Plasmonic Hybridization Induced Trapping and Manipulation of a

Oct 10, 2014 - Hybridization in the narrow gaps between the surface plasmon polaritons (SPPs) along a metal surface and the localized surface plasmons...
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Plasmonic Hybridization Induced Trapping and Manipulation of a Single Au Nanowire on a Metallic Surface Yuquan Zhang,†,# Jian Wang,†,# Junfeng Shen,† Zhongsheng Man,† Wei Shi,† Changjun Min,*,‡ Guanghui Yuan,§ Siwei Zhu,∥ H. Paul Urbach,⊥ and Xiaocong Yuan*,‡ †

Institute of Modern Optics, Nankai University, Tianjin 300071, China Institute of Micro & Nano Optics, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China § Centre for Disruptive Photonic Technologies, Nanyang Technological University, Nanyang Link 21, Singapore ∥ Institute of Oncology, Tianjin Union Medicine Centre, Tianjin 300121, China ⊥ Optics Research Group, Delft University of Technology, Lorentzweg 1, 2628CJ Delft, The Netherlands ‡

S Supporting Information *

ABSTRACT: Hybridization in the narrow gaps between the surface plasmon polaritons (SPPs) along a metal surface and the localized surface plasmons on metallic nano-objects strongly enhance the electromagnetic field. Here, we employ plasmonic hybridization to achieve dynamic trapping and manipulation of a single metallic nanowire on a flat metal surface. We reveal that the plasmonic hybridization achieved by exciting plasmonic tweezers with a linearly polarized laser beam could induce strong trapping forces and large rotational torques on a single metallic nanowire. The position and orientation of the nanowire could dynamically be controlled by the hybridization-enhanced nonisotropic electric field in the gap. Experimental results further verify that a single Au nanowire could robustly be trapped at the center of an excited SPP field by the induced forces and then rotated by the torques. Finally, a plasmonic swallow tail structure is built to demonstrate its potential in the fabrication of lab-on-a-chip plasmonic devices. KEYWORDS: plasmonic hybridization, plasmonic tweezers, dynamic manipulation, Au nanowire large field enhancement has been achieved in the wellknown plasmonic structure composed of a metallic nanoparticle located close to a flat metallic surface. This was caused by the hybridization effect of the surface plasmon polaritons (SPPs) propagating along the metal surface and the localized surface plasmons (LSPs) around the nanoparticles.1−6 Similarly, when a metal nanowire/rod was located above a metallic surface, the incident light was extremely concentrated in the region between the nanowire/rod and the metallic surface, which also led to strong enhancement of the field.6 Because of the elongated shape of the nanowire, the plasmonic enhanced structure is able to route and manipulate light and plasmons at the nanoscale. Thus, this structure has great potential in extensive applications including nanotechnology, microfabrication, spectroscopy, sensing, catalysis, biotechnology, and medical science.7−18 The position and orientation of a nanowire on a metallic surface are important to its chemical and physical properties. This motivated the development of manipulation technologies for nanowires. Conventional optical tweezers, in which field the gradient force drags an object toward the laser’s focal point, provide a versatile approach to trap and manipulate the orientation of elongated nanowire structures.19−26 However, these types of manipulations have been achieved in aquatic environments and at the water-dielectric interface, but not on

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© XXXX American Chemical Society

metallic surfaces, resulting in the inapplicability for plasmonic hybridization and field enhancement. In our previous works,3,27 focused plasmonic tweezers were demonstrated to be a convenient tool to manipulate metallic particles on a flat metallic surface with a high enhancement factor for surfaceenhanced Raman scattering (SERS) spectroscopy. This provides a potential means to manipulate nanowires on metallic surfaces with strong plasmonic field enhancement. In this Letter, we report the dynamic trapping and manipulation of a single metallic nanowire by controlling the hybridized plasmonic field. Plasmonic hybridization was achieved and controlled using dynamic linearly polarized laser excited plasmonic tweezers (LP-PT) to position and rotate the orientation of a single Au nanowire (Au nanowire) on a smooth Au film. To study the hybridization induced forces and torques in the LP-PT system, numerical analysis on the electric field distributions around the Au nanowires in the SPPs field excited with a linearly polarized laser (LP-SPPs) was performed using a three-dimensional finite-difference time-domain (3D-FDTD) method. The forces and torques were calculated by the Maxwell stress tensor (MST) method. The theoretical results revealed Received: August 3, 2014 Revised: September 30, 2014

