and does not cause a shift in wavelength (figure 3-e). We notice that for small gaps, the field enhancement in the gap increases as we approach pointed edges, which is in agreement with results in the literature [16]. However, for bigger gaps this effect is not so powerful (figure 3-f) because the change in the tip geometry is negligible with respect to the gap size. 3.2 Integrated Double Bowtie / Ring Grating Structure Upon integrating a ring grating structure with a double bowtie nanoantenna, the electric field in the double bowtie gap is expected to increase more than with the double bowtie nanoantenna alone, which will produce an enhanced fluorescence emission from emitters placed in the centre. We make use of the FDTD simulations presented in figure 3, and choose to study a gold double bowtie with L = 900 nm, s = 30 nm, and d = 30 nm on a gold substrate. The electric field intensity (arbitrary units) in the gap of this structure has a resonant peak at 705 nm, which is 3.2 times larger than what is obtained by a single bowtie nanoantenna on gold (figure 4-a). We then add a ring grating with four concentric rings around the double bowtie nanoantenna. The inner diameter of the smallest ring is μm and the grating period a is 620 nm so that

4 64 N. Rahbany et al. a) b) c) d) e) f) Figure 3: Gold double bowtie on gold substrate: Electric field intensity in the gap as a function of wavelength for varying a) gap size b) thickness c) side length d) material e) tip radius of curvature for 30 nm gap f) tip radius of curvature for 60 nm gap.

5 Integrated plasmonic double bowtie / ring grating structure for enhanced electric field confinement 65 the resonance wavelength is kept the same as the double bowtie (Eq. 1). All other parameters are kept exactly the same in the simulations to allow for direct comparison between the data of figures 4-a and 4-b. The electric field intensity in the gap of a double bowtie coupled to the ring grating structure is obtained and compared to that of a double bowtie nanoantenna alone. The hybrid coupled structure is shown to increase the electric field intensity by a factor of 77, as shown in figure 4-b. The bowtie side length and the ring diameter are chosen so that the two structures are as close to each other as possible (5 nm apart) as shown in Figure 5-b. This is important since the electric field enhancement appears to decrease as the rings move away from the double bowtie. Figure 5-a shows the relationship between the maximum electric field in the gap of the integrated structure as a function of the distance x between the ring and the double bowtie arms. This decrease could be due to increased losses in the system because the two structures start acting as independent plasmonic nanoantennas instead of a single, more efficient, one. 4 Conclusion In this work, we provided a parametric study on the local electric field enhancement of a double bowtie nanoantenna on gold substrate with varying gap size, thickness, side length, material, and tip angle of curvature. We showed that the electric field intensity in the gap of a double bowtie nanoantenna is 3.2 times higher than that of a single bowtie, surpassing what has been obtained in the literature for the magnetic field enhancement [5]. We also demonstrated that integrating a double bowtie a) b) Figure 4: Electric field intensity as a function of wavelength in the centre of a) a single and a double bowtie nanoantenna b) the integrated ring-bowtie structure for x=5 nm. a) b) Figure 5: a) Electric field intensity in the centre of an integrated ring-bowtie structure as a function of the ring-bowtie distance x, upon excitation with a circularly polarized source of wavelength range of 400 nm nm b) Schematic of the integrated structure with diameter D=1.835 µm, period a=620 nm, and distance x=5 nm between the ring and the double bowtie arms.

6 66 N. Rahbany et al. nanoantenna with a ring grating structure enhances the electric field intensity in the gap by a factor of 77 due to a double cavity effect. This enhancement also exceeds what has been obtained in the literature to date [9,10]. Further work can be done concerning placing emitters in the gap of the double bowtie nanoantenna of our proposed integrated structure. The dramatic increase in the electric field is expected to enhance the emitter s quantum yield which might lead to the strong coupling regime between the emitter and the ring-bowtie nanocavity. Acknowledgments: N. R. would like to thank the French Ministry of Education for her PhD grant. C. C., S. B. and R. B. would like to acknowledge the financial support by the CNRS PEPS project InteQ. C. C. thanks the partial funding by the Champagne-Ardenne region via the project NanoGain. The authors would like to thank Aurélien Bruyant and Gérard Colas des Francs for helpful discussions. References [1] Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W. E. Nat. Photonics 2009, 3 (11), [2] Farahani, J. N.; Pohl, D. W.; Eisler, H.-J.; Hecht, B. Phys. Rev. Lett. 2005, 95 (1), [3] Aouani, H.; Rahmani, M.; Navarro-Cía, M.; Maier, S. A. Nat. Nanotechnol. 2014, 9 (4), [4] Biagioni, P.; Huang, J.; Duò, L.; Finazzi, M.; Hecht, B. Phys. Rev. Lett. 2009, 102 (25), [5] Gao, Z.; Shen, L.; Li, E.; Xu, L.; Wang, Z. J. Light. Technol. 2012, 30 (6), [6] Kumar V., D.; Bhardwaj, A.; Mishra, D. Micro Nano Lett. 2011, 6 (2), 94. [7] Di Martino, G.; Sonnefraud, Y.; Kéna-Cohen, S.; Tame, M.; Özdemir, Ş. K.; Kim, M. S.; Maier, S. A. Nano Lett. 2012, 12 (5), [8] Steele, J. M.; Liu, Z.; Wang, Y.; Zhang, X. Opt. Express 2006, 14 (12), [9] Wang, D.; Yang, T.; Crozier, K. B. Opt. Express 2011, 19 (3), [10] Kinzel, E. C.; Srisungsitthisunti, P.; Li, Y.; Raman, A.; Xu, X. Appl. Phys. Lett. 2010, 96 (21), [11] Chang, D.; Sørensen, A.; Hemmer, P.; Lukin, M. Phys. Rev. B 2007, 76 (3), [12] Törmä, P.; Barnes, W. L. ArXiv Prepr. ArXiv [13] Bellessa, J.; Bonnand, C.; Plenet, J.; Mugnier, J. Phys. Rev. Lett. 2004, 93 (3). [14] Stokes, J. L.; Yu, Y.; Yuan, Z. H.; Pugh, J. R.; Lopez-Garcia, M.; Ahmad, N.; Cryan, M. J. J. Opt. Soc. Am. B 2014, 31 (2), 302. [15] Hatab, N. A.; Hsueh, C.-H.; Gaddis, A. L.; Retterer, S. T.; Li, J.-H.; Eres, G.; Zhang, Z.; Gu, B. Nano Lett. 2010, 10 (12), [16] Jiunn-Woei Liaw. IEEE J. Sel. Top. Quantum Electron. 2008, 14 (6), [17] Dodson, S.; Haggui, M.; Bachelot, R.; Plain, J.; Li, S.; Xiong, Q. J. Phys. Chem. Lett. 2013, 4 (3), [18] Grosjean, T.; Mivelle, M.; Baida, F. I.; Burr, G. W.; Fischer, U. C. Nano Lett. 2011, 11 (3),