Plasmonic Trapping with a Gold Nanopillar

Transcription

1 DOI: /cphc Plasmonic Trapping with a Gold Nanopillar Kai Wang and Kenneth B. Crozier* [a] An improved ability to manipulate nanoscale objects could spur the field of nanotechnology. Optical tweezers offer the compelling advantage that manipulation is performed in a non-invasive manner. However, traditional optical tweezers based on laser beams focused with microscope lenses face limitations due to the diffraction limit, which states that conventional lenses can focus light to spots no smaller than roughly half the wavelength. This has motivated recent work on optical trapping based on the sub-wavelength field distributions of surface plasmon nanostructures. This approach offers the benefits of higher precision and resolution, and the possibility of large-scale parallelization. Herein, we discuss the fundamentals of optical manipulation using surface plasmon resonance structures. We describe two important issues in plasmonic trapping: optical design and thermal management strategies. Finally, we describe a surface plasmon nanostructure, consisting of a gold nanopillar that takes these issues into consideration. It is shown to enable the trapping and rotation (manual and passive) of nanoparticles. Methods by which this concept can be extended are discussed. 1. Introduction As predicted by Maxwell s equations, light fields carry momentum and thus can apply forces on objects through scattering and absorption processes. However, the relatively small momenta that occur for non-damaging optical powers mean that these forces are of little consequence for most situations. For example, a mirror in vacuum will experience an optical force of F optical ¼ 2 P=c when reflecting a normally-incident laser beam of power P by 100 % (c is the light speed in vacuum). The magnitude of optical force can therefore be estimated to be in the piconewton range for a laser beam with a power of 1 mw. While this is of little importance in most situations, this is not the case for many investigations in nanoscience and related areas, where forces in the piconewton range are highly [1, 2] suitable. The optical trapping and manipulation of colloidal particles was pioneered by Arthur Ashkin in the early 1970s. He invented the now well-known method of optical tweezers, in which a tightly focused laser beam is used to trap small objects near its focus in all three dimensions. [3] Because of its non-invasive nature and highly spatially localized interaction, optical tweezers soon proved to be powerful tools for a wide range of applications, such as single molecule force spectroscopy, particle sorting and for driving microscale motors. [1, 4 6] Optical trapping based on far-fields, for example, traditional optical tweezers based on optical microscopes, faces performance limitations due to the diffraction limit. [7] This makes it difficult to trap and position objects, and has led to a burgeoning interest in nearfield-based optical-trapping methods. [8 16] Satoshi Kawata et al. demonstrated the optical trapping and propulsion of particles by evanescent fields generated by total internal reflection. [8] Optical manipulation has been performed using waveguides, ring resonators, photonic crystal resonators, and channel/slot waveguide junctions. [9 16] Theoretical investigations have been carried out on trapping with these types of structures. [17] In addition to enhanced optical performance, these structures present the opportunity for large-scale integration, that is, optical trapping on-a-chip. This possibility also motivated the development of microfluidic chips with integrated Fresnel zone plates for optical trapping. [18 20] Compared to photonic structures based on dielectric materials, surface plasmon resonance structures permit light to be strongly localized at deep sub-wavelength scales. Surface plasmon structures offer additional advantages for their use in integrated optical manipulation chips. The resonances are broad-band, fabrication does not require many steps, and the devices have small footprints. The concept of nanometric plasmonic tweezers was proposed by Novotny and et al. [21] and many efforts have been made in this field since then. [22 28] However, the heating associated with the Ohmic loss accompanying plasmon excitation can have substantial effects, making tight and stable plasmonic trapping difficult. [29,30] In some cases, the illumination of a plasmon structure with a laser beam of moderate power can even result in boiling the water, in which the optical trapping is performed. [31] This motivates the development of thermal management approaches for plasmonic optical traps. [31] Herein, we first describe the theory of optical trapping using the dipole approximation. Based on this theory, we then discuss the general optical design considerations for plasmonic trapping, and how the combination of these considerations with those relating to thermal management leads to a plasmonic trap design comprising a gold nanopillar protruding from a gold film. We describe how these devices can be fabricated using template stripping. Finally, we present experimental results for which the device is used in three basic modes of [a] Dr. K. Wang, Dr. K. B. Crozier School of Engineering and Applied Sciences Harvard University, 29 Oxford St. Cambridge MA (USA) kcrozier@seas.harvard.edu ChemPhysChem 2012, 13, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2639