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Figure 1. Schematic of the experimental setup. (a) Experimental setup of the linearly polarized laser that excited the plasmonic tweezers system. The inset is a transmission electron microscope image of the sample. (b) Schematic of a metallic nanowire being manipulated with the LP-PT. The incident linearly polarized direction is along the y axis. The Au nanowire in the excited SPP field was trapped at the center and was rotated by changing the direction of the linearly polarized light. (c) Three-dimensional distribution of the excited LP-SPP field on the Au film. In all cases, the incident wavelength was 1064 nm, the thickness of Au film was 45 nm and the Au nanowire had an axial diameter of 75 nm and a length of 1 μm.

samples from the top and they were directly imaged with a CCD camera. In the configuration, the most percentage of incident light will be reflected by the film back into the objective. When the linearly polarized laser beam was focused onto the bottom of the Au film, SPPs were excited at the surface plasmon resonance angle, resulting in a near-zero reflection at the excitation positions (see Supporting Information, Figure S1). As the Au film was located below the focal plane of the objective lens, the excited LP-SPP propagated toward the center and interfered constructively to form a uniform intensity distribution on the surface of the Au film (Figure 1c). This provided the potential required to trap an elongated nanowire at the two focal spots and orientate it by rotating the linear polarization direction of the incident light. Theoretical Calculation Methods. The electromagnetic hybridization between the LP-SPP and the LSP strongly influences the optical forces and torques exerted on a nanowire. Therefore, numerical analysis of the electric field distribution around the Au nanowires was performed at different positions and orientations in the LP-SPP field using a 3D-FDTD method. All of the FDTD simulation parameters were chosen to match the experimental conditions. The sample nanowire was a 1 μm long Au nanowire with a diameter of 75 nm and the Au film was 45 nm thick. As the distance between the nanowire and the film could not be determined exactly, a 5 nm gap (a typical length for electrostatic interactions, often used in similar computations3) was chosen in the model and will be shown to be appropriate in the following section. In all simulations, the Au nanowire was fixed along the y axis and the included angle between the long axis of the nanowire and the polarization direction was controlled by changing the polarization of the incident light, which was accomplished by rotating the 1/2 waveplate in the experiments. Based on the electric field distribution found using the FDTD method, the MST formula was employed to calculate the electromagnetic forces exerted on the Au nanowire. For the MST method, the three force components were denoted Fx, Fy, Fz. The time-average force, ⟨F⟩ can be written as28

that the hybridization effect between the LP-SPPs on a metal film and the LSPs on the nanowire led to strong trapping forces and large rotational torques on the nanowire. This was caused by the hybridization enhanced electric field in the narrow gap between the nanowire and the Au film. Using experiments, we show that an Au nanowire in a LP-SPP field in aquatic environment can be trapped at the center of the field and then rotated following the polarization direction of the incident laser beam, verifying the theorized expectations. Finally, a swallow tail structure composed of nanowires on a metal film is built to demonstrate the great potential of this proposed method in the fabrication of plasmonic structures for lab-on-a-chip nanodevices. Single Au Nanowire Manipulate Configuration. A schematic of the LP-PT configuration is shown in Figure 1, depicting a linearly polarized laser beam focused through a high-numerical-aperture oil immersion objective lens onto a smooth 45-nm-thick Au film to excite the LP-SPPs. Details on the experimental setup are outlined in Figure 1a. Figure 1b depicts a schematic of the manipulation of a metallic nanowire with the LP-PT system. When a linearly polarized laser beam was focused on the bottom of the Au film, the excited SPP propagated toward the center and constructively interfered on the Au surface to form two focal spots of the same size with uniform intensity distributions (Figure 1c) with an effective wavelength of 754 nm.27 This LP-SPP field has been verified experimentally previously.2 The positions of the two focal spots could be controlled by rotating the polarization direction of the incident light. A metallic nanowire near the LP-SPP field could be trapped at the center of the two focal spots and orientated by rotating the polarization direction of the incident light (Figure 1b). The incident laser beam (1064 nm) was linearly polarized and then expanded through a telescopic system to fit through the back aperture of the objective lens. Finally, its polarization direction was controlled by a rotatable 1/2 waveplate to manipulate the excited SPP field. The objective lens was an oil immersion with a high numerical aperture of 1.49 and a maximum incident angle of ∼79.6°, which satisfies the requirements for SPP excitation. The samples were Au nanowires (Nanopartz Inc., US) with an axial diameter of 75 nm and a length of 1 μm (shown in the inset in Figure 1a). They were diluted in water and injected into an in-housefabricated well. A white-light source was used to illuminate the

