Plasmonic Coupling of Bow Tie Antennas with Ag Nanowire

LETTER pubs.acs.org/NanoLett

Plasmonic Coupling of Bow Tie Antennas with Ag Nanowire Zheyu Fang,† Linran Fan,† Chenfang Lin,† Dai Zhang,§ Alfred J. Meixner,§ and Xing Zhu*,†,‡ †

School of Physics, State Key Laboratory for Mesoscopic Physics, Peking University, Beijing 100871, China National Center for Nanoscience and Technology, Beijing 100190, China § Institute of Physics & Theoretical Chemistry, University T€ubingen, T€ubingen D-72076, Germany ‡

bS Supporting Information ABSTRACT: Ag nanowire with the receiving and transmitting Ag bow tie antenna pairs at its incident and emission ends was patterned on the SiO2 substrate to realize an enhanced surface plasmon emission with a factor of 45 compared to the single Ag nanowire without antenna pairs. The receiving and transmitting bow tie antenna pairs enhanced the plasmon coupling and emission efficiencies of the Ag nanowire. And the maximum plasmon emission sensitively depended on the length of Ag nanowire, the arm length of bow tie antennas, and the incident angle of optical excitation. This enhanced plasmon emission was confirmed by finite-difference time-domain simulations and explored with analytical calculations using the impedance matching theory at optical frequency. KEYWORDS: Surface plasmon, impedance matching, bow tie antennas, Ag nanowire, scanning near-field optical microscopy, finite-difference time domain

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oupling of light wave and a nanoscale optical component, such as plasmonic waveguiding, is a topic of intense theoretical and experimental interest.1-3 Metallic nanowires,4-7 dielectric-loaded plasmonic structures,8-10 and grooves etched in a metal film11-13 were explored as subwavelength waveguides for light confinement and coupling. Waveguide bends, splitters, and junctions were used to extend plasmonic waveguides to a plasmon coupling network.12,14,15 Nanoscale confinement and guiding of light were also realized by integrating multiple Ag nanowires with polymer optical waveguides.16 However, efficient plasmon propagation and enhanced emission cannot be achieved by only control of the size, shape, or network of plasmon waveguides; it is also important to pursue the mixed plasmon states in a complex configuration to realize an enhanced plasmon hybridization, such as the optical nanocircuit responsible for the coupling between propagating plasmons supported by metallic nanowires and the discrete plasmons at adjacent metallic nanostructures like the isolated dipole antennas.17-19 Ag nanoparticles were reported to be efficient antennas for coupling of visible light into propagating plasmons of an Ag nanowire.3,6,20 The intensity of the plasmon emission from the nanowire terminus was demonstrated dependent on the incident polarization and also strongly with the nanowire geometry and the position of the vicinal nanoparticles. However, in practical applications, the control of the size, shape, and position of the nanoparticles related to the nanowire represents a significant challenge since these nanoparticles can only be obtained as random defects during the nanowire synthesis. On the other r 2011 American Chemical Society

hand, the underlyng mechanism for this antenna-induced enhanced plasmon emission is still unclear.6,20 There is therefore an apparent demand to develop an efficient method that can enable precise control of optical antennas to build an antenna-wire nanocircuit which can be investigated as an original plasmonic building block to realize a maximum plasmon emission. In this Letter, we transferred the Ag nanowire to the feed gaps of prefabricated receiving and transmitting bow tie antenna pairs to investigate the enhanced plasmon emission. The influences of the length of nanowire, the arm length of bow tie antennas, and the incident angle of excitation laser on the maximum plasmon emission were systematically characterized using scanning nearfield optical microscopy (SNOM). Finite-difference timedomain (FDTD) simulations and the principle of the classical transmission line theory, the impedance matching, were carried out for a deeper understanding of the photophysics mechanism of our experimental observations. Crystalline Ag nanowires with a diameter around 100 nm and lengths ranging from 5 to 20 μm were synthesized using roomtemperature chemical fabrication.7,16,21 Ag bow tie antennas, with an 53
apex angle, 100 nm height, and various arm lengths from 50 to 800 nm (a single arm), were fabricated using electronbeam lithography (Nava 200, Nanolab, FEI Co.) and lift-off techniques and were patterned on a 100 nm thickness SiO2 Received: January 17, 2011 Revised: February 10, 2011 Published: February 23, 2011 1676

