Generation and modulation of terahertz gradient force in the interactions of two-color laser pulses with magnetized plasmas editors-pick

Xiao-Bo Zhang, Xin Qiao, Ai-Xia Zhang, and Ju-Kui Xue

Terahertz (THz) waves, as far-infrared light, offer new opportunities for the optical trapping and manipulation of single cells, in contrast to the other light sources. We present an efficient scheme to flexibly control multiple THz field distribution patterns generated by the laser–plasma interaction in a magnetized plasma. An analytical THz radiation field and two-dimensional particle-in-cell simulation are constructed to verify the feasibility of the scheme. Modulation of the THz gradient force and the energy flux by an asymmetrical THz field is investigated for the purpose of trapping and manipulating particles and cells. In particular, the stabilities of flexibly controlled THz radiation are investigated carefully in the form of the strong and short laser and super-strong magnetic field induced significant spatial structure instabilities and frequency instabilities of terahertz radiation.

DOI

Strong optomechanical coupling of light and highly confined acoustic phonons in slot dual-beam phoxonic crystal cavities

Tzy-Rong Lin, Chieh-Chun Chang, and Jin-Chen Hsu

In this study, strong optomechanical (OM) coupling is proposed through the introduction of a slot in a dual-beam phoxonic crystal cavity structure. The structure can support a confined optical slot mode and localized phononic cavity modes to interact effectively through the slot surfaces. In addition to using conventional OM coupling rates for evaluating the coupling strength of photonic and phononic modes, we consider the optical forces induced by the optical slot mode to discuss the coupling between different mode pairs and the underlying mechanisms of the strong OM coupling. The optical field can induce radiation pressure and electrostrictive forces in the structure. We demonstrate that the radiation pressure dominates the coupling enhancement of photonic and phononic modes because of the slot, whereas the electrostrictive surface pressure plays a minor role and the electrostrictive body force has a negligible contribution. On the basis of the optical forces, we can then calculate the acoustic phonon spectrum through optical excitation. The spectrum indicates the appearance of strong OM coupling in the additional phononic cavity modes. The results suggest that the slot dual-beam cavity structure can be a promising choice for tailoring effective optical forces in micro- and nano-optomechanical systems for enhancing OM coupling.

DOI

2 × 2 microparticles curvilinear transport channel based on dual self-accelerating beams

Zhihai Liu, Tong Wang, Yu Zhang, Xiaoyun Tang, Wenjie Su, Wanming Dong, Siyu Lin, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan

We propose and demonstrate a 2 × 2 microparticles curvilinear transport channel based on a dual self-accelerating beam generator. The device is composed of a dual-core fiber, a hollow capillary fiber, and a coreless silica fiber. The dual self-accelerating beams produced by the device propagate along the curvilinear trajectory due to the transverse accelerating property and then cross at the front of the fiber probe. The experimental results show that the yeast cell is transported along the curvilinear trajectory. By adjusting the optical power ratio of dual beams, we may control and ensure the yeast cell steers at the cross point. The proposed 2 × 2 curvilinear microparticles transport channel based on dual self-accelerating beams can realize microparticles sorting and obstacle avoidance, which means that it would be a useful tool in biology and colloidal science.

DOI

Theory of optical forces on small particles by multiple plane waves

Ehsan Mobini, Aso Rahimzadegan, Carsten Rockstuhl, and Rasoul Alaee

We theoretically investigate the optical force exerted on an isotropic particle illuminated by a superposition of plane waves. We derive explicit analytical expressions for the exerted force up to quadrupolar polarizabilities. Based on these analytical expressions, we demonstrate that an illumination consisting of two tilted plane waves can provide a full control on the optical force. In particular, optical pulling, pushing, and lateral forces can be obtained by the proper tuning of illumination parameters. Our findings might unlock multiple applications based on a deterministic control of the spatial motion of small particles.

DOI

Single charging events on colloidal particles in a nonpolar liquid with surfactant

Caspar Schreuer, Stijn Vandewiele, Toon Brans, Filip Strubbe, Kristiaan Neyts, and Filip Beunis
Electrical charging of colloidal particles in nonpolar liquids due to surfactant additives is investigated intensively, motivated by its importance in a variety of applications. Most methods rely on average electrophoretic mobility measurements of many particles, which provide only indirect information on the charging mechanism. In the present work, we present a method that allows us to obtain direct information on the charging mechanism, by measuring the charge fluctuations on individual particles with a precision higher than the elementary charge using optical trapping electrophoresis. We demonstrate the capabilities of the method by studying the influence of added surfactant OLOA 11000 on the charging of single colloidal PMMA particles in dodecane. The particle charge and the frequency of charging events are investigated both below and above the critical micelle concentration (CMC) and with or without applying a DC offset voltage. It is found that at least two separate charging mechanisms are present below the critical micelle concentration. One mechanism is a process where the particle is stripped from negatively charged ionic molecules. An increase in the charging frequency with increased surfactant concentration suggests a second mechanism that involves single surfactant molecules. Above the CMC, neutral inverse micelles can also be involved in the charging process.

