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Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration., Comment: 181 pages, 61 figures

Metamaterial tweezers for trapping of polystyrene particles with 20nm diameter are presented. Proper fabrication of nanostructures on a gold thin film lead to the metamaterials that exhibit Fano interference and are being used for trapping of nanoparticles. The use of metamaterial tweezers significantly enhanced the trapping conditions and demonstrate a trapping stiffness much higher than previously reported in various cases of plasmonic tweezers.

We demonstrated optical trapping of 20 nm particles using a Fano-resonance-assisted plasmonic tweezers based on arrays of asymmetrical split nanoapertures on a 50-nm gold thin film. By transmission and reflection spectra measurements, the close-mode Fano-type excitation peak was estimated at 928 nm. We investigated the trapping performance through power- and wavelength-dependent characterization. We determined the trap stiffness using the transient time method and a linear dependence of the trap stiffness for low incident laser intensities under off-resonance conditions was observed. For the on-resonance condition, a large normalized trap stiffness of 8.65 fN/nm/mW was obtained which enables our system to improved motion control of the trapped nanoparticle. Furthermore, the trap stiffness on-resonance was enhanced by a factor of 63 compared to that of off-resonance. We conclude that this enhancement is due to the ultrasmall mode volume and a cavity effect contribution. Our approach opens new avenues for steady and dynamic optical trapping, making a variety of lab-on-chip applications possible.

We fabricated an array of nanorings of various inner diameters on a 50 nm thick Au film. Herein, we present nanoparticle trapping using this plasmonic array and we investigate the trap stiffness as a function of the inner diameter and the intensity of the trapping laser.

Elliptically polarized evanescent electromagnetic fields can exert a transverse optical force on scattering objects immersed in such fields. We demonstrate this experimentally by setting isotropic, dielectric microspheres into orbital motion around a single-mode ultrathin fiber waveguide. In accordance with the theoretical results, the observed orbiting frequency is proportional to the helicity parameter, which is controlled by the degree of circular polarization of the light coupled to the fiber. The microparticles are rotating against the direction of the energy flow circulation around the fiber, thus verifying the theoretically expected negative optical torque. An important prerequisite for the measurements was the complete polarization control for the ultrathin fiber.

Nanofibre-based optical cavities are particularly useful for quantum optics application, such as the development of integrated single-photon sources, and for studying fundamental light-matter interactions in cavity quantum electrodynamics (cQED). Although several techniques have been used to produce nanofibre-based optical cavities, focussed ion beam (FIB) milling is becoming popular; it can be used for the fabrication of complex structures directly in the nanofibre. This technique uses a highly accelerated ion beam to remove atoms from the target material with high resolution. However, it is challenging to mill insulating materials with highly-curved structures and large aspect ratios, such as silica nanofibres, due to charge accumulation in the material that leads to mechanical vibrations and misalignment issues. In this article, we highlight the main features of nanofibres and briefly review cQED with nanofibre-based optical cavities. An overview of the milling process is given with a summary of different FIB milled devices and their applications. Finally, we present our technique to produce nanofibre cavities by FIB milling. To overcome the aforementioned challenges, we present a specially designed base plate with an indium tin oxide (ITO)-coated Si substrate and outline our procedure, which improves stability during milling and increases repeatability., Comment: The manuscript has 16 pages, 9 figures, and 3 tables, and it will be submitted to Applied Physics B: Laser and Optics

Plasmonic nanostructures can overcome Abbe's diffraction limit to generate strong gradient fields, enabling efficient optical trapping of nano-sized particles. However, it remains challenging to achieve stable trapping with low incident laser intensity. Here, we demonstrate a Fano resonance-assisted plasmonic optical tweezers (FAPOT), for single nanoparticle trapping in an array of asymmetrical split nano-apertures, milled on a 50 nm gold thin film. Stable trapping is achieved by tuning the trapping wavelength and varying the incident trapping laser intensity. A very large normalized trap stiffness of 8.65 fN/nm/mW for 20 nm polystyrene particles at a near-resonance trapping wavelength of 930 nm was achieved. We show that trap stiffness on resonance is enhanced by a factor of 63 compared to off-resonance conditions. This can be attributed to the ultra-small mode volume, which enables large near-field strengths and a cavity Purcell effect contribution. These results should facilitate strong trapping with low incident trapping laser intensity, thereby providing new options for studying transition paths of single molecules, such as proteins, DNA, or viruses., Comment: 28 pages, 7 figures

This publisher’s note reports a correction to the funding section of Optica 7, 59 (2020)OPTIC82334-253610.1364/OPTICA.374441.

