Broadband Surface Plasmon Polariton Directional Coupling via

Jan 24, 2014 - Broadband unidirectional launching of SPPs is achieved, and the extinction ratio ... KEYWORDS: Surface plasmon polaritons, optical slot...
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Broadband Surface Plasmon Polariton Directional Coupling via Asymmetric Optical Slot Nanoantenna Pair Jing Yang,† Xiao Xiao,† Chuang Hu,† Weiwei Zhang,† Shuxiang Zhou,† and Jiasen Zhang*,†,‡ †

State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, China Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

‡

S Supporting Information *

ABSTRACT: Surface plasmon polariton (SPP) coupling is a basic subject for plasmonic study and applications. Optical nanoantennas enable downscaling of the SPP coupling to subwavelength scales. In this study, asymmetric optical slot nanoantenna pairs composed of two optical slot nanoantennas with different lengths are proposed for SPP directional coupling. Broadband unidirectional launching of SPPs is achieved, and the extinction ratio obtained experimentally reaches up to 44. The bandwidth is larger than 157 nm. Furthermore, SPP direction-selective radiation is demonstrated using the asymmetric optical slot nanoantenna pairs. A novel plasmonic display device showing the propagation direction of SPPs is achieved by employing asymmetric optical slot nanoantenna pairs without any electric device. Asymmetric optical slot nanoantenna pairs have large potential in the directional control of SPP launching and radiation and can be very useful in compact optical circuits and other photonic integrations. KEYWORDS: Surface plasmon polaritons, optical slot nanoantennas, broadband, directional coupling, plasmonic display devices

S

enhancement or far-field emission, and the interconversion between light and SPPs has not been thoroughly explored28 because most optical antennas are composed of metallic nanoparticles. Baron et al.6 proposed a compact antenna composed of grooves with different depths and widths to achieve unidirectional SPP launching and decoupling. Liu et al.29 proposed a resonant magnetic antenna based on metal− insulator−metal structure for unidirectional SPP excitation. In the present work, we propose a method to achieve directional control of SPP launching and radiation using an asymmetric optical slot nanoantenna pair (AOSNP) composed of two slot nanoantennas with different lengths and different resonant properties. Broadband unidirectional SPP launching was achieved experimentally with an extinction ratio as high as 44 and a bandwidth larger than 157 nm. Furthermore, the AOSNPs were demonstrated to be useful for SPP directionselective radiation. A novel plasmonic display device was presented to show the propagation directions of SPPs using AOSNPs without any electric device. Figure 1a and b shows the respective schematics of the topview and sectional view of the proposed AOSNP structure, which is composed of two slot nanoantennas fabricated in a gold film deposited on a silica substrate. The two nanoantennas have the same width w and different lengths L1 and L2, and the distance between them is d. The optical slot nanoantennas can implement conversion between free-propagating light and

urface plasmon polaritons (SPPs), which are confined electromagnetic fields at the metal−dielectric interface, have elicited much research interest in the past decades in fields such as physics, chemistry, biology, and materials science.1−5 Numerous photonic devices based on SPPs have been proposed, such as SPP launchers,6 lens,7 waveguides,8 and demultiplexers.9 One of the most important fundamentals for SPP applications is SPP coupling. Prisms and gratings10 are conventionally used for SPP coupling. However, prisms are too bulky to be integrated, and gratings still occupy large areas. Metallic nanostructures have been proposed and demonstrated to couple SPPs efficiently. Slits and ridges11−14 have been used to excite SPPs. By employing asymmetric structures,11,15,16 incident angles,14,17 or positions,13 unidirectional SPP launching can be achieved. Small holes18−21 and hole arrays22 in metallic films have also been studied to couple light and SPPs, but most of them are nonresonant, and the coupling efficiency is low. SPP launching and radiation involve the coupling between confined SPPs and free-propagating light fields, which can be achieved using optical antennas. In radio frequency and microwave regimes, antennas are widely used as devices for interconversion between free electromagnetic wave and guided electric current. Optical antennas composed of metallic nanostructures have been proposed to achieve interconversion between light and SPPs and have drawn much interest.23−27 Different shapes of optical antennas have been investigated, such as bowtie antennas,23 dipole antennas,24 Yagi-Uda antennas,25 monopole antennas,26 and patch antennas.27 However, most of these studies concentrate on near-field © 2014 American Chemical Society