⟨F⟩ =

∮ {Re[(E·n)E*] − 4ε (E·E*)n +

B

μ μ Re[μ(H ·n)H*] − (H ·H*)n ds 2 4

}

(1)

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along the Au film was not confined to a small volume. A larger enhancement was observed in a single Au nanowire (Figure 2b), owing to the excited LSP mode that was confined around the nanowire. The largest enhancement occurred in the nanowire−film integrated configuration (Figure 2c), which was over 20 times higher than the Au film and approximately 3 times higher than the Au nanowire. This was because the hybridized field composed of the excited SPP and LSP modes was confined within the narrow gap between the nanowire and the thin film. In addition, the gap could act as a cavity, providing a periodic resonant mode (the periodic electric field distribution in the gap is shown in Figure 2c), further contributing to the field enhancement. Such a strong electric field in a narrow gap could lead to polarized electric charges and optical force distributions and demonstrates the potential for use in advanced scientific areas.12,18 Forces Exerted on an Off-Centered Au Nanowire. As mentioned above, strong plasmonic hybridization leads to optical forces acting on the Au nanowire. In most cases, the nanowire is off-center to the LP-SPP field. Here, a situation where the Au nanowire is fixed along the y axis, but 1 μm from the center with an included angle of 45° between the long axis of the nanowire and the polarization direction, is considered (Figure 3a). Figure 3b presents the corresponding force distribution around the off-centered nanowire in the y-z plane with the electric field distribution as the background. All of the other conditions were kept consistent with the above calculations. For the off-centered Au nanowire in Figure 3b, the total force along the y axis attracted the nanowire to the center of the LPSPP field and the total force along the z axis dragged the nanowire toward the Au film. Both forces were caused by the electric field gradient around the nanowire, which was enhanced by plasmonic hybridization, similar to our previous works on trapping metallic particles.3,27 As a result, all of the Au nanowires in the LP-SPP field were attracted and then stabilized close to the metal surface, at the center. Figure 3c shows sequential snapshots extracted from a CCD video (see Supporting Information Video 1) of the trapping of a single Au nanowire with the LP-PT system. Also, in Figure 3c, the orientation of the Au nanowire was rotated clockwise during the trapping process by the rotational torque in the x−y plane, which will be discussed in the following sections. Forces Acting on Au Nanowire at the Center of the Plasmonic Field. The experiments above verified that the off-

where ε and μ are the relative permittivity and relative permeability of the medium around the wire, respectively, n is the unit normal to the integral area, ds, which is outward pointing with respect to the Au nanowire. All of the electromagnetic field components required in the MST method were obtained directly from the FDTD simulation data. Plasmonic Hybridization Induced Field Enhancement. It is well known that an electric field can be enhanced by plasmonic hybridization.3,29−31 Here, the proposed geometry, featuring a long narrow gap between an Au nanowire and an Au film, promises to show strong plasmonic hybridization induced enhancement of the electric field confined to the small volume of the gap. To confirm this, the electric field intensity distributions of a thin flat Au film (Figure 2a), a single Au