Optical manipulation has attracted remarkable interest owing to its versatile and non-invasive nature. However, conventional optical trapping remains inefficient for the nanoscopic world. The emergence of plasmonics in recent years has brought a revolutionary change in overcoming limitations due to diffraction and the requirements for high trapping laser powers. Among the near-field optical trapping cavity-based systems, Fano resonant optical tweezers have a robust trapping capability. In this work, we experimentally demonstrate sequential trapping of 20 nm particles through the use of metamaterial plasmonic optical tweezers. We investigate the multiple trapping via trap stiffness measurements for various trapping positions at low and high incident laser intensities. Our configuration could be used as a light-driven nanoscale sorting device under low laser excitation. Our results provide an alternative approach to trap multiple nanoparticles at distinct hotspots, enabling new ways to control mass transport on the nanoscale., 5

The manipulation of microparticles using optical forces has led to many applications in the life and physical sciences. To extend optical trapping towards the nano-regime, in this work we demonstrate trapping of single nanoparticles in arrays of plasmonic coaxial nano-apertures with various inner disk sizes and theoretically estimate the associated forces. A high normalized experimental trap stiffness of 3.50 fN nm−1 mW−1 μm−2 for 20 nm polystyrene particles is observed for an optimum design of 149 nm for the nanodisk diameter at a trapping wavelength of 980 nm. Theoretical simulations are used to interpret the enhancement of the observed trap stiffness. A quick particle trapping time of less than 8 s is obtained at a concentration of 14 × 1011 particles ml−1 with low incident laser intensity of 0.59 mW μm−2. This good trapping performance with fast delivery of nanoparticles to multiple trapping sites emerges from a combination of the enhanced electromagnetic near-field and spatial temperature increase. This work has applications in nanoparticle delivery and trapping with high accuracy, and bridges the gap between optical manipulation and nanofluidics.

Evanescent electromagnetic fields near a waveguide can exert a transverse radiation force on scattering objects. To prove this experimentally, we demonstrate light-induced orbiting of isotropic, dielectric microparticles around an optical nanofiber that guides elliptically polarized, fundamental modes. The orbit frequency is proportional to the helicity of the coupled light. Interestingly, the observed motion is opposite to the energy flow circulation around the fiber. This result verifies the theoretically predicted negative optical torque on a sufficiently large particle in the vicinity of a nanofiber.

We have used a gold nanohole array to trap single polystyrene nanoparticles, with a mean diameter of 30 nm, into separated hot spots located at connecting nanoslot regions. A high trap stiffness of approximately 0.85 fN/(nm·mW) at a low-incident laser intensity of ∼0.51 mW/μm2 at 980 nm was obtained. The experimental results were compared to the simulated trapping force, and a reasonable match was achieved. This plasmonic array is useful for lab-on-a-chip applications and has particular appeal for trapping multiple nanoparticles with predefined separations or arranged in patterns in order to study interactions between them.

We study spontaneous emission from a rubidium atom into the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We show that the spontaneous emission rate depends on the magnetic sublevel, the type of modes, the orientation of the quantization axis, the position of the atom, and the fiber radius. We find that the rate of spontaneous emission into the TE modes is always symmetric with respect to the propagation directions. Directional asymmetry of spontaneous emission into other modes may appear when the quantization axis does not lie in the meridional plane containing the position of the atom. When the fiber radius is in the range from 330 to 450 nm, the spontaneous emission from an atom on the fiber surface into the ${\mathrm{HE}}_{21}$ modes is stronger than into the ${\mathrm{HE}}_{11}$, ${\mathrm{TE}}_{01}$, and ${\mathrm{TM}}_{01}$ modes. At the cutoff for higher-order modes, the rates of spontaneous emission into guided and radiation modes undergo steep variations, which are caused by the changes in the mode structure. We show that the spontaneous emission from the upper level of the cyclic transition into the TM modes is unidirectional when the quantization axis lies at an appropriate azimuthal angle in the fiber transverse plane.