Received: October 22, 2013 Revised: January 20, 2014 Published: January 24, 2014 704

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surfaces A1 and A2, respectively, that is, E0(1) = CA1 and E0(2) = CA2, where C is a constant. In this study, the scattering of the SPPs by the neighboring slot nanoantennas is ignored. Given that the propagation length of SPPs at the gold/air interface is considerably larger than the distance between the two nanoantennas (δ ≫ d), in the far field (|x| ≫ d), the intensities in the +x direction I+SPP and the −x direction I−SPP can be expressed using the following formulas: exp(−x /δ) d exp{i[k SPP(x + ) + φ1]} x 2 exp( −x /δ) d + CA 2 exp{i[k SPP(x − ) + φ2]}|2 x 2

I+SPP ≈ |CA1

(2a)

exp(x /δ) exp{i[−k SPP(x + −x exp(x /δ) − CA 2 exp{i[−k SPP(x − −x

I −SPP ≈ |−CA1

d ) + φ1]} 2 d ) + φ2]}|2 2 (2b)

The initial phases φ1 and φ2 depend on the parameters of the nanoantennas and the wavelength of the incident light. If A1 = A2 and (φ2 − φ1) + kSPPd = π, then I−SPP = 0 and the launching of SPPs propagating along the −x direction is forbidden, which originates from the destructive interference. As a result, unidirectional SPPs propagating along the +x direction are launched. To maximize the intensity I+SPP, another phase condition φ2 − φ1 = kSPPd must be satisfied. Therefore, both φ2 − φ1 = π/2 and kSPPd = π/2 should be satisfied to obtain the largest extinction ratio. An optical slot nanoantenna is the complementary structure of a dipole nanoantenna, and its resonance is localized surface plasmon resonance. The initial phase of the launched SPPs at the gold/air interface depends on the geometric parameters and the incident wavelength, which is similar to the case of the electric field dipolar oscillation in a metallic nanorod.31 The initial phase difference between the SPPs excited at the two slot nanoantennas can be tuned by changing the parameters of the slot nanoantennas. If the film thickness h is kept unchanged, only two geometrical parameters of the slot nanoantenna have to be tuned, that is, antenna length L and width w. The equal excitation amplitudes (A1 = A2) and a specific phase difference (φ2 − φ1 = π/2) cannot be obtained simultaneously in a broadband. In the experiment, the extinction ratio I+SPP/I−SPP is highly sensitive to I−SPP. For unidirectional SPP launching, I−SPP is preferred to have a value close to 0 instead of maximizing I+SPP. If (φ2 − φ1) + kSPPd = π, a large extinction ratio can be obtained. We can change the distance d between the two slot nanoantennas to satisfy this condition and the specific phase difference of π/2 (φ2 − φ1 = π/2) is not necessary. Equal excitation amplitudes can be obtained by changing the lengths of the nanoantennas. Both the antenna length and width influence evidently the resonance of the slot nanoantenna. Changing the antenna length will bring a large phase variation while maintaining nearly equal excitation amplitudes of the SPP field in broadband. More detailed calculation results and discussion about the influence of the antenna length and width on the resonance of the slot nanoantenna are given in the Supporting Information. In this work, the nanoantenna widths are kept unchanged, and the nanoantenna lengths are tuned to obtain equal excitation amplitudes (A1 = A2). Afterward, the

Figure 1. Schematics and resonances of AOSNPs. (a) Schematic of an AOSNP (top view). (b) Sectional view (y = 0 plane) of an AOSNP. (c) Electric field |Ex| versus wavelength; inset: magnetic field intensity | H|2 distribution on the top surface of the gold film (z = 0 plane) for the optical slot antenna with length L1 = 225 nm. (d) Phase difference Δφ = φ2 − φ1 versus wavelength of the two optical slot antennas (L1 = 225 nm, L2 = 260 nm). (e) The electric field |E| distribution on the top surface of the Au film (z = 0 plane) for the AOSNP (L1 = 225 nm, L2 = 260 nm, and d = 320 nm). The incident wavelength is 830 nm. The number 100 in the scale bar represents an electric field amplitude of 0.2 V/m.

