Plasmonic Manipulation of Targeted Metallic Particles by Polarization

May 18, 2018 - As a tool in the manipulation of micro- and nano-objects, optical tweezers are found in applications in many areas. However, selective ...
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Plasmonic Manipulation of Targeted Metallic Particles by Polarization-Sensitive Metalens Xianyou Wang, Yanmeng Dai, Yuquan Zhang, changjun min, and Xiaocong Yuan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00282 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Plasmonic Manipulation of Targeted Metallic Particles by Polarization-Sensitive Metalens Xianyou Wang†, Yanmeng Dai†, Yuquan zhang*, Changjun Min, and Xiaocong Yuan* Nanophotonics Research Center, Shenzhey Laboratory of Micro-Scale Optical Information Technology, Shenzhen University & Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, China, 518060

ABSTRACT: As a tool in the manipulation of micro- and nano-objects, optical tweezers are found in applications in many areas. However, selective trapping still poses challenges. Recently, a meta-surface technique offers an approach to improve optical trapping and manipulation capabilities. Here, we demonstrate the selective trapping of metallic nanoparticles with tailored plasmonic fields using a polarization sensitive metalens. We show, both by theory and experiments, modulated trapping and anti-trapping forces when beam polarizations are tuned. Combining the effects of two orthogonal circular polarizations, single target particles were stably trapped in the center while all other particles were repelled. This particle isolation points toward targeted manipulations that may find applications in single-particle assistant molecular Raman detection and assembly of plasmonic structures. KEYWORDS: Optical Trapping; Polarization Sensitive Metalens; Plasmonics.

The optical tweezers technique, by either laser or plasmonic trapping, has attracted extensive interest and is playing an important function in manipulating nano/micro-objects in many fields.1-5 However, the number of trapped particles at the focus is always imprecise in classical techniques, with no guarantee of control over the cluster that forms,6-8 especially for plasmonic traps. Most implementations are narrowed to a specific precise control of a single target object, for example, the precise manipulation is of vital importance for precision molecular detection, on-chip functional structure construction, and many other situations.3, 9-13 As a result, it is necessary and essential to get a targeting manipulation for such widespread applications. As optical trapping commences with distribution of the optical field, tailoring the distribution of the field is a direct way to modulate the optical force and trapping results. Many approaches have been proposed and implemented to establish specific forces within traps,14-15 by modulating a divergent surface plasmonic field and another simultaneously focussing one to trap a single particle while rejecting others. However, control of the trapping dynamics is still lacking. In our previous work, we succeeded in accurately manipulating a single nanoparticle by combining two independent laser sources.7 In fact, a strict combination of two independent beams was still indispensable in its achievement, and ma-

nipulations sharply became increasingly difficult. Consequently, a simple and direct way to achieve selective trapping is still urgently needed. Recently, the rapidly developed metasurface technique provides an alternative approach.16-18 Because of its high flexibility and efficiency, liquid crystal-based metasurfaces have created a surge in the development of optical field modulation. Herein, we describe the design of a polarization-sensitive metalens (PSML) that tailors the optical field by focusing the left-circular polarization (LCP) but produces a diverging right-circular polarization (RCP) beam with opposite focal length. The two orthogonal components focus at different planes behind the objective lens creating a tightly focused optical system. Consequently, two independent plasmonic fields are excited at the surface plasmon resonance (SPR) angle, and propagate along opposite directions that establish trapping and anti-trapping forces simultaneously. The excited plasmonic fields may be specially tailored by adjusting the focal length of the PSML and the relative distance between the thin silver film and the objective lens, to achieve trapping of a targeted particle while blocking others. Both theoretical simulations and experimental results validate the effectiveness of the proposed method. We envisage this configuration to be a stable, convenient, and practical tool for physical and chemical researches.

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Figure 1. (a) Schematics of the polarization-directed flat metalens, insert is its polarization imaging; (b) experimental setup to create the tailored plasmonic trapping system using the PSML; and (c) the tailored plasmonic field and trapping of the targeted particle.