Precise control of particle positioning is desirable in many optical propulsion and sorting applications. Here, we develop an integrated platform for particle manipulation consisting of a combined optical nanofiber and optical tweezers system. We show that consistent and reversible transmission modulations arise when individual silica microspheres are introduced to the nanofiber surface using the optical tweezers. The observed transmission changes depend on both particle and fiber diameter and can be used as a reference point for in situ nanofiber or particle size measurement. Thence, we combine scanning electron microscope (SEM) size measurements with nanofiber transmission data to provide calibration for particle-based fiber assessment. This integrated optical platform provides a method for selective evanescent field manipulation of micron-sized particles and facilitates studies of optical binding and light-particle interaction dynamics.

We investigated the optical binding between dielectric microparticles in the evanescent fields of the first group of higher order microfiber modes. Particle groups consisting of up to five particles were propelled along the fiber and neighboring interactions were experimentally investigated and supported by numerical simulation.

We present a systematic treatment of higher-order modes of vacuum-clad ultrathin optical fibers. We show that, for a given fiber, the higher-order modes have larger penetration lengths, larger effective mode radii, and larger fractional powers outside the fiber than the fundamental mode. We calculate, both analytically and numerically, the Poynting vector, propagating power, energy, angular momentum, and helicity (or chirality) of the guided light. The axial and azimuthal components of the Poynting vector can be negative with respect to the direction of propagation and the direction of phase circulation, respectively, depending on the position, the mode type, and the fiber parameters. The orbital and spin parts of the Poynting vector may also have opposite signs in some regions of space. We show that the angular momentum per photon decreases with increasing fiber radius and increases with increasing azimuthal mode order. The orbital part of angular momentum of guided light depends not only on the phase gradient but also on the field polarization, and is positive with respect to the direction of the phase circulation axis. Meanwhile, depending on the mode type, the spin and surface parts of angular momentum and the helicity of the field can be negative with respect to the direction of the phase circulation axis., 24 pages, 22 figures

A plasmonic nanoparticle trap based on an array of nanoring structures with a 160 nm inner disk inside a 300 nm nanohole was demonstrated. Based on the extinction coefficient spectrum, 980 nm incident light was selected to trap 500 nm polystyrene particles. The transmitted intensity was collected for the power spectral density calculation to obtain the corner frequency. Compared to a conventional optical tweezers, approximately 20 times lower incident power is needed for this nanoring device to achieve the same trapping strength. Note from the author: With further experiments, we realized that at a higher incident power (as in the original proceeding, 1.45 mW) two-particle trapping events could happen and result in a higher value for the trap stiffness for the plasmonic tweezers. To eliminate two-particle trapping events, we have applied a lower incident power (0.6 mW) to guarantee single particle trapping and checked images of the trapped particle with a CCD camera. For a proper comparison to conventional optical tweezers, we updated the value of trap stiffness for our plasmonic tweezers for single, 0.5 µm polystyrene particle trapping at low incident power.

Trapping and detection of micro- and nano-objects at low incident laser powers in the near-infrared region (NIR) is crucial for many biological applications [1]. Using a conventional optical tweezer method, trapping is limited to dielectric particles larger than 100nm in size with high incident laser powers. For this reason, plasmonic nanostructures, which have recently attracted research attentions, offered enhanced trapping fields at their resonant wavelengths, and subsequently can be used to trap and detect silica dielectric particles down to 12nm and single 3.4nm bovine serum albumin proteins using low incident powers (< 1mW) [2].

Evanescent fields in optical micro- and nanofibres (MNF) have been proposed as efficient means for multiple microparticle trapping and prolusion [1, 2]. Here, we demonstrate the propulsion of 3 µm polystyrene particle chains in the evanescent fields of the fundamental and first higher order modes in an MNF system. The power at the waist was estimated to be 25mW in both cases Self-assembly and speed variation of particle chains was observed. This observation implies that strong optical interactions between trapped particles within a particle chain occur along the fibre waist region The effect is associated with the long-range, one dimensional optical binding. The numerical finite element and analytical scattering-matrix approaches were used to investigate the field dynamics surrounding the trapped particles. The optical forces and velocities of bound particle chains were calculated using these particle-field dynamics. Both the analytical and numerical analyses show good agreements with the experimental data The observation reveal that higher order modes in a microfibre offer stable multiple particle trapping faster particle propulsion speeds, and allow for the capability to better control each individual trapped object in particle ensembles near the microfibre surface compare to the fundamental mode [3].

