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We propose a novel V-shaped Ag nanowire structure as a subwavelength polarization beam splitter. When an incident light is focused onto the junction of the two branches, two surface plasmon polaritons (SPPs) are launched and propagate along the two branches. The polarizations of the emission light from the two ends are always parallel to the directions of the branches and the splitting ratio can be adjusted by changing the polarization of the incident light. The polarization characteristic originates from the fact that only single plasmonic waveguide mode exists in the thin nanowire and high order modes are cutoff. The near-field coupling between the two branches dominates the SPPs launching and the splitting ratio, which are very different with the single nanowire case. The V-shaped nanowire structure will have many potential applications in the integration of plasmonic devices, such as plasmonic router or polarizer.
©2013 Optical Society of America
1. Introduction
Strong confinement of electromagnetic energy below the diffraction limit can be realized at optical frequencies by exploiting surface plasmon polaritons (SPPs) [1]. Plasmonic waveguide is regarded as one of the ideal candidates for highly integrated nanophotonic circuits due to their ability to guide and manipulate light at deep sub-wavelength scales [2, 3]. Over the past few decades, several kinds of SPP waveguides have been proposed, such as metallic stripes [3, 4], metallic grooves [5, 6], metal–insulator–metal (MIM) waveguides [7, 8] and nanowires [9, 10]. Among these various SPP waveguides, the silver (Ag) nanowire can be considered as an excellent one because of some unique properties, such as the higher SPP excitation efficiency and the lower propagation dissipation due to their atomically smooth surface [9–13]. For the Ag nanowires, different devices including directional couplers [14, 15], Y-branch splitters [16, 17] and detectors [12] have been demonstrated. The high efficiency excitation, oriented transmission and mutual coupling have been realized. Among these nanowire-based structures, the ones which can realize the flexible control on the intensity and polarization of SPP, are significant in the integration of plasmonic devices and have attracted a lot of attention. For example, the correlation between incident and emission polarization in the Ag nanowire has been studied and it was demonstrated that the polarization directions of the emission light are variable with the change of polarization of incident light [18].
In this paper, we propose a SPP polarization beam splitter (PBS) based on a V-shaped Ag nanowire structure. Due to the single mode transmission in the nanowires, the polarizations are controlled by the directions of the nanowires. The influence of the near-field coupling between the two branches of the V-shaped nanowire on the splitting ratio is studied. The results show a new way to manipulate the polarization and intensity at the nanoscale.
2. Experiment setup
The Ag nanowires with smooth surfaces are synthesized and washed using the methods as have been reported [19, 20]. The mean diameters of these nanowires are about 120nm and the lengths rang from one to tens of micrometers. The ethanolic suspension of Ag nanowires is then spin-coated on glass slides and dried under ambient condition. The V-shaped Ag nanowire structure shown in Fig. 1(a) is formed by bending one Ag nanowire and we can see that the junction is approximately symmetrical. The inset is the 45°-titled scanning electron microscopy (SEM) image of focused ion beam milled cross section of a silver nanowire. The cross section of the nanowire is nearly circular.
Fig. 1 (a) SEM image of the V-shaped Ag nanowire. The inset is the 45°-titled SEM image of focused ion beam milled cross section of a silver nanowire. (b) Sketch of the experiment setup. (c) Coordinate system.
The experiment setup is an optical microscope system as illustrated in Fig. 1(b). A Ti:sapphire laser at 750nm is used as the light source, and its polarization is controlled using a polarization beam splitter (PBS) and a half-wave plate. The excitation light is focused at the junction of the V-shaped nanowire with an objective (Olympus UPlanApo, 100 × , N.A. 0.80) to excite SPPs. The emission light at the two ends of the branches is collected by the same objective, and the optical images of the samples are recorded by a CCD detector. The polarization of the emission light can be checked through the polarizer in front of the CCD detector. The coordinate system is shown in Fig. 1(c). The origin is located at the junction and one branch is along the x-axis. The angle between the polarization of the excitation light and the x-axis is θ. The angle between the two branches is β. The angle between the polarizing direction of polarizer and the x-axis is α.
3. V-shaped nanowire as a subwavelength polarization beam splitter
We focus the laser beam at the junction of the V-shaped nanowire to launch SPPs. The optical microscope images of the V-shaped nanowires with intersection angles of 79° and 148° are shown in Figs. 2(a) and 2(d), respectively. The intensities and polarizations of the emission light from the two ends are measured. The plots of intensity of emission light as a function of α corresponding to two different θ are shown in Figs. 2(b), 2(c), 2(e), and 2(f). For the structure with β = 79° [Fig. 2(a)], the polarizations of the emission light at end a and b are always ~3° and ~75°, respectively, for θ = 90° and 160°, which deviate the directions of the two branches 3° and 4°, respectively. Similar results are obtained for the structure with β = 148° [Fig. 2(b)] and α = 4° and 143° at the end a and b, respectively, for θ = 100° and 150°. The deviation angles from the directions of the two branches are 4° and 5°, respectively. The results show that the polarizations of the emission light at the two ends are always nearly parallel to the directions of the two branches, respectively, and invariable with changing θ. Thus, the V-shaped Ag nanowire structure can act as a PBS, which can divide the incident light into two emission light with different polarizations. The polarization of the emission light can be adjusted by changing the direction of the branched wire.