The polarization sensitive optical elements generate diverse modulations for different polarized components, providing a simple and convenient potential for tailoring the optical field distribution in a direct way. Here, the PSML is a planar lens made of photo-alignment liquid crystal with radially varying fast axis orientations of local birefringent (Figure 1). 19 The polarization sensitive response of the PSML can be described with Jones calculus by   J ∙  ,

(1)

where  and  are the Jones vectors of the incident and 2 2 output light, J   is the Jones matrix of 2  2 ⁄ the PSML,     4 is the orientation of the liquid crystal molecules along radial coordinate r , f0 is a tunable focal length of the PSML. It can be shown that, for incident light with LCP (  1; ), the transmitted field is modulated as 1;  !"#   ⁄2 $, a RCP beam with a divergent wavefront; while for the case with RCP (  1; ), it is 1;  !"#    ⁄2 $. Thus, a helicitydependent Pancharatnam–Berry phase modulation, φ#$  &    ⁄2 , (2) is imparted to the incident light, along with a reversal of helicity.20 That is, the transmitted LCP light produces a positive focal length, whereas RCP light produces a focal length of opposite sign. Figure 1 (a) shows schematic of the polarization-directed flat lens, and insert is the polarization imaging. Generally, the incident laser beam will produce simultaneously, two output waves that are circularly polarized and orthogonal, one with a positive focal length and the other negative. In experiments, a focused LCP with a coaxial divergent RCP component is formed after the PSML. Subsequently, the two orthogonally polarized beams are incident on an objective lens and focused at different focus positions in the focal region of the objective lens. Benefitting from the designed PSML, an inverted microscopic system was established to achieve a stable

plasmonic trap for targeted particles. The configuration [Figure 1 (b)] involves the PSML modulating the exciting beam to tailor the plasmonic fields after an oil-immersed objective lens (NA=1.49). Using a polarizer (P1) and a quarter-wave plate (QWP), the polarization of the incident beam is elliptically tuned. The beam is then expanded by a telescope system with two convex lenses (L1 and L2). The expanded beam is next decomposed into LCP and RCP components using the PSML; their decomposition is altered by rotating the QWP. A 4f setup (L3 and L4) is used to project the light beam emerging from the PSML to the back focal plane (BFP) of the objective lens. A 50-nm silver film coated on the glass substrate excites the plasmonic field. The gap between the objective lens and the glass substrate is filled with index-matching oil to satisfy the exciting condition. Water diffused with Au particles (0.3–1.5 μm) is dripped onto the silver film. For dark field illumination, in-house built elements consisting of an objective (NA=0.9) and camera CCD2 are used to capture the experimental process from the upside. Camera CCD1 is used to image the back-Fourier plane of the objective lens and to capture the particle dynamics. As tightly focused circular polarized beams have a transverse magnetic component with respect to the metal film, a plasmonic field can be excited on the metal film at the surface plasmon resonance (SPR) angle,8-9, 21 where momentum matching holds. Furthermore, the convergence angle may be controlled by tuning the focal length of the PSML, and the position of the excitation may be adjusted through the relative position of the PSML and the silver film. When the film is located at the plane between the two foci, two independent counter-propagating plasmonic fields are excited as the two exciting laser components are orthogonal to each other; see Figure 1(c). To illustrate the disparate behaviors when the two light beams of opposite helicity are tightly focused, we used the Finite-Difference Time-Domain (FDTD) method to calculate the electric field distributions at a silver–water interface. The complex amplitudes 1⁄√2# ( &  ) $ *+#,-$ are

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used for the LCP (sign +) and the RCP (sign -) beams; here φ# $  &   ⁄6 are the phase distributions along the radial coordinate   / sin , /  1.80 mm is the focal length of the objective lens,  the angle of refracted ray with its maximum determined by numerical aperture 78  1.49 and refractive index :  1.52 of the oilimmersion objective lens; specifically,  #78⁄: $. The Fresnel transmission coefficients of the layered glass/silver/water structures are also taken into account.22 For a wavelength of 532 nm, the refractive indexes of silver and water are 0.13 + 3.20i and 1.33,23 respectively. The thickness of the silver film is set to 50 nm. The silver–water interface is positioned at the focal plane of the objective lens.

