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Holographic plasmonic nano-tweezers for dynamic trapping and manipulation Preston R Huft, Joshua D Kolbow, Jonathan T Thweatt, and Nathan C Lindquist Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04289 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017
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Nano Letters
Holographic plasmonic nano-tweezers for dynamic trapping and manipulation Preston R. Huft†, Joshua D. Kolbow†, Jonathan T. Thweatt, and Nathan C. Lindquist* Physics Department, Bethel University, St Paul, MN 55112, USA *
[email protected],
†
http://sites.google.com/a/bethel.edu/ncl48757-nanolab/
Equal Contribution
Abstract. We demonstrate dynamic trapping and manipulation of nanoparticles with plasmonic holograms. By tailoring the illumination pattern of an incident light beam with a computer-controlled spatial light modulator, constructive and destructive interference of plasmon waves create a focused hotspot that can be moved across a surface. Specifically, a computer-generated hologram illuminating the perimeter of a silver Bull’s Eye nanostructure generates surface plasmons that propagate towards the center. Shifting the phase of the plasmon waves as a function of space gives complete control over the location of the focus. We show that 200 nm diameter nanoparticles trapped in this focus can be moved in arbitrary patterns. This allows, for example, circular motion with linearly-polarized light. These results show the versatility of holographically-generated surface plasmon waves for advanced trapping and manipulation of nanoparticles.
Keywords. Plasmonic tweezing, computer-generated holograms, holographic optical tweezing, plasmonics.
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Nano Letters
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Optical trapping and holographic optical tweezing have become indispensable noninvasive methods in biology, physical chemistry, nano-science and atomic physics.1 However, the diffraction limit of light makes it difficult to manipulate subwavelength objects with nanoscale precision2 and the requirement of bulky optics such as high numerical aperture objectives makes conventional three-dimensional optical tweezing incompatible with planar chip geometry and high-density integration.3 As one solution, the field of plasmonics allows the manipulation of optical fields on deep sub-wavelength scales.4 Surface plasmons (SPs) are electromagnetic surface waves bound to a metaldielectric interface by coupling to free electrons.5 Because of the coupling between photons and electrons, the SP wave is confined to within 10~100 nm of the metaldielectric interface in the form of an exponentially decaying evanescent field. By harnessing these qualities, metallic nanostructures can squeeze light into nanometer-scale volumes, enhancing the local field intensity and light-matter interactions. With proper patterning, the plasmons can then be controlled, squeezed, re-directed, or otherwise manipulated depending on the shape of the nanostructures and surrounding materials. This has generated intense interest for applications as wide ranging as photovoltaics, spectroscopy, imaging, and sensing.6-18 Research into plasmonic trapping has been motivated fundamentally by its potential for enhanced optical forces. Due to increased field intensity and larger field gradients, plasmons can offer several significant advantages over conventional optical tweezers such as greatly reduced laser intensity requirements for trapping delicate samples and the trapping of subwavelength particles or even single molecules.3,19-26 Propagating surface plasmons have a smaller wavelength compared to free-space light and can be focused into tighter spots. The localized fields or
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“hotspots” in and around metallic nanostructures are not diffraction-limited spatially and can be designed to operate at a laser wavelength of choice. Indeed, many different metallic nanostructures such as gaps,21-23,27 apertures,24,25 bumps,26 and flat particles3 have been used for plasmonic trapping and the field has grown rapidly.28 However, static illumination of pre-fabricated nanostructures is inherently limited for dynamic plasmonic tweezing, real-time sorting, assembly, and on-chip applications. While holographic optical tweezing has become a powerful technique1 that allows realtime manipulation of single particles or arrays of particles,29,30 precise spatial control of focused plasmons for dynamic nano-tweezing applications has not yet been realized. Some works have studied the transport of trapped particles31 but the trapped particles were allowed to move only passively in a lattice. Other experiments have been done to transport or rotate trapped particles by changing the polarization of the incident light but the location of the traps was still determined by the pre-fabricated nanostructure26,32,33 or the trapping sites were fixed in space.34 Higher-order laser modes can excite various distributions of plasmonic hotspots, but experiments are again limited to the fixed location of the predetermined nanoparticle modes.35 Furthermore, while changing the location of focused plasmons by changing the illumination pattern has recently generated significant interest,35-41 these techniques have not yet been used for the trapping and manipulation of nanoparticles. Therefore, while some recent effort has gone into developing plasmonic tweezers with more dynamic or tunable properties,42-44 the ability to control and precisely direct the arbitrary sub-wavelength placement of focused plasmons for real-time dynamic nano-particle manipulation under full computer control remains a significant challenge.
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In this paper, we show active control of focused plasmonic hotspots for the holographic manipulation of nanoparticles. The locations of the focused plasmons are not confined to a particular pre-fabricated geometry but can rather be placed at arbitrary locations within a region of interest. Figure 1 shows the experimental scheme that was used. A plasmonic “Bull’s Eye” structure was used to convert incident light into converging plasmon waves. These waves interfered in the center of the Bull’s Eye to form a plasmonic trapping site (figure 1a). By changing the illumination pattern as shown in figure 1b, the trapping location can be moved around. The entire setup (figure 1c) used a standard inverted optical microscope with two lasers and a Spatial Light Modulator (SLM). The 532 nm illuminating laser was used to image 200-nm diameter fluorescent spheres over a wide field-of-view. The 660 nm trapping laser was reflected from the SLM that was programmed with a computer-generated holographic pattern tailored for trapping on the Bull’s Eye structure. The two beams were then combined and sent into the microscope with a lens focusing conjugate to the back aperture of the microscope objective. This caused the illuminating laser to cover a wide area whereas the trapping laser was programmed to focus onto the Bull’s Eye pattern. Figures 1d,e show scanning electron microscope (SEM) images of representative Bull’s Eye devices. Fabrication details and schematics are provided in the Supporting Information. Creating a focused plasmon in the center of the Bull’s Eye requires that all plasmons scattered from the rings of the Bull’s Eye interfere constructively in the center. This means that for x-polarized light, the two halves of the Bull’s Eye should be illuminated πout-of-phase with respect to each other. This is accomplished with an appropriate pattern on the SLM. Furthermore, if the phase of the scattered plasmons are designed to not
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interfere constructively in the exact center of the Bull’s Eye but rather in any arbitrary location (x, y) near the center, the phase ϕ of the illumination pattern needs to change continuously as a function of polar angle θ around the Bull’s Eye as follows: , , =
2 cos + + sin + ,
−
