Vertical plasmonic nanowires for 3D nanoparticle trapping

Transcription

1 Vertical plasmonic nanowires for 3D nanoparticle trapping Jingzhi Wu, Xiaosong Gan * Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, PO Box 218, Hawthorn, VIC-3122, Australia ABSTRACT Nanoparticle trapping is considered to be more challenging than trapping micron-sized objects because of the diffraction limit of light and the severe Brownian motion of the nanoparticles. We introduce a nanoparticle trapping approach based on plasmonic nanostructures, which consist of nanopillars with high aspect ratio. The plasmonic nanopillars behave as plasmonic resonators that rely on paired nano-pillars supporting gap plasmon modes. The localized surface plasmon resonance effect provides strong electromagnetic field enhancement and enables confinement of nanoparticles in three dimensional space. Numerical simulations indicate that the plasmonic structure provides stronger optical forces for trapping nanoparticles. The study of thermal effect of the plasmonic structure shows that the impact of the thermal force is significant, which may determine the outcome of the nanoparticle trapping. Keywords: Plasmonics, nanoparticle, plasmon resonance 1. INTRODUCTION Plasmonic nanostructures have been applied in subwavelength optics, metamaterials and nanoparticles manipulation [1]. Metallic nanostructures can be engineered to efficiently couple to electromagnetic field and confine it into subwavelength volume with enhancement [2-3]. Because of the electromagnetic field confinement and enhancement induced by surface plasmon resonance of nanostructures, plasmon-based optical trapping has been proposed and implemented [4]. However, since plasmon resonance is confined in the near field of plasmonic structures, accurate threedimensional manipulation of nanoparticles is still a highly desirable technique [2]. Metallic nanorods or nanowires support surface plasmon polaritons which are able to generate localized and enhanced electromagnetic field [5]. Nanowires can act as surface plasmon resonators, which enhance electromagnetic waves of certain frequencies, provided that the wire end faces reflect incident surface plasmons, and the length of the resonator determines the resonant frequency [6]. Heating effects generated by absorption of the plasmonic nanostructure is one of key issues associated with trapping with nanoplasmonic structures [7]. Metallic nanoparticles excited by laser with the frequency close to its plasmon resonance can efficiently convert electromagnetic energy into thermal energy, which may raise the temperature in the surrounding medium. Due to the relative low thermal conductivity of dielectric materials compared with metals, thermal gradient will build up in the surrounding media consequently. For nanoparticle trapping in a nano/micro-environment, the thermal gradient produced by the plasmonic structure can create significant disturbance [2]. Here we demonstrate a nanoparticle trapping approach based on a plasmonic structure comprising of two vertically aligned metallic nanorods placed on the substrate, as shown in Figure 1. A plane wave is employed to excite localized plasmonic resonance of the nanostructure. Plasmonic absorption of the nanoplasmonic structures and its consequent contribution to the temperature variation of the local micro-environment are investigated using the Finite Difference Time Domain (FDTD) method and the thermal conduction model [2]. The thermal force is estimated based on the temperature gradient [8]. *xgan@swin.edu.au Smart Nano-Micro Materials and Devices, edited by Saulius Juodkazis, Min Gu, Proc. of SPIE Vol. 8204, 82040T 2011 SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol T-1

2 2. THEORY Paired metallic nanorods support propagation of gap plasmon modes when the separation of the nanorods is so small that the interaction can not be ignored. Plasmon resonances occur provided that the height h of the nanorods satisfies the following condition, ksph= mπ + ϕ (1) ksp is the plasmon wavevector which can be approximated as the wavevector for surface plasmon polaritons propagate on a metal/dielectric interface, ϕ is the phase change induced by reflection of the two ends, and m = 0, 1, 2 is the resonance order. The heat generation density q in a plasmonic structure in an electromagnetic field can be given by 1 2 () r Im{ } E() r (2) 2 q = ωε 0 ε r where ε 0 is the permittivity of vacuum, ε r is the relative permittivity of the plasmonic material, Im{} denotes the imaginary part, Er () is the electric field at position r. Due to the presence of thermal gradients, thermophoresis will occur, which is a phenomenon describes nanoparticle drift in addition to random Brownian motion [8]. Depending on the properties of nanoparticles and surrounding medium, the particles move either towards the cold or the hot zones. For low concentration of nanoparticles, the net force f acting on the particle by the solvent due to the present of the temperature gradient can be approximated as linearly proportional to the temperature gradient [8] S T f = T (3) β where S T is the Soret coefficient, β = 1 kt B, k B is the Boltzmann constant. The Soret coefficient does not vary significantly if the temperature increase is less than 10 K. However it is noted that S T depends on temperature and particle size, and can even vary from positive to negative at various temperature for different nanoparticle suspensions. Figure 1. A schematic diagram of the metallic nanostructure: A linearly polarized (in the x direction) laser beam propagates in the z direction from the substrate (n s = 1.78) into water (n w = 1.33). The plasmonic structure consists of two vertically aligned silver nanorods immersed in water. The plasmonic rods are 820 nm in height h, 70 nm in diameter d, and the gap between the two rods is 70 nm. Proc. of SPIE Vol T-2