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Figure 1. Procedure of the PMMA-mediated transfer printing technique22 to move an Ag nanowire to the feed gaps of receiving and transmitting bow tie antenna pairs.

substrate. Using the ploy(methyl methacrylate)-mediated (PMMA) nanotransfer method,22 an individual Ag nanowire was successfully transferred to the feed gap of the bow tie antennas as illustrated in Figure 1: (i) The Ag nanowire with desired geometry on a source substrate was loaded onto the PMMA mediator surface. (ii) The mediator was driven to contact with the target substrate at the required location with marks and an aligning system (XYZ stage). (iii) The PMMA film was dissolved and to release the Ag nanowire onto the target surface. (iv) A fine adjusting of the Ag wire position with nanometer accuracy was performed using the scanning probe. Figure 2a is the scanning electron microscopy (SEM) image of a part of a single Ag nanowire with one end placed at the feed gap of the receiving antenna pair. The whole length of the nanowire is around 20 μm. The incident end of the Ag nanowire is nearly touched with tip end of the bowtie antennas. Taking account of the plasmon damping, this nanowire can be considered as an infinitely long waveguide, and the amplitude of the reflected plasmon wave from the terminal facet of nanowire can be negligible. (See Supporting Information.) The receiving antenna pair was illuminated with a focused beam (spot size = 1 μm, λ = 672 nm) with its polarization parallel to the antenna pair. To avoid the artifacts induced by the incident laser, the plasmon field enhancement was compared at two positions with distances of 1 and 3 μm down the Ag nanowire (points P and Q). The experimental near-field intensity corresponding to various receiving antenna arm lengths is plotted in Figure 2b. For the case of point Q, the antenna-wire circuit showed a lower intensity enhancement but exhibited a resonance at about the same length of 250 nm as point P and the simulated resonance of an isolated bow tie antenna pair (black dash-dot curve). It implied that the antenna resonance and the coupling plasmon intensity could be optimized for the same receiving antenna arm length of 250 nm, which was kept constant in our further experiments. Compared to the same position P of a single Ag nanowire without receiving antenna pair, the plasmon intensity improves by a factor of FC = 13.6 in this infinitely long antenna-wire configuration. (See Supporting Information.) Figure 3a illustrates a 10 μm long Ag nanowire being accurately placed into the feed gaps of both receiving and

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Figure 2. (a) SEM image of the top part of the 20 μm long Ag nanowire with one end placed at the feed gap of the receiving antenna pair. (b) Near-field intensity as a function of the arm length of the receiving antennas for an isolated bow tie antenna pair (calculated, black dash-dot line) and the equivalent positions P (experimental, red circles) and Q (experimental, blue triangles) with distances of 1 and 3 μm down the bow tie, respectively. The maximum intensity is observed at the arm length = 250 nm.

transmitting bow tie antenna pairs. The distance between the nanowire ends and the tip of the bow tie antennas is less than 10 nm. Such geometry allowed us to examine the antenna-wire coupling, the plasmon propagation, and the SPP emission mechanisms using SNOM23 in one integrated setup. The length of Ag nanowire, the arm length of the transmitting antenna, and the angle of laser excitation were systematically varied in order to investigate their influences on the plasmon emission maximum. As the first step, Ag nanowires with different lengths were illuminated by a halogen lamp, and the optical spectra were acquired by the CCD camera setup combined with a monochromator (iHR550, Jobin Yvon Co.). The curve in Figure 3b represents the dependence of the measured resonant wavelengths on different nanowire lengths. It can be seen that, for the incident laser of 672 nm, the matching length of the Ag nanowire is around 10 μm. Thus, in further investigations, the 10 μm long Ag nanowire was chosen to build the finitely long antenna-wire nanocircuit. These nanocircuits were also tested with transmitting antenna of different arm lengths varied from 50 to 800 nm. The arm length of the receiving antennas was kept unchanged as 250 nm as mentioned above. Figure 3c represents the detected emission intensity against a series of arm length of specific transmitting antennas. The arm length increasing from 50 to 250 nm led to an increased plasmon emission. However, further increasing the arm length from 250 to 800 nm dramatically decreased the plasmon emission. The optimized emission intensity was obtained when the arm length of the transmitting antenna was chosen as approximately 250 nm. To investigate the influence of the incident angle and the emission intensity, we varied the incident angles of the linearly polarized laser beam from 15
to 60
. The emission intensity for a given incident angle was determined by averaging five brightest pixels at the site. Data were collected from different angles of the same nanowire to ensure that the emission intensity curves resulted from the coupling geometry and were not artifacts. The emission intensities were plotted against different incident angles in Figure 3c. The maximum emission intensity is achieved 1677

