Nano-optical Trapping Research Articles

Opto-thermoelectric trapping of fluorescent nanodiamonds on plasmonic nanostructures.

Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this Letter, we show how chemically prepared gold nanoparticles and silver nanowires can facilitate an opto-thermoelectric force to trap individual entities of FNDs using a long working distance lens, low power-density illumination (532-nm laser, 12 µW/µm2). Our trapping configuration combines the thermoplasmonic fields generated by individual plasmonic nanoparticles and the opto-thermoelectric effect facilitated by the surfactant to realize a nano-optical trap down to a single FND that is 120 nm in diameter. We use the same trapping excitation source to capture the spectral signatures of single FNDs and track their position. By tracking the FND, we observe the differences in the dynamics of the FND around different plasmonic structures. We envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells.

Multiplexed Long-Range Electrohydrodynamic Transport and Nano-Optical Trapping with Cascaded Bowtie Photonic Crystal Nanobeams.

Photonic crystal cavities with bowtie defects that combine ultrahigh Q and ultralow mode volume are theoretically studied for low-power nanoscale optical trapping. By harnessing the localized heating of the water layer near the bowtie region, combined with an applied alternating current electric field, this system provides long-range electrohydrodynamic transport of particles with average radial velocities of 30 μm/s towards the bowtie region on demand by switching the input wavelength. Once transported to a given bowtie region, synergistic interaction of optical gradient and attractive negative thermophoretic forces stably trap a 10nm quantum dot in a potential well with a depth of 10 k_{B}T using a mW input power.

Optical Trapping, Sensing, and Imaging by Photonic Nanojets

The optical trapping, sensing, and imaging of nanostructures and biological samples are research hotspots in the fields of biomedicine and nanophotonics. However, because of the diffraction limit of light, traditional optical tweezers and microscopy are difficult to use to trap and observe objects smaller than 200 nm. Near-field scanning probes, metamaterial superlenses, and photonic crystals have been designed to overcome the diffraction limit, and thus are used for nanoscale optical trapping, sensing, and imaging. Additionally, photonic nanojets that are simply generated by dielectric microspheres can break the diffraction limit and enhance optical forces, detection signals, and imaging resolution. In this review, we summarize the current types of microsphere lenses, as well as their principles and applications in nano-optical trapping, signal enhancement, and super-resolution imaging, with particular attention paid to research progress in photonic nanojets for the trapping, sensing, and imaging of biological cells and tissues.

Single Photon Source from a Nanoantenna-Trapped Single Quantum Dot.

Single photon sources with high brightness and subnanosecond lifetimes are key components for quantum technologies. Optical nanoantennas can enhance the emission properties of single quantum emitters, but this approach requires accurate nanoscale positioning of the source at the plasmonic hotspot. Here, we use plasmonic nanoantennas to simultaneously trap single colloidal quantum dots and enhance their photoluminescence. The nano-optical trapping automatically locates the quantum emitter at the nanoantenna hotspot without further processing. Our dedicated nanoantenna design achieves a high trap stiffness of 0.6 (fN/nm)/mW for quantum dot trapping, together with a relatively low trapping power of 2 mW/μm2. The emission from the nanoantenna-trapped single quantum dot shows 7× increased brightness, 50× reduced blinking, 2× shortened lifetime, and a clear antibunching below 0.5 demonstrating true single photon emission. Combining nano-optical tweezers with plasmonic enhancement is a promising route for quantum technologies and spectroscopy of single nano-objects.

Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping.

Plasmonic nanotweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nanotweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nanotweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

Plasmonic optical tweezers based on nanostructures: fundamentals, advances and prospects

Abstract The ability of metallic nanostructures to confine light at the sub-wavelength scale enables new perspectives and opportunities in the field of nanotechnology. Making use of this unique advantage, nano-optical trapping techniques have been developed to tackle new challenges in a wide range of areas from biology to quantum optics. In this work, starting from basic theories, we present a review of research progress in near-field optical manipulation techniques based on metallic nanostructures, with an emphasis on some of the most promising advances in molecular technology, such as the precise control of single biomolecules. We also provide an overview of possible future research directions of nanomanipulation techniques.