Fig. 2 (a) Microscope optical image of the V-shaped nanowire structure with β = 79°. (b) Intensity of the emission light at end a as a function of α corresponding to two different polarizations of incident light for the structure in (a). (c) Intensity of the emission light at end b as a function of α corresponding to two different polarizations of incident light for the structure in (a). (d) Microscope optical image of the V-shaped nanowire structure with β = 148°. (e) Intensity of the emission light at end a as a function of α corresponding to two different polarizations of incident light for the structure in (d). (f) Intensity of the emission light at end a as a function of α corresponding to two different polarizations of incident light for the structure in (d).
To understand the polarization characteristic of the V-shaped Ag nanowire structure which is greatly different from that reported in [18], we calculate the SPP mode in a nanowire by using the finite-difference time-domain (FDTD) method. As shown in Fig. 3(a) , the diameter and the length of the single nanowire are about 120nm and 5μm, respectively. Although the ends of chemically synthesized crystalline nanowires have ðve {111} facets, they are usually axisymmetric [21]. Therefore, it is reasonable that we consider one cylindrical nanowire with elliptic termination in the simulation. The nanowire is put on a silica substrate. A light beam with a wavelength 750nm and a polarization angle θ is focused on the left end along the -y axis to launch SPPs. The polarization of the incident light can be divided into two orthogonal directions: x-polarization and z-polarization. The current and electric field intensity distributions in the nanowire are calculated. Figures 3(b) and 3(d) show the current distributions in the x-z cross section of the nanowire 25nm away from the substrate for the incident light with z- and x-polarization, respectively. Figures 3(c) and 3(e) show the electric field intensity distributions in the x-y cross section of the nanowire for the incident light with z- and x-polarization, respectively. As shown in Fig. 3(b), when the nanowire is illuminated by the incident light with a parallel polarization, the SPPs can propagate along the nanowire efficiently. According to Fig. 3(c), m = 0 mode is launched in this situation. However, it is quite different for the perpendicular polarization case as shown in Figs. 3(d) and 3(e). The m = 1 mode can be seen and the intensity of the current along the propagation direction is exponential decay with an attenuation length smaller than one wavelength, which means that the m = 1 mode is cutoff. As a result, single mode exists in the nanowire, which dominates the polarization characteristic of the emission light from the ends of the nanowire waveguide.
Fig. 3 (a) Structure and coordinate system used in the simulation. (b) Intensity of the current along the nanowire when the polarization of the incident light is parallel to the nanowire. The inset is the current distribution in the x-z plane. (c) Electric field intensity distribution in the x-y plane when the polarization of the incident light is parallel to the nanowire. (d) Intensity of the current along the nanowire when the polarization of the incident light is perpendicular to the nanowire. The inset is the current distribution in the x-z plane. (e) Electric field intensity distribution in the x-y plane when the polarization of the incident light is perpendicular to the nanowire.
To study the emission character of the SPP waveguide modes, we calculate the angular distributions of the far field emission light scatted at the end for the m = 0 mode in Fig. 3(c) using FDTD method [22] and the results for the intensities of the x, y, and z components of the electric field are shown in Figs. 4(a) -4(c), respectively. The white dashed circles indicate the emission angles corresponding to the collection angle of the objective used in the experiment (N.A. = 0.8). We can see that the polarization of the emission light is dominated by the z-component, which explains that the polarization of the emission light is nearly parallel to the direction of the nanowire. Thus, the V-shaped Ag nanowire with a small diameter can serve as a subwavelength PBS. The structure will have great potential application in the integration of plasmon devices.
Fig. 4 Far field angular distributions of (a) x, (b) y, and (c) z component of the emission light in the direction of the collecting objective. The white dashed circles indicate the collection angle of the objective used in the experiment (N.A. 0.8).
