Optical radiation forces of focused Gaussian beams on the three-layered microgel particles with near-infrared responses

Wei Ju, Jie Yao, He-Ping Ding, Da-Jian Wu

Optical manipulation of three-layered microgel particles with near-infrared responses is important for drug delivery and release in vivo. In this work, we investigate the optical radiation force (ORF) on a three-layered SiO2–Au–pNIPAM particle in a focused Gaussian beam (FGB) based on Mie scattering theory. As the radius of inner SiO2 core increases to 43 nm, the dipole plasmon resonance of the SiO2–Au–pNIPAM particle moves to ~ 800 nm. In the vicinity of this wavelength, the ORF of the FGB on the SiO2–Au–pNIPAM is always positive due to the strong scattering. As the working wavelength is larger than ~ 1100 nm, the gradient force on the particle becomes stronger than the scattering force, and thus the negative ORF is realized. We focus on negative ORF on the SiO2–Au–pNIPAM, and find that the beam waist radius, the outer gel shell, and the embedding medium all influence the ORF on the SiO2–Au–pNIPAM particle. The present work may be helpful for manipulating three-layered microgel particles, with the negative ORF being particularly important for particle trapping.

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Light-driven self-assembly of hetero-shaped gold nanorods

Jiunn-Woei Liaw, Hsueh-Yu Chao, Cheng-Wei Huang, Mao-Kuen Kuo

Light-driven self-assembly and coalescence of two nearby hetero-shaped gold nanorods (GNRs) with different lengths are studied theoretically. The optical forces and torques, in terms of Maxwell’s stress tensor, upon these GNRs provided by a linearly polarized (LP) plane wave are analyzed using the multiple multipole (MMP) method. Numerical results show that the optical torque dominates their alignments and the optical force their attraction. The most likely outcome of the plasmon-mediated light–matter interaction is wavelength dependent. Three different coalescences of the two GNRs could be induced by a LP light in three different wavelength regimes, respectively. For example, the side-by-side coalescence of two GNRs with radius of 15 nm and different lengths (120 and 240 nm) is induced in water as irradiated by a LP light at 633 nm, the T-shaped one at 1064 nm, and the end-to-end one at 1700 nm. The plasmonic attractive force and heating power densities inside GNRs with different gaps are also studied; the smaller the gap, the larger the attractive force and heating power. The results imply that the plasmonic coalescence and heating of two discrete GNRs may cause the local fusion at the junction of the assembly and the subsequent annealing (even recrystallization). Because the heating makes the two discrete GNRs fused to become a new nanostructure, the plasmonic coalescence of optical manipulation is irreversible.

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Force, torque, linear momentum, and angular momentum in classical electrodynamics

Masud Mansuripur

The classical theory of electrodynamics is built upon Maxwell’s equations and the concepts of electromagnetic (EM) field, force, energy, and momentum, which are intimately tied together by Poynting’s theorem and by the Lorentz force law. Whereas Maxwell’s equations relate the fields to their material sources, Poynting’s theorem governs the flow of EM energy and its exchange between fields and material media, while the Lorentz law regulates the back-and-forth transfer of momentum between the media and the fields. An alternative force law, first proposed by Einstein and Laub, exists that is consistent with Maxwell’s equations and complies with the conservation laws as well as with the requirements of special relativity. While the Lorentz law requires the introduction of hidden energy and hidden momentum in situations where an electric field acts on a magnetized medium, the Einstein–Laub (E–L) formulation of EM force and torque does not invoke hidden entities under such circumstances. Moreover, total force/torque exerted by EM fields on any given object turns out to be independent of whether the density of force/torque is evaluated using the law of Lorentz or that of Einstein and Laub. Hidden entities aside, the two formulations differ only in their predicted force and torque distributions inside matter. Such differences in distribution are occasionally measurable, and could serve as a guide in deciding which formulation, if either, corresponds to physical reality.

