December 15, 2014 / Vol. 39, No. 24 / OPTICS LETTERS
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Plasmonic spiderweb nanoantenna surface for broadband hotspot generation Rüştü Umut Tok1,2 and Kürşat Şendur1,* 1
2
Faculty of Engineering and Natural Sciences, Sabanci University, Orhanli-Tuzla,34956, Istanbul, Turkey Inowatt Plasmonics and Renewable Energy Technologies, GOSB Teknopark, Gebze, 41480, Kocaeli, Turkey *Corresponding author: [email protected] Received September 19, 2014; revised November 10, 2014; accepted November 16, 2014; posted November 18, 2014 (Doc. ID 223442); published December 15, 2014
In this study, we demonstrate a general framework for obtaining a plasmonic nanoantenna surface with a broadband polarization-independent response. The plasmonic spiderweb nanoantenna surface is composed of unit cells, which form multiple resonance paths due to patterning of the metallic conductor such that electrons can find multiple ways to oscillate between the poles of the conductor. The tailoring of the conductor paths and shapes of the unit cells’ patterns results in a broadband spectral response. At various resonance frequencies, the electrons oscillate along different paths between the poles of the antenna, generating broadband hot spots around those poles. © 2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (250.5403) Plasmonics; (350.6050) Solar energy; (160.3918) Metamaterials. http://dx.doi.org/10.1364/OL.39.006977
It is well-known that metallic particles [1] and nanoantennas [2] support narrowband surface plasmon resonances. However, in many emerging practical applications, such as thin-film solar cells [3,4], single molecule spectroscopy [5,6], single molecule detection [5], nonlinear optics [7,8], near-field imaging [9], pulse shaping [10], optical information processing [11], and light bending [12], broadband plasmonic structures are essential. There have been many attempts to fill this gap, and some broadband plasmonic structures have been demonstrated in the form of single antenna [8,13], plasmonic surfaces [14,15], multilayer structures [4], metamaterials [16–18], metallodielectric composites [17], and particle suspensions [19]. The studies to obtain broadband plasmonic structures in the literature can be classified as: (i) bringing particles with different resonances into a single unit cell [13–15]; (ii) using transformation optics [8]; (iii) using an effective medium approach [17]; and (iv) designing structures that support coupling of multiple modes [20–23]. In many applications such as high harmonic generation [8] surface-enhanced spectroscopy techniques [24], and spontaneous two photon emission [7], a broadband hotspot is crucial. There are some structures able to produce broadband hotspots [8,13–15,21], however, these structures do not produce a polarization-independent broadband hotspot in a close-packed geometric form. A close-packed design can maximize the signal by maximizing the number of hotspots per unit area, which in turn increases the sensitivity of the surface [15]. In this work, we demonstrate a general framework for obtaining a plasmonic nanoantenna surface with a broadband polarization-independent response. To illustrate the idea of a multiresonance plasmonic surface with alternative resonance paths, a close-packed plasmonic spiderweb nanoantenna surface is formed. By using the proposed antenna array surface, we demonstrated ultra-broadband hot spot generation at the gap regions with field enhancement greater than one order of magnitude compared to the incident field amplitude. In this study, the physical mechanism behind the broadband characteristic and field localization is explained. 0146-9592/14/246977-04$15.00/0
To obtain broadband field enhancement, the plasmonic spiderweb nanoantenna surface shown in Fig. 1 is used. The plasmonic spiderweb nanoantenna surface is composed of unit cells, which form multiple resonance paths. Figure 1(a) illustrates the plasmonic spiderweb nanoantenna surface composed of unit cells, and Fig. 1(b) represents the unit cells of four different multiresonance path antenna array designs with one (black), two (red), three (blue) and four (green) resonance paths. The antenna material is chosen as gold for its plasmonic properties. Gold nanoantennas are placed on top of a BK7 glass, and there is a chromium adhesion layer E
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Fig. 1. (a) Top view of plasmonic spiderweb nanoantenna surface composed of unit cells and (b) top view of the unit cell of one (black), two (red), three (blue), and four (green)-resonance path antenna arrays. (c) Unit cell of one-resonance path antenna array with its corresponding geometrical parameters. tg and tc are the thickness of the gold and chromium layer, respectively. L is the length of the anchor part of the antenna in the radial-direction. L1 is the length of the arms in the azimuthal-direction between anchor parts, which is illustrated for a one-resonance loop antenna. w1 and w2 are the widths of the anchor part and square loops, respectively. g is the gap diameter between adjacent antennas. © 2014 Optical Society of America