SPPs. When an x-polarized plane wave normally impinges on the nanoantennas from the substrate side, SPPs are launched at each nanoantenna both at the air/gold and silica/gold interfaces. In this study the gold film is 200 nm thick and there is no coupling between the SPPs at the two interfaces. Only SPPs launched at the air/gold interface are considered, and the initial phases of SPPs at the two nanoantennas are labeled as φ1 and φ2. Considering that the size of the antennas is considerably smaller than the wavelength, the two optical slot nanoantennas serve as two SPP point sources in the far field, and the SPP field can be written as a dipolar form Ex(r,θ) = E0 cos2θ exp(−r/δ) cos(ωt − 2πr/λSPP)/(r1/2),30 where δ is the propagation length of the SPP field. The SPPs propagate along the ± x directions and interfere, and the intensities in the far field on the x-axis can be written using the following equation: d

ISPP = |E0

(1)

exp( −|x + 2 | /δ) x+

d 2

exp[i(k SPP|x +

d | + φ1)] 2

d

+ E0

(2)

exp( −|x − 2 | /δ) x−

d 2

exp[i(k SPP|x −

d | + φ2)]|2 2 (1)

where E0(1) and E0(2) represent the amplitudes of SPP electric field launched by the two slot nanoantennas, which are proportional to the amplitudes of the electric field in the x direction at the centers of the slot nanoantennas on the top 705

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distance d is optimized to match the phase condition (φ2 − φ1) + kSPPd = π, resulting in I−SPP→0. Numerical simulations were performed using the finite difference time domain (FDTD) method to show the properties of AOSNPs and the excitation of SPPs. The geometric parameters of a designed AOSNP are L1 = 225 nm, L2 = 260 nm, and w = 80 nm. The depth of the holes is the same as the thickness of the gold film, with h = 200 nm. A plane wave is normally incident from the bottom of the silica substrate along the +z-axis, and its polarization is parallel to the short axis of the slots, that is, along the x axis. The initial phase of SPPs is obtained from the angle of the complex electric field Ex at the center of the slot nanoantenna on the top surface. The near-field electric field amplitudes |Ex| at the center points of the holes on the top surface and the initial phase difference Δφ = φ2 − φ1 are calculated for the two nanoantennas with L1 = 225 nm and L2 = 260 nm, as shown in Figure 1c and d, respectively. The resonant wavelengths for the two nanoantennas are 830 and 930 nm. The distribution of magnetic field intensity |H|2 on the top surface for the nanoantenna with L1 = 225 nm at the resonant wavelength 830 nm is shown in the inset of Figure 1c. In the 800−900 nm range, the near-field electric field amplitudes |Ex| of the two nanoantennas are nearly the same. At λ = 830 nm, a phase difference of φ2 − φ1 = 38° is obtained. Thus, we design the AOSNP with d = 320 nm to satisfy the condition (φ2 − φ1) + kSPPd = π. For λ = 830 nm, the electric field amplitude |E| on the air/gold surface is calculated and shown in Figure 1e. Notably, the SPPs propagating along the +x direction are considerably larger than the SPPs propagating along the −x direction. The extinct ratio of the SPP intensities at x = ±1 μm reaches 107. The results show the validity of the design of unidirectional SPP launching based on eq 2b. The SPP excitation efficiency for a single AOSNP at the air/gold interface was also calculated by integrating the Poynting vector along the lateral surface of an imaginary rectangle box.18 The excitation efficiency reaches 54% at 830 nm when the launched SPP energy at the air/gold interface is normalized to the incident energy illuminated upon the AOSNP area (260 nm × 400 nm, Figure S2b in the Supporting Information). The high efficiency can be attributed to the resonance nature of the slot nanoantennas. More simulation details can be found in the Supporting Information. To demonstrate unidirectional SPP launching experimentally, AOSNPs were fabricated using focused ion beam milling in a 200 nm thick gold film deposited on a glass substrate. Two similar gratings were fabricated to measure the intensity ratio of the launched SPPs. Figure 2a shows the scanning electron microscope (SEM) image of the sample. To increase the intensity of the launched SPPs, 20 AOSNPs are linearly located at the center of the sample with geometric parameters L1 = 225 nm, L2 = 260 nm, w = 80 nm, and d = 320 nm, which are the same as the designed parameters. The two gratings have a period of 814 nm and a separation of 50 μm lying symmetrically on the two sides of the AOSNPs. The close-up view of the AOSNP array is shown in Figure 2b. For comparison, 20 optical slot nanoantennas are also fabricated in the same gold film with length L = 225 nm and width w = 80 nm (Figure 2c). A continuous wave Ti:sapphire laser was used as the light source with a tunable wavelength range of 773−930 nm. The slot nanoantennas were normally illuminated by the x-polarized laser beam from the substrate side, and SPPs on the air/gold interface were launched and propagated along the ±x