Figure 2. Optical field distributions of incident left-circular (a), right-circular (b), and hybrid polarizations (c), at the back focal planes—the polarizations are indicated at top right corner; (d)–(f) Corresponding schematics of the plasmonic excitation for different polarizations; (g)–(h) Calculated electric field intensities at the silver–water interface for three incident polarized light beams following tight focusing from the objective lens; (j)–(l) Distributions of the optical force exerted on the Au particle.

Figure 2(a–c) shows the modulated optical field distributions with incident LCP, RCP, and hybrid polarizations, respectively, at the BFP of the PSML. For these polarizations, schematics of the corresponding plasmonic excitation are given [Figure 2(d–f)]; the calculated results for the electric field intensities at the silver–water interface are presented in Figure 2(g–i). As discussed above, the PSML modulates the LCP laser beam so that it is focused along with the RCP divergent behind the 4f system. As a result, the LCP is focused in front of the focus plane of the objective lens whereas the RCP is focused behind it. Therefore, the LCP excited plasmonic field is divergent and attenuates radially, whereas for the RCP component, the excited surface plasmon propagates centripetally and is focused at the center. With opposite propagation directions, the two fields are independent with each other.

Based on the combination of the two polarized components, a central focused spot surrounded by an outer divergent ring-wall field is established. The field distribution is the origin of optical forces acting on the particle; with electric field obtained from the FDTD simulations, we can calculated optical forces exerted on the particles using Maxwell stress tensor method. The optical force along the radial direction [Figure 2(j) and (k)] is negative for a particle in the focused plasmonic field, and positive for one in the divergent field. Consequently, the focused plasmonic field confines and traps the particle at the center, whereas the particles in the divergent field experience a repulsive force. As the two fields are independent, the optical force exerted on the particle is an approximate simple superposition of the attractive and repulsive forces at each position. As the

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polarization of the laser beam may be modulated continually, their proportionality may be adjusted to modulate the intensity of the plasmonic field. Consequently, the net force may also be modulated by adjusting the fast axis orientation of the QWP.

Figure 3. Dynamic behavior of Au particles in plasmonic fields: (a) screenshots of particles located in the focused SPP field and (d) the trajectory of the labeled particles; (b) and (e) results of particles in the outward-spreading SPP field; (c) and (f) results of particles in a combined plasmonic field. The circle marks the outward-spreading field; black crosses indicate the center of the plasmonic field; colored lines are trajectories of particles, each with a color for identification. The scale bars (lines at lower right in a–c) are of length 5 μm. Based on the above simulation and theoretical analysis, we obtained the experiment results presented in Figures 3 and 4. Figure 3(a)–(c) demonstrates the dynamic behaviors of Au particles in three different plasmonic fields, two independent and one hybrid. For further analysis and