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between the gold antenna and BK7 glass only at the regions where the gold antenna is present, as shown in Fig. 1(c). Figure 1(c) illustrates the unit cell of a one-resonance path antenna array with its associated geometrical parameters as an example. The oneresonance path antenna is formed by a single square loop around the center of the unit cell. The thickness of the gold layer, tg , and the thickness of the chromium layer, tc , are 10 nm and 3 nm, respectively. The width of the anchor part, w1 , is 10 nm, and the width of the square loops, w2 , is 10 nm for all antenna designs. The length, L, of the anchor part is 320 nm for all antenna designs, and the edge length of the square loops measured from the center of the loop is denoted by Li . For the oneresonance path antenna, shown in Fig. 1(b), L1 is 110 nm. For the two-resonance path antenna L1 ; L2 100; 170 nm, for the three-resonance path antenna L1 ; L2 ;L3 80;140;200 nm, and for the four-resonance path antenna L1 ; L2 ; L3 ; L4 80; 120; 160; 200 nm. The edges of the antennas are rounded, which decreases the gap diameter between the adjacent antennas and localizes the hotspot toward the center of the gap. The gap diameter, g, between the antennas is selected as 20 nm. Material parameters of gold [25], chromium [26], and BK7 [27] glass are taken from experimental data. To illustrate the broadband characteristic of the multiresonance path antenna designs, HFSS, a 3D finiteelement method based on full-wave solution of Maxwell’s equations is used. Tetrahedral mesh elements are used to discretize the computational domain. On the tetrahedral elements, edge basis functions and second-order interpolation functions are used to expand the field distributions. An adaptive mesh refinement algorithm is employed to improve coarse solution regions with high-field intensities and large-field gradients. Periodic boundary conditions are employed on the lateral surfaces, and radiation boundary conditions are employed at the bottom and top surfaces of the unit cell, respectively. Figure 2(a) shows the absorptance (lower curves), reflectance (middle curves), and transmittance (upper curves) of one (black curves), two (red curves), three (blue curves), and four-resonance (green curves) path antenna array surfaces when they are illuminated with linearly
polarized plane waves incident perpendicularly onto the antenna. The results are the same regardless of the polarization orientation of the linear polarization on the antenna surface. Since the unit cell of the antenna array has four-fold symmetry, it is polarization independent. As can be inferred from Fig. 2(a), the number of peaks in the spectrums are equal to the number of square loops of the multiresonance path antenna designs. As the number of square loops in the azimuthal direction increases, the peaks in the spectral response merge and prevent the spectral response from decreasing below a certain level thus forming a broadband spectral response. Electric field enhancement at the center of the gap, E Gap Center ∕E i , and maximum electric field enhancement at the gap region, E max ∕E i , in the spectrum of interest are given in Figs. 2(b) and 2(c), respectively, for different multiresonance path antenna designs with the same line colors as given in Fig. 2(a). As can be observed, spectral responses in Fig. 2(a) and field enhancements in Figs. 2(b) and 2(c) are correlated. Furthermore, maximum field enhancement or field enhancement at the gap center does not decrease dramatically after the resonance at the highest wavelength. Figure 3 shows the electric field distribution of one, two, and three-resonance path antenna arrays at their mid-planes for the resonance wavelengths when the arrays are perpendicularly illuminated with polarization parallel to one of the anchor directions, as shown in Fig. 1(c). Figure 3(a) illustrates the field enhancement at the gap region for a one-resonance path antenna at its resonance wavelength of 2750 nm. In Figs. 3(b) and 2(c) electric field distributions for the two-resonance path antenna are shown for the resonance wavelengths of 1600 and 2750 nm, respectively. Similarly the field enhancements are greatest at the gap region. Finally, Figs. 3(d), 3(e), and 3(f) demonstrate the field enhancement at the gap region of a three-resonance path antenna for the resonance wavelengths of 1200, 1950, and 3200 nm. This localized field enhancement is also valid for a four-resonance path antenna array as shown in 2750nm
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Fig. 2. (a) Absorptance, reflectance, and transmittance of one (black), two (red), three (blue), and four-resonance (green) loop antenna arrays. (b) Field enhancement at the center of gap regions of one (black), two (red), three (blue), and fourresonance (green) loop antenna arrays. (c) Maximum field enhancement at the gap region for one (black), two (red), three (blue), and four-resonance (green) loop antenna arrays.