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Design of a high-performance optical tweezer for nanoparticle trapping

D. Conteduca, F. Dell’Olio, C. Ciminelli , T. F. Krauss, M. N. Armenise

Integrated optical nanotweezers offer a novel paradigm for optical trapping, as their ability to confine light at the nanoscale leads to extremely high gradient forces. To date, nanotweezers have been realized either as photonic crystal or as plasmonic nanocavities. Here, we propose a nanotweezer device based on a hybrid photonic/plasmonic cavity with the goal of achieving a very high quality factor-to-mode volume (Q/V) ratio. The structure includes a 1D photonic crystal dielectric cavity vertically coupled to a bowtie nanoantenna. A very high Q/V ~ 107 (λ/n)−3 with a resonance transmission T = 29 % at λ R = 1381.1 nm has been calculated by 3D finite element method, affording strong light–matter interaction and making the hybrid cavity suitable for optical trapping. A maximum optical force F = −4.4 pN, high values of stability S = 30 and optical stiffness k = 90 pN/nm W have been obtained with an input power P in = 1 mW, for a polystyrene nanoparticle with a diameter of 40 nm. This performance confirms the high efficiency of the optical nanotweezer and its potential for trapping living matter at the nanoscale, such as viruses, proteins and small bacteria.

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Nanothermometry using optically trapped erbium oxide nanoparticle

Susil Baral, Samuel C. Johnson, Arwa A. Alaulamie, Hugh H. Richardson
A new optical probe technique using a laser-trapped erbium oxide nanoparticle (size ~150 nm) is introduced that can measure absolute temperature with a spatial resolution on the size of the trapped nanoparticle. This technique (scanning optical probe thermometry) is used to collect a thermal image of a gold nanodot prepared with hole-mask colloidal lithography. A convolution analysis of the thermal profile shows that the point spread function of our measurement is a Gaussian with a FWHM of 165 nm. We attribute the width of this function to clustering of Er2O3 nanoparticles in solution. The scanning optical probe thermometer is used to measure the temperature where vapor nucleation occurs in degassed water (555 K), confirming that a nanoscale object heated in water will superheat the surrounding water to the spinodal decomposition temperature. Subsequently, the temperature inside the vapor bubble rises to the melting point of the gold nanostructure (~1300) where a temperature plateau is observed. The rise in temperature is attributed to inhibition of thermal transfer to the surrounding liquid by the thermal insulating vapor cocoon.

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Maxwell stress induced optical torque upon gold prolate nanospheroid

Jiunn-Woei Liaw, Ying-Syuan Chen, Mao-Kuen Kuo

This study theoretically analyzes the surface traction on an elongated Au prolate nanospheroid to examine the resultant optical torque exerted by an optical tweezers. The multiple multipole method is applied to evaluate quantitatively the electromagnetic field induced by a linearly polarized plane wave illuminating a nanospheroid, then obtaining the surface traction in terms of Maxwell stress tensor. The optical torque is calculated by the surface integral of the cross product of position vector and traction over the nanospheroid’s surface. Our results show that two pairs of positive and negative traction zones at the two apexes of the nanospheroid play a critical role. Furthermore, the resultant optical torque is wavelength-dependent. If the wavelength is shorter than the longitudinal surface plasmon resonance (LSPR) of the nanospheroid, the optical torque rotates the long axis of nanospheroid perpendicular to the polarization direction of the incident wave. In contrast, if the wavelength is longer than the LSPR the long axis is pushed parallel to the polarization direction. The turning point with a null torque, between the perpendicular and parallel modes, is at the LSPR. The optical performance of Au nanospheroid is equivalent to that of Au NR with the same volume and aspect ratio, but the LSPR of Au NR is little red-shifted from that of an equivalent prolate spheroid.

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Anomalous optical forces on radially anisotropic nanowires

H. L. Chen, L. Gao

Full-wave electromagnetic scattering theory and Maxwell stress tensor integration techniques have been established to study the optical force on the radially anisotropic nanowires. The optical forces on the isotropic nanowires are dependent on the size of the nanowire and the wave vector in the media with the Rayleigh’s law. However, the optical forces on the anisotropic nanowires have the anomalous behaviors under non-Rayleigh vanishing condition and non-Rayleigh diverging condition. Therefore, the optical forces on the anisotropic nanowires may be enhanced or reduced by tuning the anisotropic parameters. These results may promote the potential applications in the field of nanotechnology.