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Figure 3. (a) SEM image of a 10 μm long Ag nanowire placed at the feed gaps of both receiving and transmitting antenna pairs. (b) The dependence of the incident wavelength on different Ag nanowire lengths. (c) The dependence of the plasmon emission on various arm lengths of the transmitting antennas under different incident angles of the excitation laser.

Figure 4. (a) Schematic of the antenna-wire optical nanocircuit with an Ag nanowire placed at the feed gaps of both receiving and transmitting antenna pairs. (b) The FDTD simulated electric field distribution of the antenna-wire nanocircuit. (c) Scanning near-field optical image for the plasmon propagation and end emission. (d) The equivalent electrical circuit for the model configuration. The generator represents the receiving antennas illuminated by the incident laser, ZS is the characteristic impedance of the Ag nanowire and corresponding SiO2 substrate, and ZL is the load impedance of the transmitting antenna pair.

with an incident angle of 28
and the transmitting antenna arm length around 250 nm. After we obtained the enhancement factor of plasmon coupling (FC = 13.6), and recorded the plasmon emission maximum, we are interested in the plasmon total efficiency for this nanocircuit. Compared to the detected radiations at the end facet of the single Ag nanowire (10 μm) without any antennas at both ends, the plasmon emission enhanced with a factor of FE = 45 for our antenna-wire circuit. After considering the guiding losses of the Ag nanowire (0.43 dB/μm),7,16 the plasmon emission efficiency of the transmitting antenna pair can be deduced as ηE = 80.8%, and the plasmon total efficiency of the antenna-wire nanocircuit can be obtained as ηT = 30.1%. (See Supporting Information.) To investigate this antenna-induced enhanced plasmon emission mechanism, FDTD simulations were carried out to replicate

the experimental observations. A polarized Gaussian beam (λ = 672 nm, beam spot = 1 μm) was simulated as the excitation source with its polarization parallel to the antenna pair. Bow tie antennas were modeled using simple triangles with an apex angle of 53
and the height of 100 nm. The arm length for the receiving antenna was designed as 250 nm, and the transmitting antenna was varied from 50 and 800 nm. An Ag nanowire with a diameter of 100 nm and a length of 10 μm was held in the feed gaps of receiving and transmitting antenna pairs. The simulated configuration was placed on a 100 nm thickness SiO2 substrate as illustrated in Figure 4a. The plasmon standing wave was shown in a steady-state image of the electric field distribution (Figure 4b). The wavelength of the simulated SPP standing wave was found to be around λSPP/2, which has a good agreement with our experimental observations as in Figure 4c, where the incident light could be effectively coupled 1678

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experimental detection. The plasmon field A of the fundamental mode can be described as A ¼ A0 e-γx ð1 þ jΓR jeij e-2γð10 - xÞ Þ

Figure 5. (a) Dependence of the reflectivity |ΓR| as a function of the arm length of the transmitting antenna pair varied from 50 to 800 nm (orange rhombus) and corresponding experimental emission intensity recorded by SNOM (black squares). (b) Load impedance of the transmitting antenna pair in the complex ZL plane. The open star represents the position of Zs. The arrow denotes the point of best impedance matching corresponding to the closest approach of the load impedance and Zs.