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confirmation, the trajectories for the selected particles [crosses in panels (a) and (c) indicate the trapped particle; arrows indicate the repelled particle in panels (b) and (c)] were measured for all three instances [Figure 3(d–f)]. The trajectories of the particles are assigned a color; the focus point positioned at (x=0, y=0) is marked by a black cross in Figure 3(d–f). From the trajectories in Figure 3(d), we find that a stable trapping state is obtained with small fluctuations when the focus point of the focusing RCP beam is above the metal surface. In contrast, when the focus point with a defocused LCP beam is under the metal film, a divergent circular plasmonic field forms and particles lying in this field experience a repulsive optical force that form an energy barrier preventing them from incursions into the focal spot [Figure 3(b) and (e)]. This divergent annular field excited around the focal spot thus insulates the interior. Figure 3(f) presents an image of overlapped results; there are very few particles around the outer field as the Au particles are at a low concentration. Note that a precise plasmonic trap tailored for a targeted objective has been achieved. The analysis and experiments above verified that a specific trap of targeted metal particles can be achieved by tailoring the plasmonic field. In particular, when the particle concentration and incident power of the laser beam are high enough, traps employing traditional methods became disorderly and unsystematic, thereby spoiling any manipulation precision 22. In our system, however, this drawback is perfectly avoided. The exciting position of the two components may be modulated by adjusting the focal length of the PSML and the relative position of the two foci and the silver film. The final tailored field distribution is determined by the relative excitation position of the two components. When the divergent LCP light is just focused on the silver film, the excited divergent plasmonic field originates from the center, becoming a hybrid plasmonic field on the film surface. As a result, all particles experience both the trapping and anti-trapping forces. When the proportion of the two components is dynamically adjusted by rotating the QWP, the intensity of the two plasmonic field varies. With the proportion of the LCP component decreasing [Figure 4(a)], the trapping force exerted on the particle increases and the repelling force weakens. Consequently, the repelled particles initially experience an increased trapping force attracting them to the center. The opposite occurs when the LCP component is increased [Figure 4(b)]. As a result, the balance position is tunable to elicit an interesting behavior. When the film deviates from the LCP focus, only focused surface plasmons are present to exert attractive forces on the particles in the area inside the divergent plasmonic field. However, in the outer areas, the two fields are still composited. As has been discussed above, the outer divergent plasmonic field propagates radially to exert a repulsive scattering force on the particles, but the focused field results in an attracting force. When a focused field is excited outside the divergent field, two op-

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posite fields are present in the annular-shaped overlapping area between the two excited circular positions. In addition to the trapping force excited by the focused plasmonic field, there is also a negative gradient force for the divergent field to restrain the repulsive effect. When a metal particle is repelled, the gradient and trapping forces

drags it back. During balance, a fascinating phenomenon occurs as only the targeted particle is trapped stably in the center with the other particles being rearranged in a circle [Figure 4(c)].

Figure 4. Experimental results of Au particles in a tailored plasmonic field. Decreasing (a) and increasing (b) proportion of the LCP component produced by rotating the QWP. (c) Stable trapping of a targeted particle with other particles rearranged circularly around at high concentration. The scale bars (lines at far lower right) are of length 5 μm.

The trap thus relies on a comprehensive cooperation among various forces to form a stable optical potential well. Aside from the optical forces, thermal effects (including thermophoresis and convection) also are present in optical trapping techniques, where heat urges the particles to move towards the heat source at the optical focus center. In experiments, the incident power of the laser beam is 30 mW. Consequently, the thermodynamic effect in our system is insignificant over temperature increments of several degrees Kelvin, as demonstrated in our previous works.12, 21, 24 Although traditional optical elements such as Fresnel zone plates are also capable of demonstrating such trapping or anti-trapping of particles, there are several benefits enjoyed by the liquid crystal-based PSML. First, the relative intensities between the convergent and divergent components, and thus the distance between trapped and excluded particles, can be tuned by simply adjusting the polarization state of the incident light. Second, a 99% higher conversion efficiency in modulating the polarization of light beams can be achieved with this PSML, eliminating deleterious effects of unwanted light beams. More importantly, the spiral-phase modulation of the surface plasmon arising from the spin–orbit interaction of light in tightly focused systems25 can be compensated easily with a more-sophisticated designed meta-surface.26 Furthermore, PSML-assisted trapping is notably repeatable and easy to reestablish. Therefore, a stable means for targeted particle manipulation is at hand.

In conclusion, we have in constructing a special plasmonic trap through a designed PSML to modulate the optical field that enables the focal lengths of the LCP and RCP components to be controlled in opposite ways. By focusing the beams onto a thin silver film through an objective, a tailored plasmonic field was excited at the SPR angle. The excitation position may be adjusted by selectively setting the focal length of the PSML and the relative position of the film and objective lens, to achieve ultimately the selective trapping of a targeted particle. Theoretical and experimental results demonstrate that, by ameliorating nearby disturbances, this system has the capability to achieve precise dynamic manipulations of the targeted particle, which is crucial in molecular detection and dynamic structure assemblies. The designed PSML provides a stable, convenient, and practical tool for specialized trapping and isolation of the target object. We believe this PSML will play a significant role in various applications, where precise manipulations of a single nanoparticle are needed.