DOI

Using light scattering to resolve Brownian rotation dynamics of optically trapped Au nanorods

Ana Andres-Arroyo and Peter J. Reece

Optically trapped Au nanorods are known to adopt a preferential orientation when trapped in three dimensions at the focus of linearly polarised optical tweezers. Trapped nanorods experience both translational and rotational perturbations due to Brownian motion that are governed by the strength of the trap and associated shape-dependent hydrodynamic properties. In this study, we make use of the strong angular dependent light scattering of the localised surface plasmon resonances to interrogate the rotational dynamics of trapped nanorods principally aligned along the propagation axis of the trapping laser. Our measurements reveal that significant rotational dynamics can be observed whilst maintaining stable translational trapping at low powers.

DOI

A semi-analytical model of a near-field optical trapping potential well

Mohammad Asif Zaman, Punnag Padhy, and Lambertus Hesselink

A semi-analytical model is proposed to describe the force generated by a near-field optical trap. The model contains fitting parameters that can be adjusted to resemble a reference force-field. The model parameters for a plasmonic near-field trap consisting of a C-shaped engraving are determined using least squares regression. The reference values required for the regression analysis are calculated using the Maxwell stress tensor method. The speed and accuracy of the proposed model are compared with the conventional method. The model is found to be significantly faster with an acceptable level of accuracy.

DOI

Ponderomotive convection in water induced by a CW laser

M. N. Shneider and V. V. Semak

An optically induced convection during IR laser interaction with water or any absorbing liquid is described theoretically. The numerical simulations performed using the developed model show that the optical pressure and ponderomotive forces produce water flow in the direction of the laser beam propagation. In the later stage of interaction, when the water temperature rises, the Archimedes force becomes comparable and, ultimately, dominant, producing convection directed against the vector of gravitational acceleration (upward). The theoretical estimates and numerical simulations predict fluid dynamics similar to what is observed in previous experiments.

DOI

An optical tweezer in asymmetrical vortex Bessel-Gaussian beams

V. V. Kotlyar, A. A. Kovalev and A. P. Porfirev

We study an optical micromanipulation that comprises trapping, rotating, and transporting 5-μm polystyrene microbeads in asymmetric Bessel-Gaussian (BG) laser beams. The beams that carry orbital angular momentum are generated by means of a liquid crystal microdisplay and focused by a microobjective with a numerical aperture of NA = 0.85. We experimentally show that given a constant topological charge, the rate of microparticle motion increases near linearly with increasing asymmetry of the BG beam. Asymmetric BG beams can be used instead of conventional Gaussian beam for trapping and transferring live cells without thermal damage.

DOI

Coupled electrostatic and material surface stresses yield anomalous particle interactions and deformation

B. A. Kemp, I. Nikolayev and C. J. Sheppard

Like-charges repel, and opposite charges attract. This fundamental tenet is a result of Coulomb's law. However, the electrostatic interactions between dielectric particles remain topical due to observations of like-charged particle attraction and the self-assembly of colloidal systems. Here, we show, using both an approximate description and an exact solution of Maxwell's equations, that nonlinear charged particle forces result even for linear material systems and can be responsible for anomalous electrostatic interactions such as like-charged particle attraction and oppositely charged particle repulsion. Furthermore, these electrostatic interactions and the deformation of such particles have fundamental implications for our understanding of macroscopic electrodynamics.

DOI

Comparison of plasmonic structures in terms of temperature increase under equivalent maximal trapping forces

Yong-Jun Yang and Yong-Gu Lee

Plasmonic optical trapping is a new approach that can potentially overcome some of the limitations associated with conventional optical trapping. Plasmonic tweezers generate heat because of the absorption of light at the surface of metals, and this is one of the contributions to the failure of stable trapping. Heating problems and the trapping forces tend to differ with the geometry of the plasmonicstructures. Nanodisk structures can generally deliver stronger trapping forces than nanohole structures. However, the nanodisk structures also lead to greater heat generation, which can cause the medium to boil and eventually produce bubbles that can potentially push trapped particles away from the trap. Concentrated local heat can also melt the plasmonic features or instantaneously vaporize the medium. In this paper, we have closely examined this heat generation problem for two typical plasmonicstructures, nanodisks and nanoholes, and provided a detailed analysis. For identical force generations, it is shown that the nanohole structures exhibit less heat generation.