2 K. B. Crozier and K. Wang nanoparticle manipulation: trapping, manual rotation and passive rotation. 2. Optical Design To facilitate the understanding of optical trapping, optical forces are traditionally decomposed into two components: 1) a scattering force, in the direction of light propagation and 2) a gradient force, in the direction of the spatial gradient of the intensity. The concepts of scattering and gradient force are especially useful for the theoretical description of optical trapping of small particles, where the Rayleigh scattering approximation holds and the dipole approximation can be applied. In this case, the optical forces can be calculated by treating the particle as a point dipole. Under an external field, the dipole moment of a particle that is small compared to the wavelength is given in Equation (1): n o P * ðr; tþ ¼Re a 0 þ ia 00 ÞE * ðrþe iwt where P * is the induced dipole moment, a and a are the real and imaginary parts of the particle s polarizability, and E * ðrþ is the external electric field. The instantaneous optical force experienced by the induced dipole can be calculated as the total Lorentz force on the two point charges that comprise the dipole and is written as shown in Equation (2): [32] F * ðr; tþ ¼ P * ðr; tþ ðr; tþr E * B * ðr; tþ where B * ðr; tþ is the magnetic field. In a time-harmonic field at frequency w, the averaged optical force can be written in phasors [Eq. (3)]: ð1þ ð2þ E * ðrþr E * * ðrþ ¼ re * ðr; ¼ þ ðrþ re * * ðrþ E* The first part of Equation (3) originates from field inhomogeneities and is proportional to the electric field intensity gradient. It is therefore termed the gradient force. The second part is related to the imaginary part of the polarizability, and termed the scattering force. With radiation damping taken into account, the polarizability of a small sphere with radius of a (a l) is given by Equation (6): [33] A a ¼ 4pe e ik3 A, where A ¼ e a3 p e e, k ¼ 2p e p þ 2e e l e e and e p are the dielectric constants of the environment and the particle, respectively. It can be seen that the polarizability has an imaginary part even if e p is real, that is, the particle is made of a lossless dielectric. This imaginary part contributes to the scattering process. The gradient optical force on a small dielectric particle can be found with Equation (7): [34] < F * gradðrþ > pe e a e 3 p e e r E * ðrþ e p þ 2e e In a plane wave field of intensity I 0 propagating in direction of s *, the scattering force on this particle can be found to be [Eq. (8)]: [34] < F * scatðrþ > 128p5 a 6 3l 4 c 2 ð4þ ð5þ ð6þ ð7þ e p e 2 e I e p þ 2e 0 ðrþs * ð8þ e < F * ðr; tþ >¼ 1 h i 2 Re ðrþr E * * * ðrþþiwp ðrþb * * ðrþ P* ¼ 1 n h io 2 Re ða0 þ ia 00 Þ E * ðrþr E * * * ðrþþiwe ðrþb * * ðrþ 8 22 * 3 39 E * E* ¼ 1 >< 2 Re ða0 þ ia 00 * Þ E * E* ðrþ þ ðrþð iwþb * * * ðrþþiwe ðrþb * * >= ðrþ 6 4 E* 7 5 >: * E * E* ðrþ ¼ a0 >< 4 r ðrþ 2 a00 E* 2 Im 6 @z >= 7 5 >; Vector Equation (4) and Maxwell Equation (5) are used in the above derivation: ð3þ The optical gradient force, as described in Equation (7), is conservative while the scattering force is not. To achieve stable optical trapping, the gradient force needs to overcome the scattering force. This can be done by creating a large field gradient, for example by using a high numerical aperture (NA) oil immersion objective to focus the light to a diffraction limited spot. From Equation (7) it can be seen that the optical gradient force scales as the third power of the particles radius. It therefore becomes increasingly difficult to trap particles tightly with optical tweezers as the particle size decreases. Plasmon resonance structures, which are capable of focusing light to deep sub-wavelength scales, can potentially increase the optical force and the precision to which the trapped particles are localized. It is the optical-trapping potential depth, however, rather than just the optical force, that determines whether trapping is stable. The optical-trapping potential depth has to be large compared with thermal energy for stable trapping, and is the Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13,

3 Plasmonic Trapping with a Gold Nanopillar path integral of the optical restoring force. If we assume that the field decreases to zero as the distance from the surface approaches infinity, then the optical-trapping potential depth can be written according to Equation (9): U trap ¼ surface Z 1 pe e r e 3 p e e rjej 2 dx ¼ pe e p þ 2e e r e 3 p e e je e e p þ 2e surface j 2 ð9þ e From this expression, we can see that the field intensity on the surface determines the depth of the trapping potential. Thus, it is important to achieve not only a strong field confinement but also to maximize the field enhancement as much as possible. These two requirements are satisfied by the plasmon resonance structures illuminated on resonance. In summary, the strong field confinement and enhancement provided by plasmon resonance structures lead to the following advantages over traditional far-field-based optical-trapping methods: 1) The narrower optical potential of the plasmonic trap (Figure 1 a) means that trapped particles are localized more tightly. the