Figure 2. Electric field intensity distribution determined using the 3-D FDTD method. Electric field intensity distributions for (a) a simple flat thin Au film, (b) a single Au nanowire, and (c) the proposed configuration. The white box and yellow line indicate the Au nanowire and Au film, respectively. In the calculations, the incident wavelength was 1064 nm, the polarization direction was along the y axis, the thickness of flat the Au film was 45 nm, the Au nanowire had an axial diameter of 75 nm and length of 1 μm, and the gap between nanowire and Au film was 5 nm.

nanowire (Figure 2b), and a nanowire−film integrated configuration (Figure 2c) were performed using the 3DFDTD method to compare the near-field enhancements. The enhancement in the thin flat Au film (Figure 2a) was the lowest among the three because the excited SPP mode propagating

Figure 3. Attractive forces exerted on an off-centered Au nanowire. (a) Map of an off-centered Au nanowire along the y axis, 1 μm from the center. The polarization direction of the incident laser had an included angle of 45° with the Au nanowire. (b) Force (white arrows) distribution around the Au nanowire in the vertical y−z plane. The background is the amplitude of the electric field. (c) Successive images of an Au nanowire trapped by the LP-PT, recorded using a CCD camera. The black crosses indicate the position of the plasmonic center. The length of scale bar is 2 μm. C

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Figure 4. Vertical forces acting on a centrally located Au nanowire in the LP-PT system. (a) Vertical forces (Fz) exerted on the Au nanowire at an included angle of 45°. The length and orientation of the arrows represent the magnitude and direction of the force. (b) Electric charge distribution along the nanowire and in the vertical y−z plane at an included angle of 45°. The background is the amplitude of the hybridized plasmonic electric field. The white arrows indicate the magnitude and direction of the electric field. (c) Amplitude of the electric field distribution along the y axis in the gap between the Au nanowire and the metal surface at various included angles. (d) Total vertical force as a function of the included angle from 0° to 90° with a step of 5°. The inset shows the definition of the included angle (θ), where the Au nanowire was placed in the center of the LP-SPP field and fixed in the y axis direction, θ was modulated by rotating the polarization direction of the laser.

Figure 5. Horizontal forces and torques on a centrally located Au nanowire. (a) Horizontal force components exerted on an Au nanowire positioned at the center of the LP-SPP field with an included angle of 45° in the horizontal x−y plane. The background is the resultant electric field amplitude. The green arrow indicates the polarization direction of the incident laser beam. (b) Hybridized electric field and charge distribution of an Au nanowire with the same included angle as (a). (c) The torques acted on the nanowire at different included angles between the Au nanowire and the polarization direction in radial coordinates. The radii of each point define the magnitude of the torque. The red and blue spots signify that the nanowire rotates clockwise and anticlockwise, respectively, caused by the torque. (d) and (e) Successive images of a single Au nanowire rotating clockwise and anticlockwise, respectively, stimulated by changing the incident polarization direction. The rotation direction is indicated by the black arrow and the length of scale bar is 2 μm.

vertical attractive force was mainly caused by the optical gradient force, which is essentially the electric charge induced Coulomb force for the Au nanowire in this system. In Figure 4b, the charge distribution alternated between positive and negative along the long axis of the nanowire. This is consistent with the periodic cavity resonant mode (Figure 2c). The Coulomb force induced by the positive charges in the nanowire was along the direction of the electric field. Meanwhile, the Coulomb force induced by the negative charges in the nanowire was in the opposite direction. Both resulted in strong attractive forces on the nanowire, pulling it toward the metallic surface, which agrees with the vertical force distribution in Figure 4a. Similar attractive vertical forces have been reported in other plasmonic particle−film structures.3,27,28 The distribution of the electric field and the corresponding vertical force at other

centered nanowires would be trapped at the center of the plasmonic field. Figure 4 shows the vertical force distributions around the centrally positioned Au nanowire in the y−z plane. Figure 4a demonstrates the time averaged vertical forces (Fz) exerted on the centrally positioned Au nanowire with an included angle of 45°. Figure 4b shows the corresponding electric charge distribution along the gap at 45°. To clearly show the contribution from the hybridized plasmonic field, the resultant electric field amplitude distributions, calculated by FDTD, are presented as the background. Avoiding the nanowire in the light field being pushed away (Supporting Information Figure S2), the vertical force on the Au nanowire (calculated with the MST method) was mainly exerted toward the nether Au film (Figure 4a), especially at the enhanced field positions in the gap, indicating that the nanowire could be attracted to the metal surface by the plasmonic field. This D