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Coupled nano-plasmons

M. Apostol, S. Ilie, A. Petrut, M. Savu, S. Toba

A simple model of coupled plasmons arising in two neighbouring nano-particles is presented. The coupled oscillations and the corresponding eigenfrequencies are computed. It is shown that the plasmons may be periodically transferred between the two particles. For larger separation distances between the two particles the retardation is included. The oscillation eigenmodes are the polaritons in this case. There are distances for which the particles do not couple to each other, i.e. the polaritonic coupling gets damped. The van der Waals-London-Casimir force is estimated for the two particles; it is shown that for large distances the force is repulsive. We compute also the polarizabilities of the two coupled nano-particles and their cross-section under the action of an external monochromatic plane wave, which exhibit resonances indicative of light trapping and field enhancement. A resonant force is also identified, acting upon the particles both on behalf of the external field and of each other.

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Optical forces induced by metal nanoparticle clusters

Jordi Sancho-Parramon, Salvador Bosch
The strong field localization generated between closely placed metal particles excited by electromagnetic radiation induces intense forces on small polarizable objects. In this study we investigate the optical forces that can be generated in the vicinity of metal nanoparticle clusters using fully electrodynamic numerical simulations. The influence of the cluster configuration as well as of the excitation parameters is analyzed.
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Microbead dynamics in optical trap assisted nanopatterning

Romain Fardel, Yu-Cheng Tsai and Craig B. Arnold

Optical near-field techniques allow one to overcome diffraction by positioning an optical element in close proximity to the surface of interest. In optical trap assisted nanopatterning, this optical element is a microbead optically trapped above the substrate in a liquid environment. Using high-speed microscopy, we show that under certain conditions, the laser pulse creates a gas bubble under the bead and that this bubble displaces the bead before disappearing. The bead then returns to its original position under the action of the scattering force of the optical trap. We measure the bead vertical trajectory and extract its terminal velocity in order to calculate the magnitude of the trapping force exerted on the bead. This work opens the way to a better understanding of the bead-surface interactions under laser irradiation and, therefore, contributes to the development of near-field techniques.

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Nanoscale ablation through optically trapped microspheres

Romain Fardel, Euan McLeod, Yu-Cheng Tsai and Craig B. Arnold

The ability to directly create patterns with size scales below 100 nm is important for many applications where the production or repair of high resolution and density features is needed. Laser-based direct-write methods have the benefit of being able to quickly and easily modify and create structures on existing devices, but ablation can negatively impact the overall technique. In this paper we show that self-positioning of near-field objectives through the optical trap assisted nanopatterning (OTAN) method allows for ablation without harming the objective elements. Small microbeads are positioned in close proximity to a substrate where ablation is initiated. Upon ablation, these beads are temporarily displaced from the trap but rapidly return to the initial position. We analyze the range of fluence values for which this process occurs and find that there exists a critical threshold beyond which the beads are permanently ejected.

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Optical vortex beams for trapping and transport of particles in air

V. G. Shvedov, A. S. Desyatnikov, A. V. Rode, Y. V. Izdebskaya, W. Z. Krolikowski and Y. S. Kivshar

In this paper we show that laser beams containing phase singularity can be used for trapping and guiding light-absorbing particles in air. The experiments were performed with agglomerates of carbon nanoparticles with the size in the range 0.1–10 μm; the typical cw laser power was of a few mW. The stability of open-air three-dimensional trapping was within ±2 μm in both the transverse and the longitudinal directions. The particle position on the beams axis within the trap can be controlled by changing the relative intensity of two beams. The distinguishing feature of the trapping strategy is that particles are trapped at the intensity minimum of the beam, thus with minimum heating and intervention into the particle properties, which is important for direct studies of particle properties and for air-trapping of living cells.

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