evanescent field. To take this effect into consideration, a first-order correction can be applied to Equation (6), where the particle is approximated as a collection of dipoles instead of a single dipole. Since the real part of the particle s polarizability a is proportional to the volume of the particle, as shown in Equation (6), it is possible to define the polarizability density and express the optical gradient force in the form of volume integration [Eq. (10)]: [35] < F * gradðrþ >¼ 3e e 4 Z e p e e r E * 2 dv e p þ 2e e V ð10þ Where V represents the space occupied by the particle. Using this equation, three cases can be considered, where the evanescent fields have the same field intensity on the surface but different depths, as shown in Figure 2. Figure 2. Effect of the field depth in plasmonic trapping. The field depth is a) much larger, b) comparable, and c) smaller than the size of the particle. Yellow curves are the field gradient of the evanescent field. Figure 1. Comparison of optical tweezers and plasmonic trapping. a) Optical-trapping potential shape (blue curve) of traditional optical tweezers (left) and of plasmonic trap (right), and the corresponding probability distributions of the position of the trapped particle (red curves). b) Optical-trapping potential created by two closely spaced coherent optical-trapping sites (dashed curve for positions of each site) in traditional optical tweezers (left) and in plasmonic trap (right), and the corresponding probability distributions of the position of the trapped particle (red curves). 2) An optical potential with two wells that are closely-spaced (compared to the wavelength) is possible with plasmonic trapping (Figure 1 b). This may be termed as super resolution manipulation. 3) The laser power required for stable trapping is reduced with plasmonic trapping due to the field enhancement. All the analyses so far are based on the point-like dipole approximation, where the finite size of the particles is not considered. In real cases, the size of the particle will have an important effect when it is comparable or larger than the depth of For the case that the size of the particle is smaller than the evanescent field depth, as shown in Figure 2 a, the integration of the field gradient over the volume (indicated by the yellow curve in Figure 2) tends to be small because the field gradient is small. Thus, the optical force on the particle is weak, as manifested in Equation (10). By increasing the localization of the field to be comparable in size with the particle, as shown in Figure 2 b, the field gradient becomes larger and the optical force is increased. If the evanescent field is further localized such that the field drops to zero over a small portion of the particle, as shown in Figure 2 c, the overall optical force is expected to decrease because the volume of the particle interacting with the field is small. From this perspective, there exists an optimal field depth to maximize the optical force on a particle with a certain size. To the accuracy of the first-order approximation, the optical force is maximized when the field depth is comparable with the size of the particle. In the similar way, the optical-trapping potential depth can also be expressed in the volume integration form, as shown in Equation (11): U trap ¼ 3e e 4 Z e p e e e p þ 2e e surface 1 Z V re * 2 dvdx ¼ 3e e e p e e 4 e p þ 2e e Z V surface * E 2 dv ð11þ Here, V surface represents the volume occupied by the particle when it is trapped on the surface. From this equation, it can ChemPhysChem 2012, 13, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4 K. B. Crozier and K. Wang be seen that the trapping potential depth U trap is proportional to the integration of electrical field intensity over the volume V surface. Among the three cases shown in Figure 2, the largest field depth, shown in Figure 2 a, gives maximum trapping potential depth if the electric field intensities on the surfaces are the same. However, the field enhancement on the surface in different designs varies greatly. In the design of plasmon resonance structures, there is a trade-off between peak value of field enhancement and the field depth. This is because, if the energy stored in the near-field is fixed, a stronger peak-field enhancement means that the field depth is smaller, and vice versa. The energy stored in the near field is limited by the quality factor of the plasmonic resonator. Increasing the quality factor of the plasmonic resonator is therefore one way to improve the performance of plasmonic trapping, while another way involves engineering the near-field distribution to make it optimal for trapping a particle of a given size. When the particle has a large polarizability and disturbs the near field around the plasmon resonance structure, the firstorder approximation is no longer appropriate, because the fields are quite different, depending on whether the particle is present or absent. In this case, the optical force cannot be calculated according to Equations (7) and (10) using the field obtained with the particle absent from the plasmon resonance structure, and other methods must be used. [17] The trapping of large metal particles (e.g. gold and silver) is challenging using traditional optical tweezers because of the relatively large scattering