Figure 2. Unidirectional SPP launching using AOSNPs. (a) SEM image of the experimental sample. (b) SEM image of the AOSNPs. The inset shows the close-up view of an AOSNP. (c) SEM image of the single antenna structure. (d) CCD image of the scattering gratings when the SPPs were launched with AOSNPs (L1 = 225 nm, L2 = 260 nm, d = 320 nm) for an incident wavelength of 830 nm. (e) CCD image of the scattering gratings when the SPPs were launched with the single antenna structure (L = 225 nm) for an incident wavelength of 830 nm. The scale bars in a, b, and c denote 10, 2, and 2 μm, respectively, whereas the scale bar in the inset in b denotes 200 nm.

directions. Afterward, the SPPs were scattered by the two gratings, and the scattered light was collected with an objective (100×, NA = 0.8) from the air side. The two gratings were imaged onto a charge coupled device (CCD). The light directly passing through the nanoantennas was blocked with a spatial filter. Figure 2d shows the CCD image of the two gratings for λ = 830 nm. Only the grating at the +x direction is bright; the one at the −x direction is completely dark. The intensity extinction ratio I+SPP/I−SPP in Figure 2d reaches 39, which experimentally demonstrates unidirectional SPP launching using AOSNPs. For comparison, the CCD image in Figure 2e shows the results of the case of single slot nanoantenna structure in Figure 2c. The SPPs were symmetrically excited on both sides of the nanoantennas, and the intensity ratio I+SPP/ I−SPP is 1.04. The extinction ratio with respect to the incident wavelength is shown in Figure 3a. In the tunable wavelength range of 773− 930 nm, all of the extinction ratios are larger than 16, with a maximum of 44 at 810 nm. The wavelengths whose extinction 706

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and the smoothness of the fabricated slot nanoantennas are not perfect. We have demonstrated unidirectional SPP launching using AOSNP structures. The inverse process is SPP radiation through AOSNPs. Accordingly, SPP direction-selective radiation can be implemented using the same structure. An AOSNP can scatter SPPs into light propagating along the ±z directions. For SPPs propagating along the ±x directions, the scattered light from the two nanoantennas interferes in the far field. The far-field intensity depends on the radiation electric field amplitudes and phase difference. For the AOSNPs discussed above, the scattered lights of the SPPs propagating along the +x direction radiating from the two nanoantennas to the −z direction destructively interfere with each other, and the farfield intensity should be weak. By contrast, the scattered light of the SPPs propagating along the −x direction interferes constructively, and the far-field intensity is strong. To experimentally demonstrate SPP direction-selective radiation of the AOSNP, a sample was fabricated on the 200 nm thick gold film. The SEM image is shown in Figure 4a. An

Figure 3. Extinction ratio versus wavelength. (a) Experimental results (red circular dots) and FDTD simulation result (black solid line) for the AOSNPs with L1 = 225 nm, L2 = 260 nm, and d = 320 nm. (b) Experimental results for the AOSNPs with L1 = 210 nm, L2 = 240 nm, and d = 320 nm (black square dots) and L1 = 250 nm, L2 = 285 nm, and d = 330 nm (blue circular dots).

