Optical Binding Research Articles

Non-Hermitian physics for optical manipulation uncovers inherent instability of large clusters

Intense light traps and binds small particles, offering unique control to the microscopic world. With incoming illumination and radiative losses, optical forces are inherently nonconservative, thus non-Hermitian. Contrary to conventional systems, the operator governing time evolution is real and asymmetric (i.e., non-Hermitian), which inevitably yield complex eigenvalues when driven beyond the exceptional points, where light pumps in energy that eventually “melts” the light-bound structures. Surprisingly, unstable complex eigenvalues are prevalent for clusters with ~10 or more particles, and in the many-particle limit, their presence is inevitable. As such, optical forces alone fail to bind a large cluster. Our conclusion does not contradict with the observation of large optically-bound cluster in a fluid, where the ambient damping can take away the excess energy and restore the stability. The non-Hermitian theory overturns the understanding of optical trapping and binding, and unveils the critical role played by non-Hermiticity and exceptional points, paving the way for large-scale manipulation.

Effects of Halogen Substitution on the Optoelectronic Properties of Two-Dimensional All-Inorganic Double Perovskite Cs4AgBiX8 ( X=Cl,Br,I ) with Ruddlesden-Popper Structure

Due to the influence of dimension, two-dimensional (2D) all-inorganic double perovskites may be superior to their three-dimensional (3D) counterparts as environmentally friendly and efficient optoelectronic materials. Nevertheless, how the halogens affect the optoelectronic performance of 2D all-inorganic double perovskite is still unknown. Here, the photoelectric properties, including band structures, optical absorption spectra, carrier mobilities, and exciton binding energies, of 2D all-inorganic double perovskites ${\mathrm{Cs}}_{4}\mathrm{Ag}\mathrm{Bi}{X}_{8}$ ($X$ = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$) with the Ruddlesden-Popper structure are studied via density-functional theory along with the spin-orbit coupling effect. Considering the influence of the exciton effect in low-dimensional materials, we also calculate light absorption by using the GW Bethe-Salpeter equation method. The obtained results show that the substitution of $\mathrm{Cl}$ with $\mathrm{Br}$ or $\mathrm{I}$ atoms decreases the band gap (from 2.706 eV to 2.221 and 1.715 eV) for ${\mathrm{Cs}}_{4}{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_{8}$, enhances the light-absorption performance, increases the mobility of carriers, and reduces the exciton binding energy (from 1529.90 meV to 1268.70 and 941.71 meV). Moreover, the outcomes demonstrate that 2D all-inorganic double perovskites ${\mathrm{Cs}}_{4}\mathrm{Ag}\mathrm{Bi}{X}_{8}$ ($X$ = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$) might be better candidates for luminescent devices than photovoltaic materials, and the excellent performance of ${\mathrm{Cs}}_{4}{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_{8}$ makes it the optimal among the three. Our study will expand theoretical explorations for research into 2D all-inorganic double perovskite materials for potential luminescent or photovoltaic applications.

Plasmonic octamer objects: reversal of near-field optical binding force without the aid of backgrounds.

In recent years, the near-field optical binding force has gained a lot of interest in the field of optical manipulation. The reversal of the near-field binding force, a new, to the best of our knowledge, kind of optical manipulation, has so far been investigated mostly between dimers and in a very few cases among tetramers by utilizing the help of suitable substrates or backgrounds. Until now, no known way to control the near-field optical binding force among octamer configurations has been found, to our knowledge. In this paper, we propose a plasmonic (silver) octamer configuration where we demonstrate the control and reversal (attraction and repulsion) of the near-field optical binding force of octamers by illuminating the system with a TM polarized Bessel beam. The control of the binding force and its reversal is explained based on the polarization and gradient forces created by the Bessel beam. As the aid of a background or substrate is not required, our proposed simplified approach has the potential to open up novel ways of manipulating multiple particles. Our investigation also implicitly suggests that for future research on controlling the reversal of the near-field optical binding force of multiple particles, Bessel beams can be the appropriate choice instead of plane waves.