DOI

Optical tweezing electrophoresis of single biotinylated colloidal particles for avidin concentration measurement

Toon Brans, Filip Strubbe, Caspar Schreuer, Kristiaan Neyts and Filip Beunis

We present a novel approach for label-free concentration measurement of a specific protein in a solution. The technique combines optical tweezers and microelectrophoresis to establish the electrophoretic mobility of a single microparticle suspended in the solution. From this mobility measurement, the amount of adsorbed protein on the particle is derived. Using this method, we determine the concentration of avidin in a buffer solution. After calibration of the setup, which accounts for electro-osmotic flow in the measurement device, the mobilities of both bare and biotinylated microspheres are measured as a function of the avidin concentration in the mixture. Two types of surface adsorption are identified: the biotinylated particles show specific adsorption, resulting from the binding of avidin molecules with biotin, at low avidin concentrations (below 0.04 μg/ml) while at concentrations of several μg/ml non-specific on both types of particles is observed. These two adsorption mechanisms are incorporated in a theoretical model describing the relation between the measured mobility and the avidin concentration in the mixture. This model describes the electrophoretic mobility of these particles accurately over four orders of magnitude of the avidin concentration.

DOI

Cross-type optical separation of elastic oblate capsules in a uniform flow

Cheong Bong Chang, Wei-Xi Huang and Hyung Jin Sung

The dynamic behavior of an elastic capsule with an initially oblate spheroidal shape during cross-type optical separation was numerically investigated. The penalty immersed boundary method was adopted for the fluid-membrane interaction, and the optical force calculation was conducted by using the ray optics method including the ray-surface intersection algorithm. The oblate elastic capsule of b/a = 0.5 with different surface Young's moduli and different initial inclination angles was considered. The oblate capsule with higher surface Young's moduli was less deformed, and was more migrated for each initial inclination angle. Unlike the oblate rigid particle, the initially inclined capsules with moderate inclination angles were similarly migrated since the oblate elastic capsule was deformed during rotation near the laser beam axis. The oblate capsules can be separated according to the surface Young's modulus, except for nearly non-inclined capsules. As the fluid velocity decreased, the migration distance increased. The maximum deformation parameter was insensitive to the fluid velocity. Furthermore, a new dimensionless number (Sec ) was introduced to predict the migration distance of the oblate elastic capsule.

DOI

Optically and elastically assembled plasmonic nanoantennae for spatially resolved characterization of chemical composition in soft matter systems using surface enhanced spontaneous and stimulated Raman scattering

Haridas Mundoor, Taewoo Lee, Derek G. Gann, Paul J. Ackerman, Bohdan Senyuk, Jao van de Lagemaat and Ivan I. Smalyukh

We present a method to locally probe spatially varying chemical composition of soft matter systems by use of optically controlled and elastically self-assembled plasmonic nanoantennae. Disc-shaped metal particles with sharp irregular edges are optically trapped, manipulated, and assembled into small clusters to provide a strong enhancement of the Raman scattering signal coming from the sample regions around and in-between these particles. As the particles are reassembled and spatially translated by computer-controlled laser tweezers, we probe chemical composition as a function of spatial coordinates. This allows us to reliably detect tiny quantities of organic molecules, such as capping ligands present on various nanoparticles, as well as to probe chemical composition of the interior of liquid crystal defect cores that can be filled with, for example, polymer chains. The strong electromagnetic field enhancement of optically manipulated nanoparticles' rough surfaces is demonstrated in different forms of spectroscopy and microscopy, including enhanced spontaneous Raman scattering, coherent anti-Stokes Raman scattering, and stimulated Raman scattering imaging modes.

DOI

Dynamics of submicron aerosol droplets in a robust optical trap formed by multiple Bessel beams

Ioannis Thanopulos, David Luckhaus, Thomas C. Preston and Ruth Signorell

In this paper, we model the three-dimensional escape dynamics of single submicron-sized aerosol droplets in optical multiple Bessel beam traps. Trapping in counter-propagating Bessel beams (CPBBs) is compared with a newly proposed quadruple Bessel beam (QBB) trap, which consists of two perpendicularly arranged CPBB traps. Calculations are performed for perfectly and imperfectly aligned traps. Mie-theory and finite-difference time-domain methods are used to calculate the optical forces. The droplet escape kinetics are obtained from the solution of the Langevin equation using a Verlet algorithm. Provided the traps are perfectly aligned, the calculations indicate very long lifetimes for droplets trapped either in the CPBB or in the QBB trap. However, minor misalignments that are hard to control experimentally already severely diminish the stability of the CPBB trap. By contrast, such minor misalignments hardly affect the extended droplet lifetimes in a QBB trap. The QBB trap is found to be a stable, robust optical trap, which should enable the experimental investigation of submicron droplets with radii down to 100 nm. Optical binding between two droplets and its potential role in preventing coagulation when loading a CPBB trap is briefly addressed.