to the plasmon propagation mode and finally emitted from the end facet of the Ag nanowire. In the theoretical analysis of two-wire transmission line (OTL) configuration,24,25 the principle of impedance matching was used to optimize the OTL equivalent electrical circuit at optical frequencies. In the OTL equivalent circuit, two transmission wires were used to support the identical equivalent current, and the current orientation in these two wires was opposite in order to create a circuit. In our investigation, we found even with a single Ag nanowire, the theory of impedance matching could also be applied when considering the induced polarization charges in the SiO2 substrate. The equivalent circuit of our configuration is shown in the inset of Figure 4d. It consists of a generator providing an ac voltage at the optical frequency to represent the receiving antenna pair illuminated by the laser beam, the Ag nanowire, and corresponding SiO2 substrate with the characteristic impedance ZS, and the load impedance ZL representing the transmitting antenna pair. In the following calculation, we only considered the propagation coefficient of the fundamental plasmon mode, because of the fact that the intensity of other higher-order modes can be negligible in comparison with the fundamental mode for the

ð1Þ

where γ = R þ iβ is the propagation constant of the mode consisting of the decay constant R and the wave vector β and ΓR = |ΓR|eij represents the complex reflection coefficient for plasmons at the nanowire terminus. These parameters can be determined with the plasmon standing-wave data obtained by FDTD simulations. (See Supporting Information.) Figure 5a showed that the reflectivity |ΓR| as a function of the arm length of the transmitting antenna varied between 50 and 800 nm with its minimum value obtained around 250 nm. Since the reflectivity was a direct measure of the impedance matching between the Ag nanowire and the transmitting antenna pair, we proved that by changing the arm length of the antenna, the impedance matching could be achieved at the point of minimum, |ΓR| = 5.6%, which represents the maximal energy transmission and results in the emission intensity maximum, and in this way, it corresponds with the experimental emission intensity peak obtained with the arm length of the transmitting antenna at 250 nm (shown as the arrow line in Figure 5a). On the other side, we also calculated the characteristic impedance ZS and the load impedance of the transmitting antenna pair (ZL) to explore how this impedance matching happened. The ZS could be defined as the ratio of the voltage and current, where the voltage was obtained as a line integral of the complex electric field from the core Rof the nanowire to the bottom of the SiO2 substrate using V = E ds, and the current at a given position was evaluated according to the Ampere’s law I = H H ds, where the typical closed-loop integration path was replaced with a sufficiently long path approximating a linear path from -¥ f þ¥. The nanowire impedance then was obtained as ZS = (138 - 3.2j)Ω. (See Supporting Information.) The load impedance ZL was deduced with the calculated ZS and ΓR by ZL = ZS(1 þ ΓR)/(1 - ΓR).26 The results of ZL were plotted in Figure 5b. Similar to the theoretical results given in ref 24, ZL presented a spiral in the complex plane for different arm lengths of the transmitting antenna with each incident angle of the excitation laser. The position for ZS was also denoted in this complex plane. The shortest distance between the spiral curve and ZS indicated the best impedance matching, which was obtained at 250 nm for the arm length of transmitting antenna and 28
for the incident angle of the exciting laser, corresponding to the minimum |ΓR| = 5.6% in Figure 5a. In conclusion, we have shown that with an optimized geometry of bow tie antennas and the incident angle, an Ag nanowire with an appropriate length enabled an efficient surface plasmon coupling and emission. An enhancement factor of 45 was recorded for the maximum plasmon emission corresponding to the 10 μm long for the Ag nanowire, 250 nm for the arm length of the bow tie antennas, and 28
for the incident angle of the excitation laser. The experimental observations were confirmed by FDTD simulations and explicated by the impedance matching theory. The optical nanocircuit consists of a metallic nanowire and receiving/transmitting antenna pairs, which can realize a great plasmon emission enhancement, providing a practical way to build future plasmon devices by using metallic nanowires that suffer huge Ohmic dissipation and low plasmon coupling and emission efficiencies. 1679

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’ ASSOCIATED CONTENT

bS

Supporting Information. Surface plasmon enhancement factor and emission efficiency, reflection coefficient of plasmons with fundamental modes, characteristic impedance of Ag nanowire, and corresponding SiO2 substrate. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

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(22) Jiao, L. Y.; Fan, B.; Xian, X. J.; Wu, Z. Y.; Zhang, J.; Liu, Z. F. J. Am. Chem. Soc. 2008, 130, 12612–12613. (23) Fang, Z. Y.; Peng, Q.; Song, W. T.; Hao, F. H.; Wang, J.; Zhu, X. Nano Lett. 2010, 11, 893–897. (24) Huang, J. S.; Feichtner, T.; Biagioni, P.; Hecht, B. Nano Lett. 2009, 9, 1897–1902. (25) Wen, J.; Romanov, S.; Peschel, U. Opt. Express 2009, 17, 5925–5932. (26) Cheng, D. K. Field and wave electromagnetics; Addison Wesley: Reading, MA, 1983.

Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program) Grant No. 2007CB936800 and National Natural Science Foundation of China (Grant No. 10574002). The T€ubingen group would like to acknowledge the financial support from Landesstiftung BW, Germany. ’ REFERENCES (1) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824–830. (2) Ozbay, E. Science 2006, 311, 189–193. (3) Kolesov, R.; Grotz, B.; Balasubramanian, G.; Stohr, R. J.; Nicolet, A. A. L.; Hemmer, P. R.; Jelezko, F.; Wrachtrup, J. Nat. Phys. 2009, 5, 470–474. (4) Solis, D.; Chang, W. S.; Khanal, B. P.; Bao, K.; Nordlander, P.; Zubarev, E. R.; Link, S. Nano Lett. 2010, 10, 3482–3485. (5) Yoon, I.; Kang, T.; Choi, W.; Kim, J.; Yoo, Y.; Joo, S. W.; Park, Q. H.; Ihee, H.; Kim, B. J. Am. Chem. Soc. 2009, 131, 758–762. (6) Lee, S. J.; Baik, J. M.; Moskovits, M. Nano Lett. 2008, 8, 3244–3247. (7) Ditlbacher, H.; Hohenau, A.; Wagner, D.; Kreibig, U.; Rogers, M.; Hofer, F.; Aussenegg, F. R.; Krenn, J. R. Phys. Rev. Lett. 2005, 95, No. 257403. (8) Fang, Z. Y.; Lin, C. F.; Ma, R. M.; Huang, S.; Zhu, X. ACS Nano 2010, 4, 75–82. (9) Oulton, R. F.; Sorger, V. J.; Zentgraf, T.; Ma, R. M.; Gladden, C.; Dai, L.; Bartal, G.; Zhang, X. Nature 2009, 461, 629–632. (10) Hu, M. S.; Chen, H. L.; Shen, C. H.; Hong, L. S.; Huang, B. R.; Chen, K. H.; Chen, L. C. Nat. Mater. 2006, 5, 102–106. (11) Volkov, V. S.; Bozhevolnyi, S. I.; Rodrigo, S. G.; MartinMoreno, L.; Garcia-Vidal, F. J.; Devaux, E.; Ebbesen, T. W. Nano Lett. 2009, 9, 1278–1282. (12) Bozhevolnyi, S. I.; Volkov, V. S.; Devaux, E.; Laluet, J. Y.; Ebbesen, T. W. Nature 2006, 440, 508–511. (13) Volkov, V. S.; Bozhevolnyi, S. I.; Devaux, E.; Laluet, J. Y.; Ebbesen, T. W. Nano Lett. 2007, 7, 880–884. (14) Fang, Y. R.; Li, Z. P.; Huang, Y. Z.; Zhang, S. P.; Nordlander, P.; Halas, N. J.; Xu, H. X. Nano Lett. 2010, 10, 1950–1954. (15) Sanders, A. W.; Routenberg, D. A.; Wiley, B. J.; Xia, Y. N.; Dufresne, E. R.; Reed, M. A. Nano Lett. 2006, 6, 1822–1826. (16) Pyayt, A. L.; Wiley, B.; Xia, Y. N.; Chen, A.; Dalton, L. Nat. Nanotechnol. 2008, 3, 660–665. (17) Schuck, P. J.; Fromm, D. P.; Sundaramurthy, A.; Kino, G. S.; Moerner, W. E. Phys. Rev. Lett. 2005, 94, No. 017402. (18) Greffet, J. J. Science 2005, 308, 1561–1563. (19) Muhlschlegel, P.; Eisler, H. J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W. Science 2005, 308, 1607–1609. (20) Knight, M. W.; Grady, N. K.; Bardhan, R.; Hao, F.; Nordlander, P.; Halas, N. J. Nano Lett. 2007, 7, 2346–2350. (21) Graff, A.; Wagner, D.; Ditlbacher, H.; Kreibig, U. Eur. Phys. J. D 2005, 34, 263–269. 1680

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