force, [35] while it can be readily carried out with plasmonic traps. [21,25] When the metal particle to be trapped is close to the plasmonic trap, the small gap between them can support a mode (gap mode). Charges with opposite signs accumulating on either side of gap provide a strong attractive force which makes trapping very efficient. To summarize, plasmonic trapping provides three main advantages over traditional optical tweezers for manipulating small particles: stronger localization, super-resolution manipulation, and a reduction in the laser power required for stable trapping. Depending on the size of the particles, there are different optical design considerations. For trapping of a particle with finite size there is a trade-off between field enhancement and field depth. For trapping of dipole-like particles, such as molecules, one would aim for the highest possible field gradient and field enhancement as the molecules are small compared to the field depth. The largest optical force and opticaltrapping potential depth are achieved when the field depth is comparable to the particle size. Additionally, plasmonic trapping could be very effective for trapping metallic particles if the resonances of the plasmonic trap and particle are both excited. 3. Thermal Management Thermal side effects are an important issue in plasmonic trapping. The excitation of surface plasmons leads to strong oscillations of electrons of the metal, resulting in heat generation. This is an important issue because it can lead to two effects that prevent stable trapping: 1) Thermal convection, also known as natural convection or free convection, occurs because the density of water varies with temperature. In plasmonic trapping experiments, the water around the plasmonic device is heated and therefore flows upward due to buoyancy, assuming that the water is above the plasmonic device. This hot water is then cooled down by heat exchange with the chamber walls and flows back to the plasmonic device, thereby forming a convective flow pattern. Since the convection flow exerts a Stokesdrag force on particles, it can destabilize the optical-trapping process. Thermal convection has been observed in many works in the field of plasmonic trapping. 2) Overheating of water may occur, even if the total heating power is low. The boiling of water can result from localized heating if the structure does not provide a mechanism for heat to dissipate. We observed that bubbles can be generated by plasmonic devices illuminated by a focused laser beam with an illumination intensity of 8 mw mm 2. [31] Figure 3. Models for numerical calculations of temperature distribution and convection-velocity field. a) 3D model of the plasmonic trapping system. b) Simplified 2D model. In order to understand, and therefore minimize, the heating accompanying plasmon excitation, we modeled it numerically. The numerical modeling of heating-induced side effects in plasmonic trapping has been developed before using a complex transient model. [36] Here, we simplified the problem using a static model. The geometry is shown in Figure 3 a. A circularly-shaped gold disk sits on a substrate in the center of a water-filled chamber. Because the geometry is axially symmetric, the three-dimensional (3D) model of Figure 3 a can be simplified to a two-dimensional (2D) model, as shown in Figure 3 b. Due to the small chamber dimensions (~1 mm) and the slow convection-flow velocity (~ 10 mms 1 ), the Reynolds number in this system is very small (~0.01). A stable convection-velocity field in the laminar region can therefore be solved by finding the steady-state solutions to the Navier Stokes equations. Using the Boussinesq approximation, it can be further simplified to the problem of incompressible flow with an extra volume force induced by buoyancy, as shown in Equation (12): [37] 1ðv * rþv * ¼ rp þ hr 2 v * þ F * ð12þ where 1 is the density of the fluid, v * is the fluid velocity, p is the pressure, h is the dynamic viscosity of the fluid and F * is Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13,

5 Plasmonic Trapping with a Gold Nanopillar the body force due to buoyancy, as given in Equation (13): [37] F * ¼ 1 0 g * bðt T 0 Þ ð13þ where 1 0 is the reference fluid density at reference temperature T 0, g * is the acceleration due to gravity, b is the volume thermal-expansion coefficient and T is the fluid temperature. Equation (12) describes the momentum conservation of the fluid. The complete description of fluid dynamics requires the mass-conservation equation. It has a simpler form for an incompressible flow: [37] rv * ¼ 0 ð14þ Equations (12) and (13) are coupled by the fluid temperature T, which is determined by the heat balance, as described by the conduction-convection Equation (15). [37] 1C p v * rt rðkrtþ ¼0 ð15þ Figure 4. Thermal modeling of large gold pad on glass. Temperature distribution and convection-velocity field obtained by FEM simulation. The gold pad has a diameter of 10 mm and a thickness of 50 nm. The simulation approximates the case of SPPs excitation on an unpatterned film with an illumination intensity of 10 mw mm 2. where C p is the specific heat capacity of the fluid and k is the thermal conductivity. Combining Equations (12) (15), both the temperature distribution Tðr * Þand the convection-velocity field