Fig. 3. Electric field distribution at the mid-plane of gold nanoantenna arrays for different multiresonance path antenna arrays at resonance wavelengths. (a) Electric field distribution for one-resonance path antenna array at λ 2750 nm. (b) and (c) Electric field distributions for two-resonance path antenna array at λ 1600 nm and λ 2750 nm, respectively. (d)–(f) Electric field distributions for three-resonance path antenna array at λ 1200 nm, λ 1950 nm, and λ 3200 nm, respectively.
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Fig. 4. Furthermore, this localized field enhancement does not occur just at resonance wavelengths. Due to the lightening-rod effect, at each wavelength, field enhancement is largest at the gap region as illustrated in Fig. 5. Therefore, besides the broad spectral response, the main advantage of the multiresonance path antenna array designs is broadband hotspot generation. To illustrate the physical mechanism of the broadband characteristic of the antenna array, scattered magnetic field distribution just above the array surfaces are plotted in Fig. 6. Since scattered magnetic fields originate from induced currents on the antennas, they can give insight into the physical mechanism of the resonances. Figure 6(a) shows the scattered magnetic field distribution for the one-resonance path antenna array at the resonance frequency. As can be inferred from the scattered magnetic field distribution, electrons oscillate both on the anchor part and square loop part of the antenna for the one-resonance path antenna array. In Figs. 6(b) and 6(c), scattered magnetic field distributions of tworesonance path antenna for resonance wavelengths are given. For the first resonance wavelength at 1600 nm, electrons oscillate on the first square loop, whereas for the second resonance wavelength at 2750 nm, electrons prefer the second square loop. A similar phenomenon can be observed for the three-resonance path antenna array. Electrons oscillate on the first, second, V/m
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and third square loops at the resonances wavelengths of 1200, 1950, and 3200 nm, respectively, as shown in Figs. 6(d), 6(e), and 7(f). This is also valid for the four-resonance path antenna array as illustrated in Fig. 7. From these observations it can be said that, on a conductor, electrons prefer the suitable path to oscillate in order for resonance to occur. Thus, by forming a multipath conductor, it is possible to obtain a multiresonance and broadband antenna structure. Furthermore, if the ends of different paths converge to a single point, broadband hotspot generation can be achieved. In conclusion, broadband hotspot generation is achieved using plasmonic spiderweb nanoantenna surfaces. When a conductor is patterned such that electrons can find multiple paths to oscillate between the two poles of the conductor, broadband hotspots are achieved at the poles. This principle was used in the plasmonic spiderweb nanoantenna surfaces to obtain broadband plasmonic surfaces through multiresonance paths formed over the antenna array surface. This work is supported by TUBITAK under project number 113F171.