3 3. RESULTS AND DISCUSSION The absorption spectrum of the plasmonic nanostructure is shown in Figure 2, from which several absorption peaks are observed. The plasmon resonance wavelength is affected the geometry of the nanostructure and the polarization of the excitation. Because the polarization is normal to the long axis of the nanorods, the major absorption peaks appear at wavelengths of 435 nm and 465 nm. To understand the plasmonic resonance of the nanostructure, the electromagnetic field distributions at these wavelengths have been calculated by FDTD method. Figure 2. Absorption of the plasmonic nanostructure at x polarized normal incidence. Figure 3. Intensity distributions of the nanostructure with gap of 70 nm, h: 820 nm, d: 70 nm, in the zx central cross section, at wavelength (a) 435 nm, (b) 450 nm and (c) 465 nm. The white line denotes the interface. Proc. of SPIE Vol T-3

4 Figure 3 shows the electric field distributions of the plasmonic nanostructure in the zx plane. Different orders of resonant modes are observed at different wavelength because of the finite length of the nanorods. With the strong plasmonic resonance modes, it is noticed that the electric field is not only significantly enhanced in term of field strength, but also better confined three-dimensionally in three dimensional space. To investigate optical trapping at the different strong absorption peak wavelengths, we calculated the maximum electromagnetic forces (EM force) on a 20 nm nanoparticle (n 1.5) in central plane of the gap, at wavelengths of 435 nm, 450 nm and 465 nm, respectively. As displayed in Figure 4, for the stronger absorption peak wavelength 435 nm, the y component of the EM force is weaker than that of the other peak absorption wavelength. The reason is that the plasmon mode distributions across the nanorods are different at these excitation wavelengths. The z component of the EM force, as shown in Figure 5, means that there are several equilibrium positions for particle trapping. Figure 4. The y component of the maximum EM force on 20 nm nanoparticle (refractive index: 1.5) at wavelengths of 435 nm, 450 nm and 465 nm. Intensity of the incident beam is 1 mw μm -2. Figure 5. The z component of the EM force on 20 nm nanoparticle (refractive index: 1.5) at wavelengths of 435 nm, 450 nm and 465 nm. Intensity of the incident beam is 1 mw μm -2. Proc. of SPIE Vol T-4

5 To create 3D nanoparticle trapping, it is useful to examine the thermal effects of the plasmonic nanostructure. Considering the experimental results published earlier, we assume that the Soret coefficient S T = K -1 for our calculations. As shown in Figure 6 and 7, the EM force is one order of magnitude stronger than the thermal force in the near field of the nanostructure. But the thermal forces decay slower and have a much broader working range than the EM force. If the Soret coefficient S T is negative, thermal force can greatly enhance the nanoparticle trapping, since its wide working range makes the capture of nanoparticles easier. However, if the Soret coefficient S T is positive, it can severely disrupt the optical trapping of nanoparticle. While the EM force provides multiple trapping equilibrium positions, the thermal force moves the nanoparticle away or towards the center of the plasmonic structure depending upon the Soret coefficient S T (Figure 7). As the result, nanoparticles can be trapped at positions balancing EM forces with the thermal force. Figure 6. Comparison of the y component of the thermal force and the EM force on 20 nm nanoparticle (refractive index: 1.5) at the positon of 680 nm from the interface. The incident beam intensity is 1 mw μm -2,wavelength 465 nm. The Soret coefficient S T used for calculation of is (K -1 ). Figure 7. Comparison of the z component of the thermal force and the EM force on 20 nm nanoparticle (refractive index: 1.5) at the position of 680 nm from the interface. The incident beam intensity is 1 mw μm -2,wavelength 465 nm. The Soret coefficient S T used for calculation of is (K -1 ). Proc. of SPIE Vol T-5

6 4. CONCLUSIONS In summary, resonances of the plasmonic structure with vertically aligned nanorods give rise to strong electromagnetic field enhancement and confinement three-dimensionally within the gap between the two nanorods. At absorption peak wavelengths of the plasmonic nanostructures, the EM forces on nanoparticles can be greatly enhanced, and confined in the gap between the two nanorods. It is shown that the thermal absorption by the nanoplasmonic structure leads to strong thermal forces that comparable to the EM forces in strength. The EM forces in the near field of the nanostructure are one order of magnitude stronger than the thermal force. However, the wide working range of the thermal force makes it a key factor in determining the outcome of trapping nanoparticles. REFERENCES [1] E. Ozbay, Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions, Science, 311(5758), (2006). [2] J. Wu, and X. Gan, Three dimensional nanoparticle trapping enhanced by surface plasmon resonance, Opt. Express, 18(26), (2010). [3] X. Gao, and X. Gan, Modulation of evanescent focus by localized surface plasmons waveguide, Opt. Express, 17(25), (2009). [4] M. L. Juan, M. Righini, and R. Quidant, Plasmon nano-optical tweezers, Nat Photon, 5(6), (2011). [5] J. M. Pitarke, V. M. Silkin, E. V. Chulkov et al., Theory of surface plasmons and surface-plasmon polaritons, Reports on Progress in Physics, 70(1), 1-87 (2007). [6] H. Ditlbacher, A. Hohenau, D. Wagner et al., Silver Nanowires as Surface Plasmon Resonators, Physical Review Letters, 95(25), (2005). [7] G. Baffou, C. Girard, and R. Quidant, Mapping Heat Origin in Plasmonic Structures, Physical Review Letters, 104(13), (2010). [8] R. Piazza, and A. Parola, Thermophoresis in colloidal suspensions, Journal of Physics: Condensed Matter, 20(15), (2008). Proc. of SPIE Vol T-6