On chip chiral and plasmonic hybrid dimer or tetramer: Generic way to reverse longitudinal and lateral optical binding forces

For both the longitudinal binding force and the lateral binding force, a generic way of controlling the mutual attraction and repulsion (usually referred to as reversal of optical binding force) between chiral and plasmonic hybrid dimers or tetramers has not been reported so far. In this paper, by using a simple plane wave and an onchip configuration, we propose a possible generic way to control the binding force for such hybrid objects in both the near-field region and the far-field region. We also investigate different inter-particle distances while varying the wavelengths of light for each inter-particle distance throughout the investigations. First of all, for the case of longitudinal binding force, we find that chiral-plasmonic hybrid dimer pairs do not exhibit any reversal of optical binding force in the near-field region nor in the far-field region when the wavelength of light is varied in an air medium. However, when the same hybrid system of nanoparticles is placed over a plasmonic substrate, a possible chip, it is possible to achieve a reversal of the longitudinal optical binding force. Later, for the case of lateral optical binding force, we investigate a setup where we place the chiral and plasmonic tetramers on a plasmonic substrate by using two chiral nanoparticles and two plasmonic nanoparticles, with the setup illuminated by a circularly polarized plane wave. By applying the left-handed and the right-handed circular polarization state of light, we also observe the near-field and the far-field reversal of lateral optical binding force for both cases. As far as we know, so far, no work has been reported in the literature on the generic way of reversing the longitudinal optical binding force and the lateral optical binding force of such hybrid objects. Such a generic way of controlling optical binding forces can have important applications in different fields of science and technology in the near future.

Magneto-optical binding in the near field

In this paper we show analytically and numerically the formation of a near-field stable optical binding between two identical plasmonic particles, induced by an incident plane wave. The equilibrium binding distance is controlled by the angle between the polarization plane of the incoming field and the dimer axis, for which we have calculated an explicit formula. We have found that the condition to achieve stable binding depends on the particle’s dielectric function and happens near the frequency of the dipole plasmonic resonance. The binding stiffness of this stable attaching interaction is four orders of magnitude larger than the usual far-field optical binding and is formed orthogonal to the propagation direction of the incident beam (transverse binding). The binding distance can be further manipulated considering the magneto-optical effect and an equation relating the desired equilibrium distance with the required external magnetic field is obtained. Finally, the effect induced by the proposed binding method is tested using molecular dynamics simulations. Our study paves the way to achieve complete control of near-field binding forces between plasmonic nanoparticles.

Excitonic States and Related Optical Susceptibility in InN/AlN Quantum Well Under the Effects of the Well Size and Impurity Position

Based on the finite difference method, linear optical susceptibility, photoluminescence peak and binding energies of three first states of an exciton trapped by a positive charge donor-impurity ( ) confined in InN/AlN quantum well are investigated in terms of well size and impurity position. The electron, heavy hole free and bound excitons allowed eigen-values and corresponding eigen-functions are obtained numerically by solving one-dimensional time-independent Schrödinger equation. Within the parabolic band and effective mass approximations, the calculations are made considering the coupling of the electron in the n-th conduction subband and the heavy hole in the m-th valence subband under the impacts of the well size and impurity position. The obtained results show clearly that the energy, binding energy and photoluminescence peak energy show a decreasing behavior according to well size for both free and bound cases. Moreover, the optical susceptibility associated to exciton transition is strongly red-shift (blue-shifted) with enhancing the well size (impurity position).

Optical Trapping of Sub−Micrometer Particles with Fiber Tapers Fabricated by Fiber Pulling Assisted Chemical Etching

Optical trapping of sub−micrometer particles in three dimensions has been attracting increasing attention in a wide variety of fields such as physics, chemistry, and biologics. Optical fibers that allow stable trapping of such particles are not readily available but beneficial in system integration and miniaturization. Here, we present a readily accessible batch fabrication method, namely fiber pulling assisted tubeless chemical etching, to obtain sharp tapered optical fibers from regular telecommunication single−mode fibers. We demonstrated the applications of such fiber tapers in two non−plasmonic optical trapping systems, namely single− and dual−fiber−taper−based trapping systems. We realized single particle trapping, multiple particle trapping, optical binding, and optical guiding with sub−micrometer silica particles. Particularly, using the dual fiber system, we observed the three−dimensional optical trapping of swarm sub−micrometer particles, which is more challenging to realize than trapping a single particle. Because of the capability of sub−micrometer particle trapping and the accessible batch fabrication method, the fiber taper−based trapping systems are highly potential tools that can find many applications in biology and physics.