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References 1. N. J. Halas, S. Lal, W.-S. Chang, S. Link, and P. Nordlander, Chem. Rev. 111, 3913 (2011). 2. P. Bharadwaj, B. Deutsch, and L. Novotny, Adv. Opt. Photon. 1, 438 (2009). 3. H. A. Atwater and A. Polman, Nat. Mater. 9, 205 (2010). 4. R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, Adv. Mater. 21, 3504 (2009). 5. K. A. Willets and P. Richard Van Duyne, Annu. Rev. Phys. Chem. 58, 267 (2007). 6. S. Ayas, H. Güner, B. Türker, O. O. Ekiz, F. Dirisaglik, A. K. Okyay, and A. Dâna, ACS Nano 6, 6852 (2012). 7. A. Nevet, N. Berkovitch, A. Hayat, P. Ginzburg, S. Ginzach, O. Sorias, and M. Orenstein, Nano Lett. 10, 1848 (2010). 8. M. Navarro-Cia and S. A. Maier, ACS Nano 6, 3537 (2012). 9. A. Hartschuh, E. J. Sánchez, X. S. Xie, and L. Novotny, Phys. Rev. Lett. 90, 095503 (2003). 10. R. U. Tok and K. Şendur, Phys. Rev. A 84, 033847 (2011). 11. I. L. Chuang and Y. Yamamoto, Phys. Rev. A 52, 3489 (1995). 12. X. Ni, N. K. Emani, A. V. Kildishev, A. Boltasseva, and V. M. Shalaev, Science 335, 427 (2012). 13. E. S. Ünlü, R. U. Tok, and K. Şendur, Opt. Express 19, 1000 (2011). 14. R. U. Tok, C. Ow-Yang, and K. Şendur, Opt. Express 19, 22731 (2011).
15. R. U. Tok and K. Şendur, J. Quant. Spectrosc. Radiat. Transfer 120, 70 (2013). 16. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, Nat. Commun. 2, 517 (2011). 17. A. V. Goncharenko, V. U. Nazarov, and K.-R. Chen, Opt. Mater. Express 3, 143 (2013). 18. J. A. Bossard, L. Lin, S. Yun, L. Liu, D. H. Werner, and T. S. Mayer, ACS Nano 8, 1517 (2014). 19. L. Shao, A. S. Susha, L. S. Cheung, T. K. Sau, A. L. Rogach, and J. Wang, Langmuir 28, 8979 (2012). 20. J. Valentine, S. Zhang, T. Zentgraf, and X. Zhang, Proc. IEEE 99, 1682 (2011). 21. S. V. Boriskina and L. D. Negro, Opt. Lett. 35, 538 (2010). 22. R. S. Penciu, K. Aydin, M. Kafesaki, T. Koschny, E. Ozbay, E. N. Economou, and C. M. Soukoulis, Opt. Express 16, 18131 (2008). 23. A. Artar, A. A. Yanik, and H. Altug, Nano Lett. 11, 1685 (2011). 24. F. Le, D. W. Brandl, Y. A. Urzhumov, H. Wang, J. Kundu, N. J. Halas, J. Aizpurua, and P. Nordlander, ACS Nano 2, 707 (2008). 25. E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1998). 26. A. D. Rakić, A. B. Djurišić, J. M. Elazar, and M. L. Majewski, Appl. Opt. 37, 5271 (1998). 27. https://www.escooptics.com/material‑data/bk7‑optical‑ glass.html.