We report on the optical manipulation of microparticles using higher order mode propagation in a tapered optical fiber. We show that the speed of particles is higher compared to those influenced by fundamental mode evanescent field.

While conventional optical trapping techniques can trap objects with submicron dimensions, the underlying limits imposed by the diffraction of light generally restrict their use to larger or higher refractive index particles. As the index and diameter decrease, the trapping difficulty rapidly increases; hence, the power requirements for stable trapping become so large as to quickly denature the trapped objects in such diffraction-limited systems. Here, we present an evanescent field based device capable of confining low index nanoscale particles using modest optical powers as low as 1.2 mW, with additional applications in the field of cold atom trapping. Our experiment uses a nanostructured optical micro-nanofiber to trap 200 nm, low index contrast, fluorescent particles within the structured region, thereby overcoming diffraction limitations. We analyze the trapping potential of this device both experimentally and theoretically, and show how strong optical traps are achieved with low input powers., Comment: 13 pages, 6 figures

We present experimental and theoretical results on chains of microparticles optically bound in the evanescent field of ultrathin optical fibers that can support the fundamental, $LP_{01}$ , and first group, $LP_{11}$ , of higher order fiber modes.

The propagation of higher modes, such as the LP 11 mode, in optical nanofibers using the exponentially tapered optical fiber as a basic model is investigated. In order to preserve the LP 11 mode in the downtaper as far as the nanofiber waist, the effect of varying the cladding-core radius ratio on the LP 11 adiabatic criterion is modeled. A Laguerre–Gaussian beam is created in free space using a spatial light modulator (SLM) and coupled to a few-mode fiber. This device allows convenient switching between the fundamental and LP 11 fiber modes. By selecting a few-mode fiber with a relatively low cladding-core ratio, the propagation of the LP 11 mode down to a submicron waist has been maintained. Furthermore, by observing the transmission profile during tapering, it is possible to decisively terminate the pulling process in order to eliminate the two degenerate HE 21 modes of the LP 11 mode. As a result, a nanofiber can be fabricated through which only the TE 01 and TM 01 components of the LP 11 mode propagate. Such a nanofiber has promising applications in the area of mode interference for controlled particle trapping sites.

We study the force of light on a two-level atom near an ultrathin optical fiber using the mode function method and the Green tensor technique. We show that the total force consists of the driving-field force, the spontaneous-emission recoil force, and the fiber-induced van der Waals potential force. Due to the existence of a nonzero axial component of the field in a guided mode, the Rabi frequency and, hence, the magnitude of the force of the guided driving field may depend on the propagation direction. When the atomic dipole rotates in the meridional plane, the spontaneous-emission recoil force may arise as a result of the asymmetric spontaneous emission with respect to opposite propagation directions. The van der Waals potential for the atom in the ground state is off-resonant and opposite to the off-resonant part of the van der Waals potential for the atom in the excited state. Unlike the potential for the ground state, the potential for the excited state may oscillate depending on the distance from the atom to the fiber surface.

In this work, we demonstrate trapping of microparticles using plasmonic tweezers based on arrays of annular apertures. The transmission spectra and the electric-field distribution are simulated to calibrate the arrays. Theoretically, we observe sharp peaks in the transmission spectra for dipole resonance modes and these are red-shifted as the size of the annular aperture is reduced. We also expect an absorption peak at approximately 1115 m for the localised plasmon resonance. Using a laser frequency between the two resonances, multiple plasmonic hot spots are created and used to trap and transport micron and submicron particles. Experimentally, we demonstrate trapping of individual 0.5 μm and 1 μm polystyrene particles and the feasibility of particle transportation over the surface of the annular apertures using less than 1.5 mW μm−2 incident laser intensity at 980 nm.