v * ðr * Þcan be obtained. We did this numerically with the finite-element method (FEM), using the fluid-dynamics module of the software package Comsol Multiphysics. In the numerical calculations, the outer boundaries were set to room temperature and the gold pillar was the only source of heat. The absorbed power, which is converted to heat, was calculated by P heat ¼ s abs I, where s abs is the absorption cross-section, obtained from the finite-difference time-domain (FDTD) simulations, and I is the illumination intensity. We made the assumption that the heating power is uniformly distributed throughout the volume of the gold nanopillar, which is reasonable due to the high thermal conductivity of gold (k gold = 318 Wm 1 K 1 ). [38] The chamber dimensions shown in the Figure 3 b were used for the simulations as these are close to those employed in the experiments. Since plasmonic structures are frequently formed on glass substrates, due to their excellent optical properties and low cost, we followed this convention and took both chamber cover and substrate to be made of glass (k glass = 1Wm 1 K 1 ). [38] The first case we considered was that of optical trapping and propulsion with surface plasmon polaritons (SPPs) excited on an unpatterned gold film. [22] To approximate this case, the configuration shown in Figure 3 b was used, with the radius of the disk taken as 10 mm. We assumed 100 % absorption of the illumination power incident onto the disk. The calculated temperature distribution and convection-velocity field are shown in Figure 4. Even with a relatively low illumination intensity of 10 mw mm 2, the excitation of SPPs could be seen to result in a strong thermal convection. As depicted in Figure 4, the convection flow forms a circulating pattern. Water flowed upward into the center of the chamber, where the heat source is located, guided by the upper chamber wall, and then flowed downward at locations away from the center. The largest convection velocity occured on top of the gold pad at the center of the chamber. The temperature rise on the surface of the gold pad was about 50 K. The second case we considered was for a configuration that has been used in the past for plasmonic trapping, and consists of a particle supporting localized surface plasmons (LSPs) on a glass substrate. The particle comprises a gold disk (R p = 100 nm, H =40 nm), and has a plasmon resonance at a freespace wavelength of 980 nm. The absorption cross-section was found to be 0.04 mm 2 using FDTD simulations. Under a moderate illumination intensity of 1 mwmm 2, the heating power was therefore 40 mw. The simulation results are summarized in Table 1. It can be seen that the temperature increase was the main problem, with thermal convection being much smaller than in the SPP trapping experiments. Table 1. Summary of modeling of convection velocity and temperature rise accompanying SPPs and LSPs excitation on different substrates. Substrate Cases R p [mm] I [mw mm 2 ] P heat [mw] n max [mms 1 ] DT max [K] Glass SPP Glass LSP Diamond SPP Diamond LSP Although the illumination intensity chosen for the SPP case was small, the power dissipated as heat was high due to the gold disk being large. Since the heating is generated over a large area with low density, the temperature rise is moderate, while the thermal convection velocity is substantial due to the high power dissipation. By comparison, in the LSP case, the temperature rise was high but the thermal convection was weak. In this case, the illumination has a high power density, but the total power dissipated as heat is small due to the gold disk having a small diameter. In conclusion, thermal convection is the major problem in plasmonic trapping experiments using SPPs, while excessive temperature increase of the water is the limiting factor for LSP trapping experiments. Various ways of reducing the thermal convection in SPP trapping experiments have been proposed and demonstrated, such as reducing the size of the gold pad to lessen the heating ChemPhysChem 2012, 13, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

6 K. B. Crozier and K. Wang power and using a shallow chamber to suppress the convective-flow velocity. However, the improvements provided by these means are limited. Moreover, neither of these two ways can be applied to LSP trapping experiments, where heat is highly localized. Here, we propose to employ a high-thermal conductivity material as substrate to reduce thermal-side effects. According to the simulation results of Table 1, the thermal-side effects can be reduced substantially if diamond (k diamond = 1000 Wm 1 K 1 ) [38] rather than glass is used as substrate. One can obtain roughly the same convection velocity and temperature rise with a diamond substrate when the illumination intensities are increased 300-fold and 250-fold compared to the glass-substrate case for the SPP and LSP trapping cases, respectively. It is demonstrated in the Experimental Results Section that the use of a high-thermal conductivity substrate is an effective and practical means for thermal management in plasmonic trapping. 