6977
Plasmonic spiderweb nanoantenna surface for broadband hotspot generation Rüştü Umut Tok1,2 and Kürşat Şendur1,* 1
2
Faculty of Engineering and Natural Sciences, Sabanci University, Orhanli-Tuzla,34956, Istanbul, Turkey Inowatt Plasmonics and Renewable Energy Technologies, GOSB Teknopark, Gebze, 41480, Kocaeli, Turkey *Corresponding author: [email protected] Received September 19, 2014; revised November 10, 2014; accepted November 16, 2014; posted November 18, 2014 (Doc. ID 223442); published December 15, 2014
In this study, we demonstrate a general framework for obtaining a plasmonic nanoantenna surface with a broadband polarization-independent response. The plasmonic spiderweb nanoantenna surface is composed of unit cells, which form multiple resonance paths due to patterning of the metallic conductor such that electrons can find multiple ways to oscillate between the poles of the conductor. The tailoring of the conductor paths and shapes of the unit cells’ patterns results in a broadband spectral response. At various resonance frequencies, the electrons oscillate along different paths between the poles of the antenna, generating broadband hot spots around those poles. © 2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (250.5403) Plasmonics; (350.6050) Solar energy; (160.3918) Metamaterials. http://dx.doi.org/10.1364/OL.39.006977
It is well-known that metallic particles [1] and nanoantennas [2] support narrowband surface plasmon resonances. However, in many emerging practical applications, such as thin-film solar cells [3,4], single molecule spectroscopy [5,6], single molecule detection [5], nonlinear optics [7,8], near-field imaging [9], pulse shaping [10], optical information processing [11], and light bending [12], broadband plasmonic structures are essential. There have been many attempts to fill this gap, and some broadband plasmonic structures have been demonstrated in the form of single antenna [8,13], plasmonic surfaces [14,15], multilayer structures [4], metamaterials [16–18], metallodielectric composites [17], and particle suspensions [19]. The studies to obtain broadband plasmonic structures in the literature can be classified as: (i) bringing particles with different resonances into a single unit cell [13–15]; (ii) using transformation optics [8]; (iii) using an effective medium approach [17]; and (iv) designing structures that support coupling of multiple modes [20–23]. In many applications such as high harmonic generation [8] surface-enhanced spectroscopy techniques [24], and spontaneous two photon emission [7], a broadband hotspot is crucial. There are some structures able to produce broadband hotspots [8,13–15,21], however, these structures do not produce a polarization-independent broadband hotspot in a close-packed geometric form. A close-packed design can maximize the signal by maximizing the number of hotspots per unit area, which in turn increases the sensitivity of the surface [15]. In this work, we demonstrate a general framework for obtaining a plasmonic nanoantenna surface with a broadband polarization-independent response. To illustrate the idea of a multiresonance plasmonic surface with alternative resonance paths, a close-packed plasmonic spiderweb nanoantenna surface is formed. By using the proposed antenna array surface, we demonstrated ultra-broadband hot spot generation at the gap regions with field enhancement greater than one order of magnitude compared to the incident field amplitude. In this study, the physical mechanism behind the broadband characteristic and field localization is explained. 0146-9592/14/246977-04$15.00/0
To obtain broadband field enhancement, the plasmonic spiderweb nanoantenna surface shown in Fig. 1 is used. The plasmonic spiderweb nanoantenna surface is composed of unit cells, which form multiple resonance paths. Figure 1(a) illustrates the plasmonic spiderweb nanoantenna surface composed of unit cells, and Fig. 1(b) represents the unit cells of four different multiresonance path antenna array designs with one (black), two (red), three (blue) and four (green) resonance paths. The antenna material is chosen as gold for its plasmonic properties. Gold nanoantennas are placed on top of a BK7 glass, and there is a chromium adhesion layer E
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Fig. 1. (a) Top view of plasmonic spiderweb nanoantenna surface composed of unit cells and (b) top view of the unit cell of one (black), two (red), three (blue), and four (green)-resonance path antenna arrays. (c) Unit cell of one-resonance path antenna array with its corresponding geometrical parameters. tg and tc are the thickness of the gold and chromium layer, respectively. L is the length of the anchor part of the antenna in the radial-direction. L1 is the length of the arms in the azimuthal-direction between anchor parts, which is illustrated for a one-resonance loop antenna. w1 and w2 are the widths of the anchor part and square loops, respectively. g is the gap diameter between adjacent antennas. © 2014 Optical Society of America