Reconfigurable millimeter-range optical binding of dielectric microparticles in hollow-core photonic crystal fiber.

Optical binding of microparticles offers a versatile playground for investigating the optomechanics of levitated multi-particle systems. We report millimeter-range optical binding of polystyrene microparticles in hollow-core photonic crystal fiber. The first particle scatters the incident LP01 mode into several LP0n modes, creating a beat pattern that exerts a position-dependent force on the second particle. Particle binding results from the interplay of the forces created by counterpropagating beams. A femtosecond trapping laser is used so that group velocity walk-off eliminates disturbance caused by higher order modes accidentally excited at the fiber input. The inter-particle distance can be optically switched over 2 orders of magnitude (from 42 µm to 3 mm), and the bound particle pairs can be translated along the fiber by unbalancing the powers in the counterpropagating trapping beams. The frequency response of a bound particle pair is investigated at low gas pressure by driving with an intensity-modulated control beam. The system offers new degrees of freedom for manipulating the dynamics and configurations of optically levitated microparticle arrays.

Evanescent wave induced polarization-insensitive self-organization of stratified single-negative materials

Optical fields can induce optical forces between macroscopic objects, giving rise to different structures. Through rigorous calculation, we show that a collection of single negative slabs which possesses either negative permittivity or negative permeability (i.e. ε < 0, μ > 0 or ε > 0, μ < 0) in water can be self-organized into one-dimensional photonic crystals, due to the coupling of propagating wave and evanescent wave. We further demonstrated that the optical binding is irrespective of the polarization and angle of the incident plane wave. We call such a phenomenon—polarization-insensitive optical binding. We also demonstrate that polarization-insensitive optical binding can be achieved on microscale and millimeter scale. Polarization and angle insensitive band edge is the key to achieve polarization and angle insensitive optical binding. This work provides a new strategy to tailor the photonic crystals containing single negative materials towards the development of fine-tuning optical devices.

Controllable axial optical chain beams using a holographic method.

Axial optical chain (optical bottle beams) beams are widely used in optical micromanipulation, atom trapping, guiding and binding of microparticles and biological cells, etc. However, the generation of axial optical chain beams are not very flexible at present, and its important characteristics such as periodicity and phase shift cannot be easily regulated. Here, we propose a holographic method to achieve the axial optical chain beams with controllable periodicity and phase. A double annular phase diagram is generated based on the gratings and lenses algorithms. The beam incident to the double annular slits was tilted from the optical axis to produce concentric double annular beams. The annular beam with different radius will produce the zero-order Bessel beam with different axial wave vector. Axial optical chain beams is produced by interference of two zero-order Bessel beams with different axial wave vectors. The phase and periodicity of the axial optical chain beams can be changed by changing the initial phase difference and radius of the double annular slits of the double annular phase diagram, respectively. The feasibility and effectiveness of the proposed method are demonstrated by theoretical numerical analysis and experiments. This method will further expand the application of axial optical chain beams in optical tweezers, optical modulation and other fields.