4. Design and Fabrication of Gold Nanopillar for Plasmonic Trapping Figure 5. Electric-field intensity enhancement ( jej 2 = je INC j 2 ) distribution around a circular gold disk on two different substrates, as calculated by FDTD. a) Substrate is glass, and the disk is 50 nm thick and 200 nm in diameter. b) Substrate is diamond (n=2.54), and the disk is 50 nm thick and 140 nm in diameter. Both disks resonate at l=980 nm, at which the field is calculated. Figure 6. Gold nanopillar tweezer. a) Plasmonic nanotweezer comprising a nanopillar formed on a gold film. The underlying copper film and silicon substrate act as heat sink, conducting heat from the nanopillar to the substrate, thereby minimizing water heating. The nanopillar diameter D P is 280 nm, and the height H is 130 nm. b and c) FDTD calculations of the electric-field intensity distribution resulting from normal-incidence plane-wave illumination polarized along x axis (E INC )atl = 974 nm. Adapted from ref. [31]. Metal structures, such as nanospheres, nanorods, and bowtie optical antennas can support strong localized surface plasmon resonances. In many applications, these are formed on dielectric substrates. However, the introduction of a dielectric substrate affects the near-field distribution, and can result in a large portion of high-intensity region of the field being embedded in the substrate, as shown in Figure 5a. This is not favorable for plasmonic trapping, as the particles to be trapped cannot access these high-intensity regions that are buried in the substrate. It is very difficult, if not impossible, to avoid this problem, whenever dielectric substrates, especially those with high refractive indexes, are employed. For example, a diamond substrate can significantly mitigate deleterious thermal effects, but has a higher refractive index (n diamond ~2.4) than glass, exacerbating the problem of the high-intensity region being buried in the substrate (Figure 5 b). To reconcile the conflict between thermal management and optical near-field optimization in plasmonic trapping, we propose a new design which comprises a gold nanopillar protruding from a gold film (Figure 6a). The nanopillar supports a localized surface plasmon resonance, enabling field confinement and enhancement (Figure 6 b, c). That the nanopillar is formed on a gold film rather than a glass substrate presents an advantage for thermal management, as gold has a thermal conductivity approximately two orders of magnitude higher than glass. Rather than fashioning the entire device from gold, we employed a copper layer and silicon substrate under the gold layer, as these have excellent thermal conductivity yet are lower in cost than gold. As the nanopillar is formed on a metal, rather than a dielectric, the surface plasmon resonance present in this structure is a little different from those reported in previous plasmonic trapping designs. [25,27,28] To gain physical insight into this kind of structure, we first considered the resonances of a gold nanopillar that is longer than the one we used later in experiments (Figure 7). From the simulations, two resonance peaks, at l = 1300 nm and l = 680 nm, could be identified. The field distributions at these wavelengths are shown in Figure 7 a, b, respectively. It was found that the gold nanopillar behaves as a vertical plasmonic waveguide, and that the gold substrate acts as a mirror. Resonance occurs when the length of the gold nanopillar is Figure 7. Optical resonance modes on a protruding gold nanopillar (diameter: 280 nm, height: 400 nm) in water. Electric-field intensity distribution around the gold nanopillar when it is illuminated on resonance by a linearly-polarized plane wave at a) l = 1300 nm and b) l=680 nm. Polarization of plane wave illumination is indicated Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13,

7 Plasmonic Trapping with a Gold Nanopillar such that a constructive interference occurs between the coupled incoming wave and the reflected wave. In Figure 7a, b, the gold pillar resonates in the first and second modes. Based on this understanding, gold pillars with various shapes and dimensions can be tuned to resonate over a range of wavelengths. To facilitate its ease in fabrication, we designed the gold nanopillar to be resonant in the first mode at a wavelength of 974 nm. In addition to trapping, the proposed gold nanopillar device can be used for two other types of nanoparticle manipulation: manual rotation and passive rotation. As shown in Figure 6 c, illumination of the nanopillar with a linearly polarized plane wave results in regions of high electric-field intensity (hot spots) aligned with the polarization direction. If the polarization of the linearly-polarized light is manually rotated, the hot spots also rotate. In this way, the trapped particle can be rotated around the gold nanopillar in a precise manner, as shown in Figure 8. Additionally, passive rotation of polarization can be achieved using circularly-polarized illumination. As shown in Figure 9, when circularly-polarized illumination is employed, the time-averaged electric-field intensity consists of a hot-spot ring, rather than two individual hot spots. In addition, there is a constant flow of energy around the nanopillar, in a clockwise Figure 8. Illustration of trapping and manual rotation of the nanosphere by the gold nanopillar with linearly-polarized illumination. D P = 280 nm and H=130 nm. Adapted from [31]. Figure 9. Surface plasmon on a gold nanopillar excited by circularly-polarized light. a) FDTD