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between the gold antenna and BK7 glass only at the regions where the gold antenna is present, as shown in Fig. 1(c). Figure 1(c) illustrates the unit cell of a one-resonance path antenna array with its associated geometrical parameters as an example. The oneresonance path antenna is formed by a single square loop around the center of the unit cell. The thickness of the gold layer, tg , and the thickness of the chromium layer, tc , are 10 nm and 3 nm, respectively. The width of the anchor part, w1 , is 10 nm, and the width of the square loops, w2 , is 10 nm for all antenna designs. The length, L, of the anchor part is 320 nm for all antenna designs, and the edge length of the square loops measured from the center of the loop is denoted by Li . For the oneresonance path antenna, shown in Fig. 1(b), L1 is 110 nm. For the two-resonance path antenna L1 ; L2 100; 170 nm, for the three-resonance path antenna L1 ; L2 ;L3 80;140;200 nm, and for the four-resonance path antenna L1 ; L2 ; L3 ; L4 80; 120; 160; 200 nm. The edges of the antennas are rounded, which decreases the gap diameter between the adjacent antennas and localizes the hotspot toward the center of the gap. The gap diameter, g, between the antennas is selected as 20 nm. Material parameters of gold [25], chromium [26], and BK7 [27] glass are taken from experimental data. To illustrate the broadband characteristic of the multiresonance path antenna designs, HFSS, a 3D finiteelement method based on full-wave solution of Maxwell’s equations is used. Tetrahedral mesh elements are used to discretize the computational domain. On the tetrahedral elements, edge basis functions and second-order interpolation functions are used to expand the field distributions. An adaptive mesh refinement algorithm is employed to improve coarse solution regions with high-field intensities and large-field gradients. Periodic boundary conditions are employed on the lateral surfaces, and radiation boundary conditions are employed at the bottom and top surfaces of the unit cell, respectively. Figure 2(a) shows the absorptance (lower curves), reflectance (middle curves), and transmittance (upper curves) of one (black curves), two (red curves), three (blue curves), and four-resonance (green curves) path antenna array surfaces when they are illuminated with linearly
polarized plane waves incident perpendicularly onto the antenna. The results are the same regardless of the polarization orientation of the linear polarization on the antenna surface. Since the unit cell of the antenna array has four-fold symmetry, it is polarization independent. As can be inferred from Fig. 2(a), the number of peaks in the spectrums are equal to the number of square loops of the multiresonance path antenna designs. As the number of square loops in the azimuthal direction increases, the peaks in the spectral response merge and prevent the spectral response from decreasing below a certain level thus forming a broadband spectral response. Electric field enhancement at the center of the gap, E Gap Center ∕E i , and maximum electric field enhancement at the gap region, E max ∕E i , in the spectrum of interest are given in Figs. 2(b) and 2(c), respectively, for different multiresonance path antenna designs with the same line colors as given in Fig. 2(a). As can be observed, spectral responses in Fig. 2(a) and field enhancements in Figs. 2(b) and 2(c) are correlated. Furthermore, maximum field enhancement or field enhancement at the gap center does not decrease dramatically after the resonance at the highest wavelength. Figure 3 shows the electric field distribution of one, two, and three-resonance path antenna arrays at their mid-planes for the resonance wavelengths when the arrays are perpendicularly illuminated with polarization parallel to one of the anchor directions, as shown in Fig. 1(c). Figure 3(a) illustrates the field enhancement at the gap region for a one-resonance path antenna at its resonance wavelength of 2750 nm. In Figs. 3(b) and 2(c) electric field distributions for the two-resonance path antenna are shown for the resonance wavelengths of 1600 and 2750 nm, respectively. Similarly the field enhancements are greatest at the gap region. Finally, Figs. 3(d), 3(e), and 3(f) demonstrate the field enhancement at the gap region of a three-resonance path antenna for the resonance wavelengths of 1200, 1950, and 3200 nm. This localized field enhancement is also valid for a four-resonance path antenna array as shown in 2750nm
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Fig. 3. Electric field distribution at the mid-plane of gold nanoantenna arrays for different multiresonance path antenna arrays at resonance wavelengths. (a) Electric field distribution for one-resonance path antenna array at λ 2750 nm. (b) and (c) Electric field distributions for two-resonance path antenna array at λ 1600 nm and λ 2750 nm, respectively. (d)–(f) Electric field distributions for three-resonance path antenna array at λ 1200 nm, λ 1950 nm, and λ 3200 nm, respectively.