DOI

Measurements of the electrokinetic forces on dielectric microparticles in nematic liquid crystals using optical trapping

A. V. Ryzhkova, F. V. Podgornov, A. Gaebler, R. Jakoby, and W. Haase

We have studied the dynamics of dielectric microparticles dispersed in a nematic liquid crystal (NLC) in the presence of an external AC electric field. Investigations were performed using optical trapping technique in the cell with in-plane electrodes. It was shown that the main driving force in the bulk of the material has electrophoretic nature. It was demonstrated that the microparticle behavior strongly depends on the distance with respect to the electrode and is influenced by the dielectrophoretic force. The model, which enables estimation of the electrokinetic forces, is proposed. The forces are found from the balance with the optical trapping force. The microparticle surface charge q ≈ 2.1×10−17 C, linear electrophoretic mobilities μ∥ ≈ 10−11 m2/(V⋅s),μ⊥ ≈ 7×10−12 m2/(V⋅s), and the NLC viscosity η ≈ (21.2±4.7)×10−3 Pa⋅s at T = 40 °C are evaluated.
DOI

Spherical vortex beams of high radial degree for enhanced single-beam tweezers

Diego Baresch, Jean-Louis Thomas, and Régis Marchiano

We demonstrate gradient optical forces in metal-dielectric hybrid plasmonic waveguides (HPWG) for the first time. The magnitude of optical force is quantified through excitation of the nanomechanical vibration of the suspended waveguides. Integrated Mach-Zehnder interferometry is utilized to transduce the mechanical motion and characterize the propagation loss of the HPWG. Compared with theory, the experimental results have confirmed the optical force enhancement, but also suggested a significantly higher optical loss in HPWG. The excessive loss is attributed to metal surface roughness and other non-idealities in the device fabrication process.

DOI

Three dimensional force detection of gold nanoparticles using backscattered light detection

Lu Huang, Honglian Guo, Kunlong Li, Yuhui Chen, Baohua Feng, and Zhi-Yuan Li
We demonstrate three-dimensional position and force detection of single gold nanosphere (GNP) and gold nanorod (GNR) particles in optical trap by combining backscattered light detection and dark field imaging. The trapping stiffness of the GNPs and GNRs for all three dimensions is measured. The results show that the spring constants in the propagation direction of the trapping laser are somewhat weaker than in other two directions for GNPs. While for GNRs, the spring constants in the polarization direction of the trapping laser are a little weaker than in other two directions. The effect of trapping laser polarization on the particles yields different spring constants in the transverse plane which is perpendicular to the propagation direction. And this effect is larger on GNRs than GNPs.
DOI

A generalization of the dipolar force

Marian Apostol, Stelian Ilie, Aurel Petrut, Marcel Savu, and Stefan Toba
The static dipolar force is generalized to time-dependent classical distributions of dipoles and electromagnetic fields. This force may exhibit a remarkable resonance character for induced dipoles, related to the pole structure of the polarizabilities. The resonance phenomenon is illustrated for two macroscopic polarizable bodies, with mutually induced polarizations, using the well-known Lorentz-Drude model for the dielectric response with optical dispersion and a characteristic (resonance) frequency. Specifically, the calculations are performed for distances much longer than the dimension of the bodies (“point-like” bodies), but shorter than the characteristic wavelength (sub-wavelength, stationary, near-field regime). The polarizations are induced via a localized external field acting upon only one body. The force is practically vanishing for distinct substances and acquires a non-vanishing value for identical substances. It falls off as the 7-th power of the distance, being reminiscent of the van der Waals-London force. The conditions of validity of this resonance phenomenon are emphasized. Particular cases corresponding to independent external fields or two isolated, interacting bodies (closed system) are also analyzed, with similar conclusions regarding the resonance character of the force.
DOI

Evaluation of rare earth doped silica sub-micrometric spheres as optically controlled temperature sensors

P. Haro-González, L. Martínez Maestro, M. Trevisani, S. Polizzi, D. Jaque, J. García Sole, and M. Bettinelli

We report on the evaluation of rare earth (Er3+, Eu3+, and Tb3+ ions) SiO2 sub-micrometric spheres as potential optically controllable temperature sensors. Details about fabrication, optical manipulation and spectroscopic characterization of the sub-micrometric spheres are presented. The fluorescence properties of the micros-spheres in the biological range (25–60 °C) have been systematically investigated. From this systematic study, the thermal resolution potentially achieved in each case has been determined and compared to previous works.

DOI