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torque, the forces exerted on the nanowire were superimposed to obtain the total forces in the x and y directions

included angles are provided in Supporting Information Figure S3. The orientation of the nanowire had an enormous influence on the plasmonic hybridization. Figure 4c compares the distributions of the electric field amplitude inside the gap with different included angles between the nanowire and the polarization direction (indicated in the inset in Figure 4d). The periodic distributions observed remained unchanged for all included angles, corresponding to the cavity resonant mode of the gap. The electric field amplitude was a maximum at 0° and a minimum at 90° because the plasmonic hybridization was strongest when the nanowire was located coincident with the two focal spots of the LP-SPP field (Figure 1c) (0°), whereas it was weakest when the nanowire was located at the normal direction of the LP-SPP (90°). Figure 4d shows the total vertical force (Fz) of the nanowire as a function of the included angles, indicating that the plasmonic hybridization induced force was in the z direction, toward the Au film, and decreased with an increasing angle. This agrees with the results for the amplitude of the electric field in Figure 4c. Here, the force in the z direction always produced an attractive force, pulling the nanowires toward the Au film. This is distinct from the radiation pressure force in conventional laser tweezers, which pushes the metallic particles away (Supporting Information Figure S2). The force was essentially the Coulomb force because the polarized electric charges were induced by plasmonic hybridization. Analyzation of the Torque on Au Nanowire. To rotate the orientation of the nanowire, the plasmonic hybridization that induced asymmetric distributions of the electric field, force and torque in x−y plane was studied (Figure 5). Similar to Figure 4a and b, Figure 5a shows the horizontal forces (Fx + Fy) exerted to the Au nanowire in the x−y plane at an included angle of 45°. Figure 5b presents the corresponding polarized electric charge distribution, where the background is the electric field amplitude distribution. The contributions from the hybridized horizontal electrical component to the forces in the x−y plane are compared in Figure 5a and b. An included angle of 45° between the LP-SPP (along the incident polarization direction) and the LSP (along the long axis of the nanowire) in the x−y plane resulted in asymmetric plasmonic hybridization, giving rise to the nonuniform electrical field distribution around the nanowire. As a result, the induced force exerted on the left side of the nanowire mainly pointed in the −x direction (Figure 5a) and pointed mainly in the +x direction on the right side. This provided a torque, causing the nanowire to rotate clockwise. This property of the horizontal force can be explained by the Coulomb force induced by the polarized charges in the nanowire. Figure 5b shows that the positive charge on the left side, near the center was mainly subjected to a Coulomb force of −x by the strong hybridized electric field. Meanwhile, the negative charge on the right side was subjected to a +x Coulomb force, in accordance with the force distribution in Figure 5a. The distributions of the force and electrical field at other included angles are shown in Supporting Information Figure S4, further confirming that the horizontal force rotated the nanowire until it was perpendicular to the polarization direction of the incident light. The asymmetric force acting on the Au nanowire could induce a rotational torque around the optical axis.32−34 The above results revealed that the vertical forces did not contribute to the torque. Therefore, the trapped nanowire could rotate only in the horizontal x−y plane. To study the properties of the

Fxi ,j =

∑ Fx

i ,j ,k

and Fy = i ,j

k

∑ Fy

i ,j ,k

k

(2)

where Fx and Fy are the forces along the x and y axes, respectively, and (i, j, k) relates to the position in threedimensional space. Now, the torque for the nanowire in x−y plane can be written as Txy = Fxi ,j ·(yi , j − y0,0 ) + Fy ·(xi , j − x0,0) i ,j

i ,j

(3)

where (x0,0, y0,0) is the center of the nanowire, (0, 0) in our coordinate system. Therefore, the total torque of the whole nanowire could be derived as Ttotal =