4. Splitting ratio of the V-shaped nanowire structure
In the above section, we have demonstrated that the polarizations of the emission light are always nearly parallel to the directions of the nanowires and invariable with changing θ. Thus, the V-shaped nanowire structure can function as the subwavelength PBS. In this section, the splitting behavior of the PBS with intersection angles of 75°, 89° and 129° are studied. The plots of intensities of the emission light at the two ends as a function of θ are shown in Figs. 5(a) -5(c). The lines are sinusoidal function fittings of the experimental results. The intensities of the emission light oscillate nearly sinusoidally with respect to θ. Figure 5(d) shows the intensity of emission light as a function of θ for the single nanowire. Comparing Figs. 5(a)-5(c) with Fig. 5(d), it can be found that the relations between the intensity of emission light and θ are greatly different. For the single nanowire, the intensity of emission light is maximum when θ~0°, i.e. the emission light is strongest when the polarization of incident light is parallel to the nanowire. However, the maximums of the intensities of the emission light at the end a for the V-shaped nanowire structure with β = 75°, 89°, and 129° happen at θ = 102°, 107°, and 66°, respectively. When β is an acute angle, the θ corresponding to the maximum emission is an obtuse angle. On the contrary, when β is an obtuse angle, θ corresponding to the maximum emission is nearly the half of β. The splitting ratios (Ia/Ib or Ib/Ia) are shown in the insets of Figs. 5(a)-5(c). The splitting ratio depends on the intersection angle β, and a maximum splitting ratio of 4.3 was obtained for β = 89°. The lengths of the two branches also influence the splitting ratio. The difference between the single nanowire and the V-shaped nanowire structure is significant, and it is important to study and confirm the relation between the intensity of emission light and θ in the V-shaped nanowire structure.
Fig. 5 Intensities of the emission light versus the polarization angle of the incident light for V-shaped nanowires with (a) β = 75°, (b) β = 89°, and (c) β = 129°. The insets are the splitting ratio versus the polarization angle of the incident light and the SEM images of the structures. (d) Intensity of the emission light versus the polarization angle of the incident light for the single nanowire. The insets are the SEM images of the structures. The lines are the sinusoidal function fittings of the experimental results. The red arrows are the unified coordinate system used in the paper.
To find the relation between the emission intensities and β in detail, we perform the FDTD calculations for the V-shaped nanowire structures with different intersection angles. The diameter of the nanowire is 120nm and the length of each branch is 5μm. The laser beam at 750nm is incident upon the junction of the V-shaped nanowire along the -y axis. During the simulation, we keep all the variables unchanged except the direction of branch b (i.e. the intersection angles). Figures 6(a) and 6(b) show the electric ðeld intensity at the cross sections of the v-shaped structures with intersection angles of 30° and 150° when illuminated by the laser beam with two different polarizations. We can observe that for the v-shaped nanowire structure with β = 30° [Fig. 6(a)], the excitation efficiency of SPP on the branches are high when the polarization of the incident light is about perpendicular with the branch a and it is small when the polarization of the incident light is about parallel with the branch a, which are in accordance with the experiment results in Fig. 5(a). For the V-shaped nanowire structure with β = 150° [Fig. 6(b)], the excitation efficiency of SPP is high when the polarization of the incident light is about parallel with the angular bisector and it is small when the polarization of the incident light is about parallel with the branch a. Figures 6(c) and 6(d) show the intensities of the emission light at end a as a function of θ for the V-shape nanowire structure with different β, and the insets show θmax, the value of θ corresponding to the maximum emission at end a, with respect toβ. When β ≤ 90°, θmax is around 90°. When β > 90°, θmax is about the half of β. The simulation results agree with the experimental results. The results show that for a V-shape nanowire the coupling between the two branches dominates the SPP launching behavior, which is different from the case of single nanowire.
Fig. 6 (a) Intensities of the electric field at the cross sections of the V-shaped nanowire structures with β = 30° corresponding to different polarizations. (b) Intensities of the electric field at the cross sections of the V-shaped nanowire structures with β = 150° corresponding to different polarizations. (c) Intensities of the emission light at end a versus θ for V-shaped nanowires with β = 15°, 45°, 75° and 90°. (d) Intensities of the emission light at end a versus θ for V-shaped nanowires with β = 120°, 135°, 150° and 165°. The insets are θmax, the value of θ corresponding to the maximum emission at end a, versus β.
5. Conclusion
We have proposed a novel V-shaped Ag nanowire structure as a SPP PBS, which can realize both the polarization and splitting ratio of emission light controllable. The single mode transmission in the thin nanowires guarantees the polarizations of the emission light at the ends being parallel to the directions of the nanowires. The V-shaped nanowire provides a new way to control the polarizations of SPPs in nanowire waveguide. The near-field coupling between the two branches of the V-shaped, which influences the splitting ratio evidently, can be used to control the SPP’s intensity in nanowire waveguide. The V-shaped nanowires are promising for many applications in nanoplasmonics, such as polarizer, router, polarization beam splitter, and coupler.
Acknowledgments
This work was supported by the National Basic Research Program of China under Grants 2010CB923202 and 2009CB623703, MOST, and the National Natural Science Foundation of China under Grants 61036005 and 11074015.
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