AUTHOR INFORMATION Corresponding Author * [email protected] (X. C. Yuan); [email protected] (Y.Q. Zhang).

Author Contributions

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X. W., Y. Z. and Y. D. developed the concept presented in this work. X.W. and Y. D. performed analytical and numerical modeling and designed the device. X. W. fabricated the device. X. W. and Y.Z. conducted measurements. X.W. and Y.Z. wrote the manuscript. X.Y., C.M. and Y.Z. supervised all of the work and oversaw the manuscript. All authors discussed the results and commented on the manuscript. †

X. Wang and Y. Dai contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT National Natural Science Foundation of China (61427819, 61490712, 91750205, U1701661, and 61605117), National Key Basic Research Program of China (973) (2015CB352004), Leading Talents of Guangdong Province Program (00201505), Natural Science Foundation of Guangdong Province (2016A030312010, 2016A030310063, 2017A030313351), National Key Research and Development Program of China (2016YFC0102401), Science and Technology Innovation Commission of Shenzhen (KQTD2017033011044403, ZDSYS201703031605029, JCYJ2017818144338999), Excellent Young Teacher Program of Guangdong Province (YQ2014151), General Financial Grant from the China Postdoctoral Science Foundation (2017M612722).

ABBREVIATIONS PSML, polarization-sensitive metalens; LCP, left-circular polarization; RCP, right-circular polarization; QWP, quarterwave plate; NA, Numerical aperture; SPR, surface plasmon resonance; BFP, back focal plane.