∑ ∑ Txy

i ,j

i

j

(4)

which is the immediate cause of the angular acceleration and is necessary to rotate and align a trapped object. Figure 5c shows the rotational torques acting on the Au nanowire at different included angles. The orientation of the Au nanowire was fixed along the y axis (see inset in Figure 4d) and modulated the included angle by rotating the incident polarization. When the angle increased from 0° to 90°, although the horizontal force in the x−y plane always decreased (similar to the vertical force in Figure 4d), the relevant torque increased from near zero at 0° to a maximum around 45°. It then declined to near zero at 90°, indicating that the Au nanowire would rotate when it was not parallel or perpendicular to the incident polarization direction. Figure 5b and Supporting Information Figure S4 show that the nanowire would be rotated perpendicular to the incident polarization. That is, the nanowire rotated clockwise when the included angle was between (0°, 90°) and (180°, 270°) and anticlockwise for all other angles. There was also a balance when the nanowire was parallel to the polarization direction. This balance was formed by two strong opposite forces (see Supporting Information Figure S4a). Thus, a slight disturbance could break the balance and cause the nanowire to rotate until it was perpendicular to the polarization direction. As a result, the nanowire would be stable when it was trapped perpendicular to the polarization direction, in concordance with a previous report on rotating elongated structures in linearly polarized laser tweezers.19 To verify the theoretical results, Figure 5d and e show experimental snapshots of an Au nanowire that was rotated clockwise and anticlockwise, following the rotation of the incident polarization direction (see Supporting Information Videos 2 and 3). This demonstrated that the orientation of the Au nanowire could be controlled by the plasmonic hybridization induced asymmetric horizontal forces in the LP-PT system. Assembly of Plasmonic Structures. Metallic nanowire structures have great potential in quantum optics, optical characterization, spectroscopy, and sensing applications.35−39 The nanocavities between the metallic nanowire and metallic substrate should be capable of achieving an enhanced spontaneous emission, which is applicable to a wide variety of emitters with high cavity quantum electrodynamics effects and enables a host of potential applications in quantum optics.35 Meanwhile, to efficiently manipulate light on the nanometer scale, the metallic nanowire itself can serve as a waveguide to propagate plasmons. This has the advantages of E

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Figure 6. Nanowire junctions and assemblies built using a LP-PT. (a) Schematic of the swallow tail structure assembly consisting of two Au nanowires. (b) Successive images of the trapping and assembly of two single Au nanowires. The blue arrows indicate the position and orientation of the manipulation-free Au nanowire and the black arrows indicate the fixed nanowire. The upper row shows the process of trapping a free nanowire near a fixed nanowire on the metal film. The lower row presents the rotation and positioning procedures. A free Au nanowire is initially trapped near a fixed one and then rotated to align with the adjacent tips on the smooth Au film. The length of scale bar is 2 μm.

to our previous work.27 Thus, the influence of thermophoresis could be ignored, though the weak convection could help to draw the nanowire to the center of LP-SPP field. Hence, in our LP-PT system, the manipulation of the nanowire was mainly caused by the forces and torques induced by the hybridized plasmonic field. Furthermore, the force distribution of multiple Au nanowires with length of 500, 800, and 1500 nm were calculated (see Supporting Information Figure S5). The results show that the length is not a decisive factor for the direction of forces and torques, and similar manipulation is still valid for other lengths. In summary, this study has demonstrated a robust configuration of plasmonic hybridization to obtain strong electromagnetic enhancement to accurately trap and manipulate a single metallic nanowire/rod on a metallic film. In the proposed LP-PT system, the SPP excited on a metal film and the LSP excited around a nanowire hybridized in the narrow gap between them, leading to a hybridized enhanced electric field, strong trapping forces, and large rotational torques on the nanowire. Trapping forces acting directly on the Au film stably captured a nanowire close to the metallic surface and the unbalanced forces in the horizontal x−y plane induced a large rotational torque to control the orientation of the trapped nanowire. Numerical analysis of the electric field distribution calculated with 3D-FDTD simulations and the forces calculated by the MST method agreed with the experimental results. Accordingly, the proposed system could be employed to position and rotate a metallic nanowire and other elongated metallic objects from the nanometer to the micrometer scale. Thus, this study has great potential for use in various applications owing to the wide use of metallic nanowires in the frontiers in science and technology. This work could provide a profound influence on manipulating elongated metallic structures and have an important contribution to plasmonic lab-on-a-chip devices for quantum optics, optical characterization, spectroscopy, and sensing applications.