Figure 4. Direction-selective SPP radiation. (a) SEM image of the AOSNPs for SPP radiation. The two insets in a are close-up views of the AOSNPs. (b) CCD image of the AOSNPs for an incident wavelength of 870 nm. The scale bar in a denotes 5 μm, whereas the scale bars in the two insets in a denote 500 nm.

SPP launching grating with a period of 814 nm and two AOSNP arrays were fabricated. Five AOSNPs with L1 = 225 nm, L2 = 260 nm, w = 80 nm, and d = 350 nm exist in each array. The distance d of the fabricated AOSNPs deviates from the designed value of 320 nm because of the imperfect fabrication. The distance between two adjacent AOSNPs is 200 nm. In these two arrays, the orders of the short and long nanoantennas in the AOSNPs are different to obtain different radiation intensities. The distance between the two arrays is 10 μm. The length of the grating is 30 μm, and the distance between the grating and the AOSNP arrays is 25 μm. A laser beam was normally incident upon the grating from the air side to launch SPPs, which propagated toward the AOSNP arrays and were scattered. The scattered light was collected at the substrate side and imaged on the CCD. For the AOSNPs on the left side in Figure 4a, the −z-direction radiations of the antennas interfere constructively, resulting in a strong output in the far field. By contrast, the output in the far field should be weak for the AOSNPs on the right side. The experimental results of the intensities of the SPP radiations from the AOSNP arrays at λ = 870 nm are shown in Figure 4b, which is consistent with the expected results. The AOSNPs on the left side are bright, whereas the AOSNPs on the right side are dark. The far-field radiation extinction ratio Ileft/Iright obtained from the CCD image is 6.8. The extinction ratio is not very large. The two nanoantennas in an AOSNP can be considered as two point light sources with different initial phases. Their positions do not overlap, resulting in imperfect destructive interference in

ratios are bigger than 10 are defined as the effective operating wavelengths of the unidirectional SPP launching structure. For the designed AOSNP with L1 = 225 nm, L2 = 260 nm, d = 320 nm, and h = 200 nm, the bandwidth is larger than the tunable range of the light source of 157 nm. The broadband response of the device originates from the frequency dependence of both the initial phase difference φ2 − φ1 and the wave vector of the SPPs kSPP. The conditions of (φ2 − φ1) + kSPPd = π and A1 = A2 can be approximately satisfied in a large wavelength range. For more detailed analysis, see the Supporting Information. To check the tolerance in terms of fabrication deviation, two other AOSNPs with different lengths (L1:L2:d = 210 nm/240 nm/320 nm and 250 nm/280 nm/330 nm) and w = 80 nm were also fabricated. The experimental extinction ratios are shown in Figure 3b. The extinction ratios are all higher than 10 in the whole measured wavelength range, which indicates that the AOSNPs have good robustness in terms of the deviation of designed parameters because of imperfect fabrication. The maximum extinction ratios are 41 and 43 for these two structures. We calculated the frequency response of the AOSNP (L1 = 225 nm, L2 = 260 nm, d = 320 nm, w = 80 nm, and h = 200 nm) using FDTD method, and the results are compared with the experimental results, as shown in Figure 3a. The relative frequency response of the simulated results agrees well with the experimental results. However, the calculated extinction ratios are slightly higher than the measured value because the shapes 707