calculation of time-averaged electric-field intensity enhancement jej 2 = je INC j 2 distribution resulting from circularly-polarized illumination at l=974 nm. b) FDTD calculation of y component of Poynting vector around the nanopillar with left circularly-polarized illumination. Colorbar is in units of W m 2, and takes the incident illumination as having an electric field with an amplitude of E INC = 1Vm 1. Adapted from [31]. or counter-clockwise direction, depending on the handedness of the incident circular polarization. In Figure 9 b, the y component of the Poynting vector is shown for left circularly-polarized illumination of the nanopillar. It can be seen that there is a counter-clockwise energy flow around the nanopillar. It should be expected, therefore, that trapped particles are driven by the scattering force to rotate in the same direction as the energy flow. Because the design called for a gold nanopillar protruding from a gold film, template stripping could be employed in the fabrication process (Figure 10 a). The silicon template was first fabricated by e-beam lithography and reactive-ion etching (RIE). Gold (500 nm) and copper (1000 nm) were then deposited by e-beam evaporation. A piece of single-crystal silicon was then stuck on to the copper film with a very thin layer of epoxy (EPO-TEK 330, Epoxy Tech. Inc.). In the final step, the structure was stripped from the template using a blade. Using this method, gold nanopillars in a large array could be fabricat- Figure 10. Fabrication of gold nanopillars by template stripping. a) Fabrication procedure. b and c) SEM of devices fabricated by template stripping. d) SEM of gold nanopillar fabricated by e-beam lithography and lift-off. e) AFM image of template-stripped gold nanopillar. Scale bars are 1 mm in (b) and 300 nm in (c) and (d). Adapted from [31]. ChemPhysChem 2012, 13, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

8 K. B. Crozier and K. Wang ed, as shown in Figure 10b. The resulting nanopillars (Figure 10 c) had more regular shapes and smoother surfaces than nanopillars fabricated by standard lift-off processes (Figure 10 d). The quality of the surface morphology is also evident from the atomic force microscope (AFM) image shown in Figure 10e. In the experimental set-up (Figure 11), the beam from a fibercoupled trapping laser (l = 974 nm) was collimated, passed through polarization-control optics, and was loosely focused by a microscope objective (Nikon, NA = 0.45, 50 ) onto the plasmonic nanotweezer device, which sits on a translation stage. The laser beam did not fill the objective back aperture, resulting in the focused spot having an e 2 intensity diameter of ~ 3 mm, which relaxed the alignment tolerance. In the light path, either a half waveplate was used to rotate the linear polarization, or a quarter waveplate was used to convert linear polarization to circular polarization. The chamber between the device and the glass cover slide was filled with a diluted colloid solution (carboxylate-modified microspheres, Invitrogen Inc) to which surfactant (0.5 % by weight, Triton X100, Sigma Aldrich Inc.) and NaCl (5 mm) were added to prevent adhesion and screen surface charges, respectively. A second laser beam (l = 532 nm) was loosely focused by the objective on the device to excite fluorescence from the colloidal particles, which was imaged onto an EMCCD camera (PhotonMax, Princeton Instruments). The set-up enabled the positions of particles to be determined with high precision. In the experiments, polystyrene particles, as small as 110 nm in diameter, were successfully trapped. The incident laser beam had an average intensity over the focused spot (e 2 intensity spot diameter of 3 mm) at the sample of I avg = 10 mw mm 2. Fluorescence images of the trapping process obtained at successive times (t1 t4) are shown in Figure 12. The sphere appears white, while the gold nanopillars appear black. At the beginning, the particle was very weakly trapped by the 5. Experimental Results Figure 11. Experimental setup. D1, D2 and D3: dichroic mirrors. EMCCD: Electron-multiplying charge-coupled device. Adapted from [31]. Figure 12. Fluorescence images, obtained at successive times, of trapping and rotating 110 nm diameter polystyrene sphere with a gold nanopillar. At time t1, the sphere is close to the nanopillar, but not trapped. At times t2, t3 and t4, the sphere is trapped by the nanopillar. The input polarization is manually rotated, resulting in a sphere rotating clockwise about the pillar. The gray dots, as indicated by arrows, represent the position and size of the nanopillars. Adapted from [31]. loosely focused laser spot. Then the gold nanopillar was moved to be aligned with the focused spot. At time t1, the sphere was close to, but not trapped by, the gold nanopillar, and moved under Brownian motion. At time t2, the sphere randomly moved sufficiently close to the nanopillar and was drawn into one of the hot spots by the gradient force. After the sphere was trapped, we manually rotated the input-beam polarization during frames t2 t4, resulting in the sphere rotating clockwise around the nanopillar. More detailed measurements were performed on trapping 200 nm diameter polystyrene particles. Trapping stiffnesses normalized to the illumination intensity were estimated to 3.8 pn mmmw 1 and 0.96 pn mmmw 1 in the radial and tangential directions. [31] According to the report on typical optical tweezers, the trapping stiffnesses of a 220 nm polystyrene particles normalized to illumination intensity are 0.04 pn mmmw 1 and pn mmmw 1, in the directions that are parallel and perpendicular to the polarization, which are more than one order of magnitude smaller than we achieved here. [39] Since two hot spots on opposite sides of the gold nanopillar are generated simultaneously when using linearly-polarized light illumination, we could trap two particles at the same time, as shown in Figure 13. Due to the limited resolution of the mi Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13,