We calculate the force of a near-resonant guided light field of an ultrathin optical fiber on a two-level atom. We show that, if the atomic dipole rotates in the meridional plane, the magnitude of the force of the guided light depends on the field propagation direction. The chirality of the force arises as a consequence of the directional dependencies of the Rabi frequency of the guided driving field and the spontaneous emission from the atom. This provides a unique method for controlling atomic motion in the vicinity of an ultrathin fiber., Comment: text and figures were revised, and a new discussion was added

The use of optical micro- and nanofibers has become commonplace in the areas of atom trapping using neutral atoms and, perhaps more relevantly, the optical trapping and propulsion of micro- and nanoscale particles. It has been shown that such fibers can be used to manipulate and trap silica and polystyrene particles in the 1-3 µm range using either the fundamental or higher order modes of the fibers, with the propulsion of smaller particle sizes also possible through the use of metallic and/or high index materials. We previously proposed using a focused ion beam nanostructured tapered optical fiber for improved atom trapping geometries; here, we present the details of how these nanostructured optical fibers can be used as a platform for submicron particle trapping. The optical fibers are tapered to approximately 1.2 µm waist diameters, using a custom-built, heat-and-pull fiber rig prior to processing using a focused ion beam. Slots of approximately 300 nm in width and 10-20 µm in length are milled clean though the waist regions of the tapered optical fibers. High fiber transmissions (> 80%) over a broad range of wavelengths (700-1100 nm) are observed. We present simulation results for the trapping of submicron particles and experimental results on the trapping of 200 nm particles. This work demonstrates even further the functionality of optical micro- and nanofibers as trapping devices across a range of regimes.

We study the optical properties of hybrid gold nanodisk and nanohole arrays and present experimental evidence of nanoparticle trapping using these devices. The fabrication procedure using electron beam lithography (EBL) is also discussed. This hybrid design exhibits a splitting of the resonance modes (low and high energy modes) due to the coupling of the electromagnetic interaction between nanohole and nanodisk plasmons. The devices demonstrate high plasmon resonance tunabilities from the visible to the near-infrared region (NIR) by varying the dimensions of the features of this design. This enhancement in the NIR is highly desirable for the purposes of biological sample manipulation where photo damage should be low. Additionally, these devices consist of grooves connecting the hybrid structures to each other. These regions provide further enhancement of the local electric fields and play the role of the trapping sites. We demonstrate multiple dielectric nanoparticle trapping in these grooves while the devices are excited by evanescent fields via the Kretschmann configuration. The results provide good evidence of the potential of this design to be used for the manipulation of biological samples with sub-diffraction limit sizes.

Optical micro- and nanofibers have attracted great interest due to their potential for trapping and manipulating micronsized particles in the evanescent field that extends from their surface. To date, most particle manipulation research has been limited to fundamental mode propagation within the ultrathin fiber. In this work, we study propulsion of polystyrene particles under the influence of the evanescent field from the first order guided modes and compare it to the fundamental mode case. Speeds of single and double particles are compared, and the optical binding effect between particles is analyzed. The dependence of the particles’ speeds on their diameter is studied for 1 μm, 3 μm, and 5 μm diameter polystyrene particles using the higher order modes of the fibers. Furthermore, we also study the material dependence of particle propulsion speeds.

Propulsion of polystyrene particles in the evanescent field of the first group of higher order modes in an optical microfiber were studied. Higher speeds were observed for higher order mode propulsion than for fundamental mode.

Optical micro- and nanofibers have recently gained popularity as tools in quantum engineering using laser-cooled, neutral atoms. In particular, atoms can be trapped around such optical fibers, and photons coupled into the fibers from the surrounding atoms could be used to transfer quantum state information within the system. It has also been demonstrated that such fibers can be used to manipulate and trap silica and polystyrene particles in the 1-3 μm range. We recently proposed using a focused ion beam nanostructured tapered optical fiber for improved atom trapping geometries1. Here, we present details on the design and fabrication of these nanostructured optical fibers and their integration into particle trapping platforms for the demonstration of submicron particle trapping. The optical fibers are tapered to approximately 1-2 μm waist diameters, using a custom-built, heat-and-pull fiber rig, prior to processing using a focused ion beam. Slots of about 300 nm in width and 10-20 μm in length are milled right though the waist regions of the tapered optical fibers. Details on the fabrication steeps necessary to ensure high optical transmission though the fiber post processing are included. Fiber transmissions of over 80% over a broad range of wavelengths, in the 700-11100 nm range, are attainable. We also present simulation results on the impact of varying the slot parameters on the trap depths achievable and milling multiple traps within a single tapered fiber. This work demonstrates even further the functionality of optical micro- and nanofibers as trapping devices across a range of regimes.