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the far field. The extinction ratio can be improved by adding more AOSNPs along the symmetrical axis direction of the AOSNPs. Based on the SPP direction-selective radiation property of the AOSNPs, a novel plasmonic display device that can show the propagation direction of SPPs without any electric devices was designed. The device is composed of AOSNPs and symmetric optical slot nanoantenna pairs (SOSNPs). In this study, the SPPs have four propagation directions of ±x and ±y directions. A dominated SPP propagation direction exists, in which the radiation intensity of the AOSNPs to the substrate side is considerably larger than that in the other three directions. In an SOSNP, the two nanoantennas have the same geometric parameters. As a result, for the SPPs with propagation directions parallel to the short axis of the nanoantennas in an SOSNP, the intensity of the radiation to the substrate side is considerably larger than that of SPPs with two other propagation directions. The SEM image of the experimental sample is shown in Figure 5a. The display device

structure of the nanoantenna pairs. When the top grating is illuminated by a laser beam at 870 nm, SPPs are launched and propagated along the −y direction. The image of the device obtained from the substrate side is shown in Figure 5c. Some of the nanoantenna pairs are bright and are composed of an arrow shape to indicate the propagation direction of the SPPs. For the SPPs propagating along the +y, −x, and +x directions, the images of the devices are shown in Figures 5d−f, respectively. The results demonstrate the plasmonic display device using the anisotropy of the radiation of AOSNPs. The properties of the device can be tuned by changing the structure and the number of the nanoantenna pairs. The observed noise in Figures 5c−f originates from the roughness of the gold film. One approach to increase the display quality is to increase the number of the nanoantenna pairs, resulting in high radiation intensity. We have proposed and demonstrated directional control of SPP launching and radiation at the nanoscale level using optical slot nanoantenna pairs. An extinction ratio as high as 44 and a bandwidth higher than 157 nm were experimentally obtained for unidirectional SPP launching. The AOSNPs also show unique radiation properties, wherein the radiation intensity depends on the propagation direction of SPPs. A novel plasmonic display device composed of AOSNPs and SOSNPs was demonstrated to display SPP propagation directions without any electric device, which presents a new way to construct the all-optical device. Compared with other plasmonic structures for unidirectional SPP launching, such as nanoslits, holes, and grooves, the AOSNPs are compact, broadband, and easy to be fabricated and integrated in the twodimensional surface of metal films. Based on the abilities in controlling light at the nanoscale level, AOSNPs have promising applications in nanophotonic and plasmonic devices and circuits, such as SPP launcher, lens, sensors, and displayer.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. Plasmonic display device. (a) SEM image of the experimental sample. (b) SEM image of the designed plasmonic display device. The inset is a close-up view of the AOSNPs on the left top border of the structure. (c−f) CCD images of the plasmonic display device when the SPPs propagated along the −y, +y, −x, and +x directions for an incident wavelength of 870 nm. The white arrows on the top right corner of the CCD images show the SPP propagation directions. The scale bars in a and b are 10 μm and 2 μm, respectively, whereas the scale bar in the inset in b denotes 200 nm.

Simulation details and broadband property analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

is located at the center, and the close-up is shown in Figure 5b. A total of 12 SOSNPs are located in two lines that comprise a cross, and the distance between two adjacent SOSNPs in a line is approximately 2 μm. A total of 16 AOSNPs exist in addition to the SOSNPs. The fabricated AOSNPs have geometric parameters of L1 = 225 nm, L2 = 260 nm, w = 80 nm, and d = 350 nm, and the SOSNPs have the same nanoantenna length of 225 nm, w = 80 nm, and d = 320 nm. The distance d of the AOSNPs deviates slightly from the designed value of 320 nm because of imperfect fabrication. The arrangement in the AOSNPs is designed to produce nanoantennas with longer lengths that are closer to the cross of the device than nanoantennas with shorter lengths in the same AOSNPs (inset in Figure 5b). Four gratings with a period of 814 nm are used to launch SPPs with propagation directions of +x, −x, +y, or −y. The distance between the gratings and the device center is 25 μm. The intensity of the scattered light to the substrate side depends on the propagation direction of the SPPs and the

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 61036005, 61377050, and 11327902) and the Research Fund for the Doctoral Program of Higher Education (grant no. 20130001110050).

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