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10 5
c
4
5
Fig. 4. Furthermore, this localized field enhancement does not occur just at resonance wavelengths. Due to the lightening-rod effect, at each wavelength, field enhancement is largest at the gap region as illustrated in Fig. 5. Therefore, besides the broad spectral response, the main advantage of the multiresonance path antenna array designs is broadband hotspot generation. To illustrate the physical mechanism of the broadband characteristic of the antenna array, scattered magnetic field distribution just above the array surfaces are plotted in Fig. 6. Since scattered magnetic fields originate from induced currents on the antennas, they can give insight into the physical mechanism of the resonances. Figure 6(a) shows the scattered magnetic field distribution for the one-resonance path antenna array at the resonance frequency. As can be inferred from the scattered magnetic field distribution, electrons oscillate both on the anchor part and square loop part of the antenna for the one-resonance path antenna array. In Figs. 6(b) and 6(c), scattered magnetic field distributions of tworesonance path antenna for resonance wavelengths are given. For the first resonance wavelength at 1600 nm, electrons oscillate on the first square loop, whereas for the second resonance wavelength at 2750 nm, electrons prefer the second square loop. A similar phenomenon can be observed for the three-resonance path antenna array. Electrons oscillate on the first, second, V/m
1500nm
V/m
2500nm
12
12
10
10
8
8 2
6
6 4
4
1
2
2
a
b 2800nm
c 3600nm
4000nm 30
14 12
25
10
20
8
30 25 20
15
15
10
10
6 4
5
2
d
V/m
14 4 3
e
3
d
d
Fig. 4. Electric field distribution at the mid-plane of four-resonance path nanoantenna array for different resonance wavelengths (a) 1150 nm, (b) 1750 nm, (c) 2400 nm, and (d) 3200 nm.
800nm
8
5
f
Fig. 5. Electric field distribution at the mid-plane of threeresonance path nanoantenna array for different off-resonance wavelengths (a) 800 nm, (b) 1500 nm, (c) 2500 nm, (d) 2800 nm, (e) 3600 nm, and (f) 4000 nm.
6 4
e
4
f
Fig. 6. Magnetic field distributions just above the gold nanoantenna arrays for different multiresonance path antenna arrays at resonance wavelengths. (a) Magnetic field distribution for one-resonance path antenna array at λ 2750 nm. (b) and (c) Magnetic field distributions for two-resonance path antenna array at λ 1600 nm and λ 2750 nm, respectively. (d)–(f) Magnetic field distributions for three-resonance path antenna array at λ 1200 nm, λ 1950 nm, and λ 3200 nm, respectively.
1150nm
A/m
−3
x 10 8
1750nm
−3
x 10 10 9
7
8
6
7 6
5
5
4
4
a
b 2400nm
A/m
−3
3
3200nm
x 10 12
−3
x 10 12
10
10
8
8 6
6
4
c
d
4
Fig. 7. Magnetic field distributions just above the four-resonance path nanoantenna array for different resonance wavelengths (a) 1150 nm, (b) 1750 nm, (c) 2400 nm, and (d) 3200 nm.
and third square loops at the resonances wavelengths of 1200, 1950, and 3200 nm, respectively, as shown in Figs. 6(d), 6(e), and 7(f). This is also valid for the four-resonance path antenna array as illustrated in Fig. 7. From these observations it can be said that, on a conductor, electrons prefer the suitable path to oscillate in order for resonance to occur. Thus, by forming a multipath conductor, it is possible to obtain a multiresonance and broadband antenna structure. Furthermore, if the ends of different paths converge to a single point, broadband hotspot generation can be achieved. In conclusion, broadband hotspot generation is achieved using plasmonic spiderweb nanoantenna surfaces. When a conductor is patterned such that electrons can find multiple paths to oscillate between the two poles of the conductor, broadband hotspots are achieved at the poles. This principle was used in the plasmonic spiderweb nanoantenna surfaces to obtain broadband plasmonic surfaces through multiresonance paths formed over the antenna array surface. This work is supported by TUBITAK under project number 113F171.
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