9 Plasmonic Trapping with a Gold Nanopillar Figure 13. Successive fluorescence images showing the simultaneous trapping and rotation of two 200 nm spheres by a gold nanopillar. Over the period t=1 s to 4 s, the input polarization is rotated. The image at t=5 sis obtained after the trapping laser is turned off, and shows the two spheres escaping the nanopillar. Adapted from [31]. croscope, the images of the two spheres, which were ~480 nm apart, merged to appear as an ellipse. It could be seen, however, that the ellipse rotated as the polarization was manually rotated, confirming that the two spheres rotate together. At the end of the experiment (t = 5 s), the laser as switched off, and the two spheres were seen to depart from the nanopillar. As mentioned in connection with Figure 9, a passive rotation can be achieved using circularly-polarized light illumination. In the experiment, we demonstrated this on a 340 nm diameter polystyrene particle. An illumination intensity of 5 mwmm 2 was used. Histograms of particle centroid positions (Figure 14) show that they follow circular trajectories, which confirms the rotation around the gold pillar. Over the measurement period of 30 s, the average rates of counter-clockwise rotation and clockwise rotation as shown in Figure 14a, b were 4.3 and 5.7 revolutions s 1, respectively. a small gap could be a good candidate. For trapping small particles with such a structure, one would aim to maximize field confinement and enhancement in the gap. Surface-enhanced Raman scattering (SERS) makes use of field enhancement for a different application (spectroscopy), and the methods developed there are likely to be highly applicable for plasmonic trapping, though one would need to modify the designs to incorporate thermal management. [40 45] In summary, the strong field confinement and enhancement provided by plasmon resonance structures make them especially suitable for near-field optical trapping if thermal management is considered hand-in-hand with the optical design. Three main advantages can be expected in plasmonic trapping: stronger localization, super-resolution manipulation, and enhanced trapping with a reduced power requirement for the trapping laser. Due to the intrinsic loss, plasmonic resonance structures have limited quality factors and there exists a tradeoff between field depth and field enhancement for trapping of particles with different sizes. The trapping performance predicted from purely optical considerations can be achieved only if deleterious thermal-side effects are minimized. Calculation predicted the substantial suppression of thermal-side effects if the substrate on which the plasmonic structures sit is made of a high-thermal conductivity material. With these considerations in mind, we introduced a plasmonic tweezer consisting of a gold nanopillar, and demonstrated enhanced trapping performance. It can be used not only to trap nanoparticles, but also to rotate them manually and passively, using linearly- and circularly-polarized excitation. Figure 14. Passive rotation of polystyrene spheres by a gold nanopillar. Centroid tracking in the x-y plane of 340 nm the sphere is trapped and rotated by the nanopillar with a) right and b) left circularly-polarized illumination, respectively. I avg = 5mWmm 2. Arrows indicate the polarization (E) and velocity (v)-rotation directions. Adapted from [31]. Since the thermal management in this design allows higher illumination intensity, there should be no difficulty in extending the trapping to smaller particles. However, the challenge lies in the quantitative measurement of particles dynamics in the trap. Additionally, the current single-nanopillar design is not optimized for trapping particles smaller than 10 nm, because the e 2 field depth of ~60 nm was much larger than the particles and the intensity enhancement of ~ 400 was relatively small compared to other plasmon resonance structures. As discussed in connection to Equations (9) and (10), the extent of the near field should be comparable to the particle size to maximize trapping efficiency. To further increase the localization of the field, two protruding gold nanopillars separated by Acknowledgements This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) N/MEMS S&T Fundamentals program under grant no. N issued by the Space and Naval Warfare Systems Center Pacific (SPAWAR), and in part by the National Science Foundation under grant no. ECCS (CAREER award). Fabrication work was carried out in the Harvard Center for Nanoscale Systems, which is supported by the NSF. 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