Optical manipulation in the vicinity of optical micro- and nanofibres has shown potential across several fields in recent years, including microparticle control, and cold atom probing and trapping. To date, most work has focussed on propagation of the fundamental mode through the fibre. However, along the maximum mode intensity axis, higher order modes have a longer evanescent field extension and larger field amplitude at the fibre waist compared to the fundamental mode, opening up new possibilities for optical manipulation and particle trapping. In this work, we demonstrate a microfibre/optical tweezers compact system for trapping and propelling dielectric particles based on the excitation of the first group of higher order modes at the fibre waist. Single polystyrene particles were trapped and propelled in the evanescent fields of higher order and fundamental modes near the surface of microfibres. Speed enhancement of particle propulsion was observed for the higher order modes compared to the fundamental mode for particles ranging from 1 {\mu}m to 5 {\mu}m in diameter. The optical propelling velocity of a single, 3 {\mu}m polystyrene particle was found to be 8 times faster under the higher order evanescent field than the fundamental mode field for a waist power of 25 mW. Experimental data and dynamic interactions between the evanescent field of these two different fibre modes and the particles are supported by theoretical calculations. This work can be extended to trapping and manipulation of laser-cooled atoms for quantum networks.

A new class of hybrid materials for efficient laser emission and amplification in the 1.55 μm eye-safe telecommunication window has been designed, elaborated and characterized, based on lanthanide-doped yttrium oxide nanoparticle guests embedded in a polymer host matrix. The average size of these nanocrystals can be finely tuned by controlling the relative quantities of an organic fuel (glycine) with respect to the yttrium- and lanthanide-containing reactants involved in the combustion reaction leading to nanoparticle synthesis. Two complementary sensitizers have been added to Er/Y 2 O 3 nanocrystals in order to improve the Erbium luminescence efficiency: cerium to increase population inversion between the emitting and the ground state by accelerating the de-excitation process from the 4 I 11/2 initially uppermost excited state at 980 nm down to 4 I 13/2 state emitting around 1.55 μm, and ytterbium to improve the pumping efficiency of the material owing to its strong absorption cross section at 980 nm and subsequent efficient energy transfer towards the 4 I 11/2 state of erbium ion. Y 2 O 3 nanoparticles with an optimized erbium–ytterbium composition have been incorporated in a PMMA polymer matrix, resulting in a hybrid amorphous material displaying high gain coefficient values (up to 30 cm −1 as from Amplified Spontaneous Emission experiments) upon moderate 980 nm pumping intensities. These materials open attractive perspectives in the domain of polymer-based telecom amplifiers, with the additional asset of multifunctional properties whereby gain originating from the inorganic Y 2 O 3 nanocrystals can be associated to nonlinear responses attached to an adequately functionalized polymer host matrix.

We experimentally realized an optical nanofiber-based cavity by combining a 1-D photonic crystal and Bragg grating structures. The cavity morphology comprises a periodic, triplex air-cube introduced at the waist of the nanofiber. The cavity has been theoretically characterized using finite-difference time-domain simulations to obtain the reflection and transmission spectra. We have also experimentally measured the transmission spectra, and a Q-factor of ∼784 ± 87 for a very short periodic structure has been observed. The structure provides strong confinement of the cavity field, and its potential for optical network integration makes it an ideal candidate for use in nanophotonic and quantum information systems.

Plasmonic tweezers based on nano-ring arrays on gold thin film are demonstrated. A cylindrical surface plasmon resonance is generated in the aperture of a nano-ring and a transmission peak results. When nano-slits are included to connect the nano-rings, the transmission peak becomes narrower. When the size of the aperture of the nano-ring is reduced, this peak is red-shifted. Both 0.5 μm and 1 μm polystyrene particles are trapped successfully by nano-ring arrays. A self-induced back-action effect is observed when a red-shifted laser beam is used. With multiple trapping sites provided by the nano-ring array, this type of plasmonic tweezers has huge potential to be integrated in lab-on-a-chip systems for life sciences research.