localizing the electromagnetic energy in subnanoscale regions and guiding the propagation direction by controlling its position and orientation.36,39 Moreover, standing surface plasmon waves along the nanowire axis (discussed in Figure 2) have potential for the application of nanowires as efficient surface plasmon Fabry−Perot resonators, constituting a basic element for a variety of nano-optical functionalities. 8 Furthermore, metallic nanorod/nanowire-structured coupled plasmonic antennas37 could produce high intensity enhancement, localized in the gap in the optical near-field, which is potentially useful in many applications including optical characterization, optical data storage and information processing, and manipulating nanostructures. Building plasmonic structures composed of nanowires on a metal film will have great potential in many applications. Our theoretical and experimental results have shown that, based on the plasmonic hybridization induced forces and torques, LP-PTs have the capacity to trap metallic nanowires at the center of a LP-SPP field and to control the position and orientation of a trapped nanowire on a metallic surface. Figure 6 shows a plasmonic swallow tail structure with two nanowires on an Au film. The controllable included angle between the two nanowires was 3π/4. Figure 6a shows a schematic of the assembly consisting of two Au nanowires. Figure 6b shows successive images of the attraction (upper row) and assembly (lower row) of Au nanowires (see Supporting Information Video 4). For simplification, an Au nanowire fixed on an Au film was chosen as the target location and a free nanowire was attracted to the target by LP-PT trapping. The position and orientation of the trapped nanowire was then manipulated by modulating the position and polarization direction of the incident light. The included angle between the two nanowires was controlled as the orientation of the polarization direction was rotated. This process could be repeated to fabricate more complex nanowirebased constructions. This provides experimental evidence of the great potential of the LP-PT system in the micro/nano fabrication for lab-on-a-chip plasmonic devices. In addition to the optical forces and torques studied above, it is well known that heating (including thermophoresis and convection) also influences some plasmonic trapping systems.40 In our system, the power of the incident linearly polarized laser ranged from 60 to 100 mW and only a small amount could couple to the SPP (shown as the dark arcs in Supporting Information Figure S1) and contribute to the heating effect. Meanwhile, the heat was rapidly conducted to the whole Au film owing to its high thermal conductivity. As a result, the temperature increase was maintained on the order of several Kelvin (2.01 K, see Supporting Information Figure S6), similar

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ASSOCIATED CONTENT

S Supporting Information *

Details about the excited LP-SPP, forces, and torque analysis on Au nanowires and the temperature distribution and thermal convection in the LP-PT system. Movie clips show manipulation processes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. F

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Author Contributions

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These authors contributed equally to this paper.

Author Contributions

X.Y. initiated and supervised all of the work and oversaw the manuscript. C.M. carried out the theoretical derivation and analysis and assisted in analyzing the results and revising the manuscript. Y.Z. carried out the experiments and prepared the manuscript. J.W. performed the theoretical calculations and contributed to drafting the manuscript. J.S., Z.M., and G.Y. performed some of the simulations. W.S. helped in carrying out some of the experiments. S.Z. and H.P.U. participated in designing the project and provided editorial input. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China under Grant numbers 61036013, 61138003, 11204141, and 61377052. X.Y. acknowledges support from the Ministry of Science and Technology of China under National Basic Research Program of China (973) grant No.2015CB352004 and the start-up funding at Shenzhen University.

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dx.doi.org/10.1021/nl502975k | Nano Lett. XXXX, XXX, XXX−XXX