Guided transport of nanoparticles by plasmonic nanowires

Controllable transportation of microparticles along structured waveguides by the plasmonic spin-hall effect

With the nanoscale integration advantage of near field photonics, controllable manipulation and transportation of micro-objects have possessed plentiful applications in the fields of physics, biology and material sciences. However, multifunctional optical manipulation like controllable transportation and synchronous routing by nano-devices are limited and rarely reported. Here we propose a new type of Y-shaped waveguide optical conveyor belt, which can transport and route particles along the structured waveguide based on the plasmonic spin-hall effect. The routing of micro-particles in different branches is determined by the optical force components difference at the center of the Y junction along the two branches of the waveguide. The influence of light source and structural parameters on the optical forces and transportation capability are numerically studied. The results illustrate that the proposed structured waveguide optical conveyor belt can transport the microparticles controllably in different branches of the waveguide. Due to the selective transportation ability of microparticles by the 2D waveguide, our work shows great application potential in the region of on-chip optical manipulation.

Optothermal pulling, trapping, and assembly of colloids using nanowire plasmons

Optical excitation of colloids can be harnessed to realize soft matter systems that are out of equilibrium. In this paper, we present our experimental studies on the dynamics of silica colloids in the vicinity of a silver nanowire propagating surface plasmon polaritons (SPPs). Due to the optothermal interaction, the colloids are directionally pulled towards the excitation point of the nanowire. Having reached this point, they are spatio-temporally trapped around the excitation location. By increasing the concentration of colloids in the system, we observe multi-particle assembly around the nanowire. This process is thermophoretically driven and assisted by the SPPs. Furthermore, we find such an assembly to be sensitive to the excitation polarization at the input of the nanowire. Numerically-simulated temperature distribution around an illuminated nanowire corroborates sensitivity to the excitation polarization. Our study will find relevance in exploration of SPP-assisted optothermal pulling, trapping and assembly of colloids, and can serve as a test-bed of plasmon-driven active matter.

Plasmon-Directed On-Wire Growth of Branched Silver Nanowires with Chiroptic Activity

Silver nanowires (Ag NWs) present prominent waveguiding properties of subwavelength light due to their nanoconfinement with propagating surface plasmons, which is of great importance for on-chip integration of nanophotonic devices and optical computation. Such propagating plasmons also exert plasmonic forces, which can be utilized to manipulate nanoparticles (NPs) beyond the diffraction limit. However, such controllability is spatially limited to the near fields, whereas a large portion of uncontrolled particles are randomly deposited on the chips, which could be detrimental to the integrated optical devices. Herein we shine continuous wave laser at one end of the Ag NW immersed in AgNO₃ solution to launch the propagating surface plasmons. The laser irradiation also induces the photoreduction of Ag⁺ ions to locally generate tiny Ag NPs, which evolve into large Ag flake branches closer to the other end of the Ag NW. Such a peculiar growth is due to the synergistic effect of plasmonic forces and the thermophoretic/thermo-osmosis forces induced by temperature gradient. These branched Ag NWs with sharp angles are intrinsically chiral, which can be partially controlled by changing the irradiation location, forming plasmonic chiral enantiomers. The circular differential scattering (CDS) response of these branched Ag NWs can be as large as 40%, which can be used for chiral enantiomer sensing with spectral dissymmetric factor up to 4 nm induced by phenylalanine. This plasmon-directed on-wire growth not only offers a facile approach for generating plasmonic chiral nanostructures with remote controllability, but also provides significant insights on the synergistic effect of plasmonic forces and thermal-induced forces, which has great implications for self-assembly and integration of on-chip optics.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Controllable transport of nanoparticles along waveguides by spin-orbit coupling of light

Waveguide optical tweezers can capture and transport nanoparticles, and have important applications in biology, physics, and materials science. However, traditional waveguide optical tweezers need to couple incident light into one end of the waveguide, which causes problems such as difficulty in alignment and low efficiency. Here, we propose a new type of waveguide optical tweezers based on spin-orbit coupling of light. Under the effect of spin-orbit coupling between the waveguide and nearby particles illuminated by a circularly polarized light, the particles experience a lateral recoil force and a strong optical gradient force, which make particles in a large area to be trapped near the waveguide and then transmitted along the waveguide, avoiding the coupling of light into one end of the waveguide. We further demonstrate that the particles can be transmitted along a curved waveguide and even rotated along a ring-shaped waveguide, and its transmission direction can be simply switched by adjusting the spin polarization of incident light. This work has significance in the research of optical on-chip nano-tweezers.

Plasmon-driven nanowire actuators for on-chip manipulation

Chemically synthesized metal nanowires are promising building blocks for next-generation photonic integrated circuits, but technological implementation in monolithic integration will be severely hampered by the lack of controllable and precise manipulation approaches, due to the strong adhesion of nanowires to substrates in non-liquid environments. Here, we demonstrate this obstacle can be removed by our proposed earthworm-like peristaltic crawling motion mechanism, based on the synergistic expansion, friction, and contraction in plasmon-driven metal nanowires in non-liquid environments. The evanescently excited surface plasmon greatly enhances the heating effect in metal nanowires, thereby generating surface acoustic waves to drive the nanowires crawling along silica microfibres. Advantages include sub-nanometer positioning accuracy, low actuation power, and self-parallel parking. We further demonstrate on-chip manipulations including transporting, positioning, orientation, and sorting, with on-situ operation, high selectivity, and great versatility. Our work paves the way to realize full co-integration of various functionalized photonic components on single chips.

Implementing metal nanowires in photonic circuits is challenging due to lack of suitable manipulation techniques. Here, the authors present an earthworm-like peristaltic crawling motion mechanism, based on surface plasmons and surface acoustic waves, and show on-chip manipulations of single nanowires.

Characterisation and Manipulation of Polarisation Response in Plasmonic and Magneto-Plasmonic Nanostructures and Metamaterials

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

Tunable optical lattices in the near-field of a few-mode nanophotonic waveguide

Due to the action of the scattering force, particles that are optically trapped at the surface of a waveguide are propelled in the direction of the light propagation. In this work, we demonstrate an original approach for creating tunable periodic arrays of optical traps along a few-mode silicon nanophotonic waveguide. We show how the near-field optical forces at the surface of the waveguide are periodically modulated when two guided modes with different propagation constants are simultaneously excited. The phenomenon is used to achieve stable trapping of a large number of dielectric particles or bacteria along a single waveguide. By controlling the light coupling conditions and the laser wavelength, we investigate several techniques for manipulating the trapped particles. Especially, we demonstrate that the period of the optical lattice can be finely tuned by adjusting the laser wavelength. This effect can be used to control the trap positions, and thus transport the trapped particles in both directions along the waveguide.

Sub-10 nm particle trapping enabled by a plasmonic dark mode

We demonstrate that a highly localized plasmonic dark mode with radial symmetry, termed quadrupole-bonded radial breathing mode, can be used for optically trapping the dielectric nanoparticles. In particular, the annular potential well produced by this dark mode shows a sufficiently large depth to stably trap the 5 nm particles under a relatively low optical power. Our results address the quest for precisely trapping sub-10 nm particles with high yield and pave the way for placing sub-10 nm particles conforming to a specific geometric pattern.

Synthesis of silver nanoplate based two-dimension plasmonic platform from 25 nm to 40 μm: growth mechanism and optical characteristic investigation in situ

We show high-purity synthesis, structural engineering and in situ optical investigation of a 2D plasmonic platform using huge silver nanoplates.