Longitudinal and lateral interparticle optical binding and extrinsic radiation force and torque on a pair of lossless dielectric cylinders of arbitrary sizes and the acoustical analogue

The purpose of this work is to develop an exact formalism and analytical equations for the interparticle electromagnetic/optical (longitudinal and lateral) binding forces arising between two lossless dielectric cylinders with different permittivities in a TE-polarized plane wave-field with an arbitrary incidence angle (in the polar plane). Additionally, the extrinsic/external optical radiation force and torque acting on the aggregate (i.e. the cluster of particle pair) are determined, and their exact mathematical expressions are derived. Based on the modal series expansion method in cylindrical coordinates in conjunction with the translational addition theorem, numerical simulations and computations for the longitudinal and lateral components of the interparticle and extrinsic forces as well as the axial component of the extrinsic torque show that attraction/repulsion, rotation clockwise or counter-clockwise, or neutrality, arise depending on the interparticle distance, the incidence angle, particle sizes and their dielectric permittivities. This investigation offers a complete analytical formalism without any approximations related to particle sizes. The numerical results computed based on exact mathematical expressions for interparticle and extrinsic physical observables can be used in the design and optimization of optical tweezers and related applications in optically-bound matter, particle manipulation, cloaking, and optically-assembled metamaterials with adjustable periodicities to name some examples. Also, the relevant analogy with the acoustic counterpart is noted.

Improved Doping and Optoelectronic Properties of Zn-Doped Cspbbr3 Perovskite through Mn Codoping Approach

Reducing the toxic Pb component in perovskites is an important step to realize environment-friendly perovskite optoelectronic devices. Herein, the structural, electronic, and optical properties of Zn and Mn codoped CsPbBr3 have been investigated based on first-principle calculations and experimental verifications. Although the Zn dopant could reduce the optical band gap and exciton binding energy and enhance the optical absorption and defect tolerance for CsPbBr3, the maximum reduction of the toxic Pb component was just about 12.5% in the experiment because the Zn dopant enlarges the formation energy of CsPb1-xZnxBr3. For the stable CsPb1-x-yMnxZnyBr3 perovskite, the largest y and corresponding (x + y) could reach up to 25% and 83% respectively, since the Mn dopant could reduce the structural disorder. Especially when (x + y) < 50%, CsPb1-x-yMnxZnyBr3 exhibits a comparable carrier lifetime and exciton binding energy with a lower band gap to those of the CsPbBr3, since the Zn dopant supplies a charge to CsPbBr3 to counteract the variation of Pb-Br bonds induced by the Mn dopant. Meanwhile, the d orbitals of the dopant increase the optical absorption. These suggest that a 50% reduction of toxic Pb could be realized for stable CsPb1-x-yMnxZnyBr3 with negligibly deteriorated optoelectronic properties. This work provides an alternative approach to achieve a Pb-less perovskite with a high performance.

Stochastic dynamics of optically bound matter levitated in vacuum

Optically levitated nanoparticles in a vacuum offer a light–matter interface with broad and easy tunability of all key system parameters. However, the majority of previously reported experimental achievements in this area have only dealt with a single levitated object. Here, we demonstrate optical binding between multiple levitated objects confined in cross-polarized counter-propagating laser beams in a vacuum. We characterize the level of interparticle interaction, quantify its nonlinearity for various configurations of the system, and demonstrate its broad tunability. Our methodology for quantitative characterization of optically bound structures is supported by an extensive theoretical description and validated by numerical simulations. We believe the presented results represent a step toward the development of a framework of levitated optomechanics of complex coupled systems with a controlled level of coupling nonlinearity for experimental studies including, for example, mesoscopic entanglement.

Transverse optical binding for a dual dipolar dielectric nanoparticle dimer

The physical origins of the transverse optical binding force and torque beyond the Rayleigh approximation have not been clearly expressed to date. Here, we present analytical expressions of the force and torque for a dual dipolar dielectric dimer illuminated by a plane wave propagating perpendicularly to the dimer axis. Using this analytical model, we explore the roles of the hybridized electric dipolar, magnetic dipolar, and electric-magnetic dipolar coupling interactions in the total force and torque on the particles. We find significant departures from the predictions of the Rayleigh approximation, particularly for high-refractive-index particles, where the force is dominated by the magnetic interaction. This results in an enhancement of the dimer stability by one to four orders of magnitude compared to the predictions of the Rayleigh approximation. For the case of torque, this is dominated by the coupling interaction and increases by an order of magnitude. Our results will help to guide future experimental work in optical binding of high-refractive-index dielectric particles.