Temperature Measurement in Plasmonic Nanoapertures Used for Optical Trapping

Plasmonic nanoapertures generate strong field gradients enabling efficient optical trapping of nano-objects. However, because the infrared laser used for trapping is also partly absorbed into the metal leading to Joule heating, plasmonic nano-optical tweezers face the issue of local temperature increase. Here, we develop three independent methods based on molecular fluorescence to quantify the temperature increase induced by a 1064 nm trapping beam focused on single and double nanoholes milled in gold films. We show that the temperature in the nanohole can be increased by 10{\deg}C even at the moderate intensities of 2 mW/$\mu$m$^2$ used for nano-optical trapping. The temperature gain is found to be largely governed by the Ohmic losses into the metal layer, independently of the aperture size, double-nanohole gap or laser polarization. The techniques developed therein can be readily extended to other structures to improve our understanding of nano-optical tweezers and explore heat-controlled chemical reactions in nanoapertures.

Prediction of metallic nano-optical trapping forces by finite element-boundary integral method.

The hybrid of finite element and boundary integral (FE-BI) method is employed to predict nano-optical trapping forces of arbitrarily shaped metallic nanostructures. A preconditioning strategy is proposed to improve the convergence of the iterative solution. Skeletonization is employed to speed up the design and optimization where iteration has to be repeated for each beam configuration. The radiation pressure force (RPF) is computed by vector flux of the Maxwell's stress tensor. Numerical simulations are performed to validate the developed method in analyzing the plasmonic effects as well as the optical trapping forces. It is shown that the proposed method is capable of predicting the trapping forces of complex metallic nanostructures accurately and efficiently.

Scaling in light scattering by sharp conical metal tips.

Using the electrostatic approximation, we analyze electromagnetic fields scattered by sharp conical metal tips, which are illuminated with light polarized along the tip axis. We establish scaling relations for the scattered field amplitude and phase, and verify the validity with numerical simulations. Analytic expressions for the wavelength at which the scattered field near the tip changes its direction and for the field decay near the tip extremity are obtained, relating these characteristics to the cone angle and metal permittivity. The results obtained have important implications for various tip-enhanced phenomena, ranging from Raman and scattering near-field imaging to photoemission spectroscopy and nano-optical trapping.

Dressing plasmon resonance with particle-microcavity architecture for efficient nano-optical trapping and sensing

We propose a particle-microcavity scheme for efficient optical trapping and sensing. When a resonant plasmonic nanoparticle (NP) is placed inside a microcavity with high Q-factor, sensitivity is enhanced in the far-field extinction while near-field around the NP is barely affected. Stable near-field and high sensitivity for optical trapping and ultrasensitive detection of nanosized targets are therefore realized simultaneously. Such a particle-microcavity system opens up a new hybrid nanophotonic device platform that combines the unique merits of conventional and plasmonic integrated photonics.

Photocatalytic nano-optical trapping using TiO2 nanosphere pairs mediated with Mie-scattered near field

The localized enhanced near field on nanostructures has been attracting much attention for a template for size-selective optical trapping (tweezers) beyond the diffraction limit. The near-field optical trapping has mainly been studied using metallic substrates such as Au nanodot pairs, periodic Al nanoslits, nanoapertures on an Au film, etc. In this paper, we newly propose a Mie-scattered-near-field optical trapping scheme for size-selective photocatalytic application using pairs of poly-rutile TiO2 nanospheres. The optical intensity distribution in a 3D-nanogap space between the nanospheres was simulated by a 3D FDTD method. The simulation system consists of the two TiO2 nanospheres placed on a silica substrate in water. The 400-nm excitation laser is used for both the near-field trapping and the photocatalyst excitation. The optical trapping forces were calculated based on the near-field optical intensity distribution. The trapping stiffness for 20-nm polystyrene sphere at a gap distance of 20 nm was 6.4 pN/nm/W. The optical force vector shows that the object like virus can be trapped with sufficient forces into the nanogap space and then is driven into the direct surface of the TiO2 sphere. This result suggests that this system works as a photocatalytic trapping for killing virus, protein, etc.