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for single molecule surface-enhanced Raman scattering. Nanoscale 2017, 9, 10694-10700. (9).Du, L. P.; Lei, D. Y.; Yuan, G. H.; Fang, H.; Zhang, X.; Wang, Q.; Tang, D. Y.; Min, C. J.; Maier, S. A.; Yuan, X. C. Mapping plasmonic near-field profiles and interferences by surfaceenhanced Raman scattering. Sci. Rep. 2013, 3, 3064. (10).Marshall, A. R. L.; Stokes, J.; Viscomi, F. N.; Proctor, J. E.; Gierschner, J.; Bouillard, J. S. G.; Adawi, A. M. Determining molecular orientation via single molecule SERS in a plasmonic nano-gap. Nanoscale 2017, 9, 17415-17421. (11).Pang, Y.; Gordon, R. Optical trapping of a single protein. Nano Lett. 2011, 12, 402–406. (12).Zhang, Y. Q.; Wang, J.; Shen, J. F.; Man, Z. S.; Shi, W.; Yuan, G. H.; Min, C. J.; Zhu, S. W.; Urbach, H. P.; Yuan, X.-C. Plasmonic hybridization induced trapping and manipulation of a single Au nanowire on a metallic surface. Nano Lett. 2014, 14, 6430–6436. (13). Reece, P. J.; Paiman, S.; Abdul-Nabi, O.; Gao, Q.; Gal, M.; Tan, H. H.; Jagadish, C. Combined optical trapping and microphotoluminescence of single InP nanowires. Appl. Phys. Lett. 2009, 95, 101109. (14).Ivinskaya, A.; Petrov, M. I.; Bogdanov, A. A.; Shishkin, I.; Ginzburg, P.; Shalin, A. S. Plasmon-assisted optical trapping and anti-trapping. Light-Sci. Appl. 2017, 6, e16258. (15).Tsai, W. Y.; Huang, J. S.; Huang, C. B. Selective Trapping or Rotation of Isotropic Dielectric Microparticles by Optical Near Field in a Plasmonic Archimedes Spiral. Nano Lett. 2014, 14, 547552. (16).Yu, N. F.; Capasso, F. Flat optics with designer metasurfaces. Nat. Mater. 2014, 13, 139-150. (17).Khorasaninejad, M.; Capasso, F. Metalenses: Versatile multifunctional photonic components. Science 2017, 358, eaam8100. (18).Sautter, J.; Staude, I.; Decker, M.; Rusak, E.; Neshev, D. N.; Brener, I.; Kivshar, Y. S. Active Tuning of All-Dielectric Metasurfaces. Acs Nano 2015, 9, 4308-4315. (19).Chen, P.; Lu, Y. Q.; Hu, W. Beam shaping via photopatterned liquid crystals. Liq. Cryst. 2016, 43, 2051-2061. (20).Bliokh, K. Y.; Rodriguez-Fortuno, F. J.; Nori, F.; Zayats, A. V. Spin-orbit interactions of light. Nat. Photon. 2015, 9, 796-808. (21).Min, C. J.; Shen, Z.; Shen, J. F.; Zhang, Y. Q.; Fang, H.; Yuan, G. H.; Du, L. P.; Zhu, S. W.; Lei, T.; Yuan, X. C. Focused plasmonic trapping of metallic particles. Nat. Commun. 2013, 4, 2891. (22).Born, M.; Wolf, E. Principles of Optics. Cambridge University Press: Cambridge, UK, 1999. (23). Prucha, E. D. Handbook of Optical Constants of Solids. Academic Press: New York, USA, 1985. (24).Zhang, L. C.; Dou, X. J.; Min, C. J.; Zhang, Y. Q.; Du, L. P.; Xie, Z. W.; Shen, J. F.; Zeng, Y. J.; Yuan, X. C. In-plane trapping and manipulation of ZnO nanowires by a hybrid plasmonic field. Nanoscale, 2016, 8, 9756-9763. (25).Rodriguez-Herrera, O. G.; Lara, D.; Bliokh, K. Y.; Ostrovskaya, E. A.; Dainty, C. Optical Nanoprobing via SpinOrbit Interaction of Light. Phys. Rev. Lett. 2010, 104, 253601. (26).Devlin, R. C.; Ambrosio, A.; Rubin, N. A.; Mueller, J. P. B.; Capasso, F. Arbitrary spin-to-orbital angular momentum conversion of light. Science 2017, 358, 896-900.

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Figure 1. (a) Schematics of the polarization-directed flat metalens, insert is its polarization imaging; (b) experimental setup to create the tailored plasmonic trapping system using the PSML; and (c) the tailored plasmonic field and trapping of the targeted particle. 382x170mm (300 x 300 DPI)

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Figure 2. Optical field distributions of incident left-circular (a), right-circular (b), and hybrid polarizations (c), at the back focal planes—the polarizations are indicated at top right corner; (d)–(f) Corresponding schematics of the plasmonic excitation for different polarizations; (g)–(h) Calculated electric field intensities at the silver–water interface for three incident polarized light beams following tight focusing from the objective lens; (j)–(l) Distributions of the optical force exerted on the Au particle. 211x237mm (300 x 300 DPI)

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Figure 3. Dynamic behavior of Au particles in plasmonic fields: (a) screenshots of particles located in the focused SPP field and (d) the trajectory of the labeled particles; (b) and (e) results of particles in the outward-spreading SPP field; (c) and (f) results of particles in a combined plasmonic field. The circle marks the outward-spreading field; black crosses indicate the center of the plasmonic field; colored lines are trajectories of particles, each with a color for identification. The scale bars (lines at lower right in a–c) are of length 5 µm. 169x292mm (300 x 300 DPI)

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Figure 4. Experimental results of Au particles in a tailored plasmonic field. Decreasing (a) and increasing (b) proportion of the LCP component produced by rotating the QWP. (c) Stable trapping of a targeted particle with other particles rearranged cir-cularly around at high concentration. The scale bars (lines at far lower right) are of length 5 µm. 152x104mm (300 x 300 DPI)

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