Giant resonant enhancement of optical binding of dielectric particles

Optical coupling of two identical dielectric particles gives rise to bonding and anti-bonding resonances. The latter is featured by significant narrowing of the resonant width and strong enhancement of the $Q$ factor for the high index micron size particles in subwavelength range. We consider particles shaped as spheres and disks under coaxial illumination of dual incoherent counter propagating Bessel beams. In the case of spheres we derive analytical expressions for the optical binding (OB) force which decreases as $1/L^2$ for large distance $L$ between the spheres and displays two periods of oscillations. For close distances the OB force enormously increases in the resonant regime. The case of two coaxial disks owing to variation of the distance between disks and aspect ratio of each disk is featured by extremal enhancement of the $Q$ factor compared to the case of two spheres. In that case we demonstrate unprecedent enhancement of the OB force up to several decades of nano Newtons. We show that the magnitude and sign of the OB force strongly depend on the longitudinal wave vector of the Bessel beams.

Giant resonant enhancement of optical binding of dielectric disks

Two-fold variation over the aspect ratio of each disk and distance between disks gives rise to numerous events of avoided crossing of resonances of individual disks. For these events, the hybridized anti-bonding resonant modes can acquire a morphology close to the Mie resonant mode with the high orbital momentum of an equivalent sphere. The Q factor of such resonance can exceed the Q factor of an isolated disk by two orders of magnitude. We show that dual incoherent counterpropagating coaxial Bessel beams with power 1 m W / µ m 2 with frequency resonant to such anti-bonding Mie-like modes result in unprecedented optical binding forces up to tens of nano-Newtons for silicon micrometer-sized disks. We show also that the magnitude and sign of optical forces depend strongly on the longitudinal wave vector of the Bessel beams.

Controlled Optofluidic Crystallization of Colloids Tethered at Interfaces.

We report experiments that show rapid crystallization of colloids tethered to an oil-water interface in response to laser illumination. This light-induced transition is due to a combination of long-ranged thermophoretic pumping and local optical binding. We show that the flow-induced force on the colloids can be described as the gradient of a potential. The nonequilibrium steady state due to local heating thus admits an effective equilibrium description. The optofluidic manipulation explored in this work opens novel ways to manipulate and assemble colloidal particles.

Angle-independent and -dependent optical binding of a one-dimensional photonic hypercrystal

We show through rigorous calculation that optical binding can organize hyperbolic metamaterial slabs into one-dimensional photonic hypercrystals. It was reported that a photonic hypercrystal can possess an angle-independent band gap and blueshifting and redshifting band gaps. We find that an angle-independent band gap could lead to angle-independent optical binding, where light can organize a collection of hyperbolic slabs into one mechanically stable photonic hypercrystal, irrespective of the angle of incidence of the incident plane wave. We call such a phenomenon incident angle-independent optical binding. In addition, the evolution of the lattice constants of the self-organized photonic hypercrystals with blueshifting and redshifting band gaps is also investigated.

Cooperative cooling in a one-dimensional chain of optically bound cold atoms

We discuss theoretically the optical binding of one-dimensional chains of cold atoms shone by a transverse pump, where particles self-organize to a distance close to an optical wavelength. As the number of particles is increased, the trapping potential increases logarithmically as the contributions from all atoms add up constructively. We identify a cooperative cooling mechanism, due to the mutual exchange of photons between atoms, which can beat the spontaneous emission for chains that are long enough. Surprisingly, the cooling is optimal very close to the resonance. This peculiar cooling mechanism thus gives new insights into the cooperative physics of low-dimensional cold atom systems.

Broadband optical properties of monolayer and bulk MoS2

Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. Using this method, we conduct a detailed study of monolayer MoS2 and its bulk crystal in the broad spectral range (290–3300 nm). In the near- and mid-infrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. In addition, the proposed approach opens a possibility to observe a previously unreported peak in the dielectric function of monolayer MoS2 induced by the use of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seeding promoters for MoS2 synthesis and thus enables its applications in chemical and biological sensing. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices.