Optical Binding Research Articles

Charge State Control of F16CoPc on h‐BN/Cu(111)

AbstractThe use of molecular materials in solar cells and nano‐electronics demands a fundamental understanding and control of their electronic properties. Particularly relevant is the molecular response to the environment, that is, the interaction with the support and adjacent molecules, as well as the influence of electrostatic gating. Here, the control of molecular level alignment and charge states of fluorinated cobalt phthalocyanines (F16CoPc) on atomically thin hexagonal boron nitride (h‐BN) sheets on Cu(111) is reported using scanning tunneling microscopy (STM) and spectroscopy (STS), as well as atomic force microscopy (AFM) and complementary density functional theory (DFT) calculations. Three parameters that govern the electronic level alignment of F16CoPc orbitals are investigated: i) template‐induced gating by the work function variation of the h‐BN/Cu(111) substrate, ii) gating by the STM tip, and iii) screening by neighboring molecules. The interplay of these parameters influences the charge distribution in the studied molecular arrangements and thus provides the possibility to tune their physicochemical behavior, for instance, the response toward electronic or optical excitation, charge transport, or binding of axial adducts.

Phase Transition and Self-Stabilization of Light-Mediated Metal Nanoparticle Assemblies.

Light-mediated self-organization of nanoparticles (NPs) offers a route to study mesoscale electrodynamics interactions in many-body systems. Here we report the phase transition and self-stabilization of dynamic assemblies with up to 101 plasmonic metal NPs in optical fields. The spatial stability of self-organized NPs is strongly influenced by the laser intensity and polarization state, where phase transition occurs when the intensity increases and the polarization changes from linear to circular. Well-organized NP arrays can form in a circularly polarized laser beam, where the center of an array is less susceptible to thermal fluctuations than the edge. Moreover, larger arrays are self-protected from fluctuation-induced instability by incorporating more NP constituents. The dynamics of NP arrays can be understood by electrodynamic simulations coupled with thermal fluctuations and by examining their potential energy surfaces. This study clearly reveals the spatial inhomogeneity of optical binding interactions in a two-dimensional multiparticle system, which is important for building large-scale optical matter assemblies with NPs.

Optical binding and lateral forces on chiral particles in linearly polarized plane waves

We demonstrate numerically and analytically the optical binding between a pair of chiral nanoparticles immersed in a plane wave of linear polarization. The numerical results based on the full wave simulation show that, when the electric field of the incident wave is polarized along the pair axis (the line connecting the particle pair), the closest dimer can be formed at particle separation around $1.25\ensuremath{\lambda}$, with $\ensuremath{\lambda}$ denoting the incident wavelength, while when the electric field is polarized normal to the pair axis, the nearest dimer is formed with a separation of one wavelength. In both cases, there exist multiple stable dimer states due to the spatially oscillatory decaying behavior of the binding force with the increase of particle distance. Interestingly, there also appears a lateral optical force exerted on each particle in the direction normal to both the pair axis and light flow, when either or both particles have chirality. Analytical analysis within the dipole approximation indicates that the binding force comes dominantly from the gradient of the optical potential, irrespective of particle chirality. The lateral force, on the other hand, has multiple origins. In addition to the gradient force, the radiation force, the curl force, and the spin force may make significant contributions dependent on the particle chirality, as the particles approach each other.

Chiral and plasmonic hybrid dimer pair: reversal of both near- and far-field optical binding forces

In both the near-field (around 10 to 250 nm interparticle distance) and far-field (around 1 µm to higher interparticle distances) regions, controlling the mutual attraction and repulsion between chiral and plasmonic hybrid dimers using light has not been reported so far to the best of our knowledge. Such control is called controlling the reversal of the optical binding force. In most setups, the reversal of the optical binding force between plasmonic heterodimers vanishes with an interparticle distance of around 100 nm and above due to the disappearance of the Fano resonance. In this paper, we have demonstrated a possible optical setup, illuminated by a linearly polarized plane wave: chiral and plasmonic hybrid dimers over a plasmonic substrate, which supports the reversal of the optical binding force in both the near- and far-field regions. First, by varying the light wavelengths, we have shown that the optical binding force does not reverse for either the chiral homodimers set and or the plasmonic homodimer set for different interparticle distances. Later, we created a hybrid dimer system by placing a plasmonic and a chiral nanoparticle together. Interestingly, at the far-field region, a strong plasmonic resonance is observed, but a reversal of the optical binding force does not occur. Finally, we have placed the same chiral–plasmonic hybrid dimer setup over a plasmonic substrate and the desired result—a reversal of the binding force—is observed due to the induced lateral force on the chiral object (in the presence of the substrate) and the Fano-type resonance in the system. Controlling such near- and far-field optical binding forces can be an important aspect for particle clustering, accumulation, crystallization, and the organization of templates for biological and colloidal sciences in the near future.

Stable Fano-like plasmonic resonance: its impact on the reversal of far- and near-field optical binding force

Even for a 100 nm interparticle distance or a small change in particle shape, optical Fano-like plasmonic resonance mode usually vanishes completely. It would be remarkable if stable Fano-like resonance could somehow be achieved in distinctly shaped nanoparticles for more than 1 μm interparticle distance, which corresponds to the far electromagnetic field region. If such far-field Fano-like plasmonic resonance can be achieved, controlling the reversal of the far-field binding force can be attained, like the currently reported reversals for near-field cases. In this work, we have proposed an optical set-up to achieve such a robust and stable Fano-like plasmonic resonance, and comparatively studied its remarkable impact on controlling the reversal of near- and far-field optical binding forces. In our proposed set-up, the distinctly shaped plasmonic tetramers are half immersed (i.e. air–benzene) in an inhomogeneous dielectric interface and illuminated by circular polarized light. We have demonstrated significant differences between near- and far-field optical binding forces along with the Lorentz force field, which partially depends on the object’s shape. A clear connection is shown between the far-field binding force and the resonant modes, along with a generic mechanism to achieve controllable Fano-like plasmonic resonance and the reversal of the optical binding force in both far- and near-field configurations.

Optical binding near hyperbolic metamaterial substrates

We study optical binding of two dielectric nanoparticles near a boundary between air and hyperbolic metamaterial. Three kinds of modes contribution are analyzed and it is shown, that evanescent waves allow formation of stable bound dimer due to hyperbolic metamaterial modes. We have found the most profitable parameters of hyperbolic metamaterial substrate, providing enhancement of the optical binding effect.

Optical manipulations via auxiliary substrates

We report flexible optomechanical manipulations by the help of surface and volumetric modes of the substrate. Optical binding effect can be sufficiently enhanced due to both surface plasmon-polariton and hyperbolic modes of the structure. Volumetric modes of the structure provide optical pulling force for inclined incident plane wave, while surface waves cause enhancement of the optical trapping force under Gaussian beam illumination. Moreover, antitrapping effect can occur for specific positions of the beam waist.

Inorganic Halide Double Perovskites with Optoelectronic Properties Modulated by Sublattice Mixing.

All-inorganic halide double perovskites have emerged as a promising class of materials that are potentially more stable and less toxic than lead-containing hybrid organic-inorganic perovskite optoelectronic materials. In this work, 311 cesium chloride double perovskites (Cs2BB'Cl6) were selected from a set of 903 compounds as likely being stable on the basis of a statistically learned tolerance factor (τ) for perovskite stability. First-principles calculations on these 311 double perovskites were then performed to assess their stability and identify candidates with band gaps appropriate for optoelectronic applications. We predict that 261 of the 311 Cs2BB'Cl6 compounds are likely synthesizable on the basis of a thermodynamic analysis of their decomposition to competing compounds (decomposition enthalpy <0.05 eV/atom). Of these 261 likely synthesizable compounds, 47 contain no toxic elements and have direct or nearly direct (within 100 meV) band gaps between 1 and 3 eV, as computed with hybrid density functional theory (HSE06). Within this set, we identify the triple-alkali perovskites Cs2[Alk]+[TM]3+Cl6, where Alk is a group 1 alkali cation and TM is a transition-metal cation, as a class of Cs2BB'Cl6 double perovskites with remarkable optical properties, including large and tunable exciton binding energies as computed by the GW-Bethe-Salpeter equation (GW-BSE) method. We attribute the unusual electronic structure of these compounds to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials with small band gaps, large exciton binding energies, and absorption spectra that are strongly influenced by the identity of the transition metal. The role of the double-perovskite structure in enabling these unique properties is probed through an analysis of the electronic structures and chemical bonding of these compounds in comparison with other transition-metal and alkali transition-metal halides.

Reversible optical binding force in a plasmonic heterodimer under radially polarized beam illumination

We investigated the optical binding force in a plasmonic heterodimer structure consisting of two nano-disks. It is found that when illuminated by a tightly focused radially polarized beam (RPB), the plasmon modes of the two nano-disks are strongly hybridized, forming bonding/antibonding modes. An interesting observation of this setup is that the direction of the optical binding force can be controlled by changing the wavelength of illumination, the location of the dimer, the diameter of the nano-disks, and the dimer gap size. Further analysis yields that the inhomogeneous polarization state of RPB can be utilized to readily control the bonding type of plasmon modes and distribute the underlying local field confined in the gap (the periphery) of the dimer, leading to a positive (negative) optical binding force. Our findings provide a clear strategy to engineer optical binding forces via changes in device geometry and its illumination profile. Thus, we envision a significant role for our device in emerging nanophotonics structures.

Prethermalization and Nonreciprocal Phonon Transport in a Levitated Optomechanical Array

AbstractRecent advances in levitated optomechanics have brought new opportunities for the study of macroscopic quantum mechanics and precision measurement. Previous studies have mainly focused on the single levitated nanoparticle, the systems of multiple levitated nanoparticles were rarely involved. Here an array of optically levitated nanospheres in vacuum is considered and nontrivial phonon transport in this system is investigated. The levitated nanospheres are coupled by optical binding. Key parameters of this system, such as the interaction range, trapping frequencies, and mechanical dissipation, are highly tunable. Due to these advantages, counter‐intuitive phenomena such as prethermalization and nonreciprocal phonon transport can be achieved by tuning the spacing between neighboring spheres, the mechanical dissipation, and the trapping frequency of each sphere. The system provides a great platform to investigate novel phonon transport and thermal energy transfer.

Stability and dynamics of chiral nanoparticles in lateral optical binding induced by high-order Bessel beams

The generalized multi-particle Mie equation (GMM) and electromagnetic momentum (EM) theory are applied to investigate the stability and dynamics of chiral nanoparticles in lateral optical binding induced by a high-order Bessel beam (HOBB). Such non-diffracting light suppressed the influence of the axial intensity profile of the illuminating beams on the self-organization process which then depended critically upon the inter-particles interactions. The illuminating HOBB is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz–Mie theories (GLMT). Utilizing the addition theorem of the vector spherical wave functions (VSWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the chiral nanoparticles is considered. The observed lateral binding force (BF) dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction when the chiral spheres degenerate into isotropic spheres. We discuss the influence of the different parameters of the incident Bessel beam and of the chiral body on lateral BF in detail. Linearly and circularly polarized incident Bessel beams are considered, and the corresponding lateral BFs are compared and analyzed. The polarizations of incident HOBB considerably influence the lateral BF of chiral nanoparticles. In binding chiral nanoparticles, the polarization of incident beams should be chosen in accordance with the chirality. This finding may provide a recipe to understand the light interaction with multiple chiral particles of arbitrary shapes with the aid of the analytical approach. It could be a promising avenue in controlling the optical micromanipulation on chiral structures self-arrangement.

Many-body effects in an MXene Ti2CO2 monolayer modified by tensile strain: GW-BSE calculations.

MXenes, two-dimensional (2D) layered transition metal carbide/nitride materials with a lot of advantages including high carrier mobility, tunable band gap, favorable mechanical properties and excellent structural stability, have attracted research interest worldwide. It is imperative to accurately understand their electronic and optical properties. Here, the electronic and optical response properties of a Ti2CO2 monolayer, a typical member of MXenes, are investigated on the basis of first-principles calculations including many-body effects. Our results show that the pristine Ti2CO2 monolayer displays an indirect quasi-particle (QP) band gap of 1.32 eV with the conduction band minimum (CBM) located at the M point and valence band maximum (VBM) located at the Γ point. The optical band gap and binding energy of the first bright exciton are calculated to be 1.26 eV and 0.56 eV, respectively. Under biaxial tensile strains, the lowest unoccupied band at the Γ point shifts downward, while the lowest unoccupied band at the M point shifts upward. Then, a direct band gap appears at the Γ point in 6%-strained Ti2CO2. Moreover, the optical band gap and binding energy of the first bright exciton decrease continuously with the increase of the strain due to the increase of the lattice parameter and the expansion of the exciton wave function. More importantly, the absorbed photon flux of Ti2CO2 is calculated to be 1.76–1.67 mA cm−2 with the variation of the strain, suggesting good sunlight optical absorbance. Our work demonstrates that Ti2CO2, as well as other MXenes, hold untapped potential for photo-detection and photovoltaic applications.

Tuning the binding energy of excitons in the MoS2 monolayer by molecular functionalization and defective engineering.

First-principle calculations within many-body perturbation theory are carried out to investigate the influence of the adsorbed molecules and sulfur (S) defects on the electronic and optical properties of the MoS2 monolayer. The exciton binding energy in the range of 0.05 eV to 1.14 eV is observed as a function of molecular coverage, when NO and 1,3,5-triazin (C3H3N3) are adsorbed on the pristine surface. These results can be explained by the interaction between the exciton and the adsorbed molecule. Furthermore, the combined effect of molecular functionalization and defective doping is studied. Our results show that both the electronic and optical band gaps of the MoS2 monolayer strongly depend on the molecular species and the defective coverage, and can be tuned up to ∼2 eV. This work demonstrates the great potential of controlling the MoS2 monolayer's excitonic properties by molecular functionalization and defective engineering.

Nanoscale Tunable Optical Binding Mediated by Hyperbolic Metamaterials

Carefully designed nanostructures can inspire a new type of optomechanical interactions and allow surpassing limitations set by classical diffractive optical elements. Apart from strong near-field localization, a nanostructured environment allows controlling scattering channels and might tailor many-body interactions. Here we investigate an effect of optical binding, where several particles demonstrate a collective mechanical behavior of bunching together in a light field. In contrast to classical binding, where separation distances between particles are diffraction limited, an auxiliary hyperbolic metasurface is shown here to break this barrier by introducing several controllable near-field interaction channels. Strong material dispersion of the hyperbolic metamaterial along with high spatial confinement of optical modes, which it supports, allows achieving superior tuning capabilities and efficient control over binding distances on the nanoscale. In addition, a careful choice of the metamaterial slab’s thickness enables decreasing optical binding distances by orders of magnitude compared to free space scenarios due to the multiple reflections of volumetric modes from the substrate. Auxiliary tunable metamaterials, which allow controlling collective optomechanical interactions on the nanoscale, open a venue for new investigations including collective nanofluidic interactions, triggered biochemical reactions, and many others.

Self-Trapped Nanoparticle Binding via Waveguide Mode

In this paper, we study a stable optomechanical system based on a nanoparticle chain coupled to a waveguide mode. Under the plane wave excitation the nanoparticles form a stable self-organized periodic chain array along the direction of the waveguide through the transverse binding effect. We show that owing to the long-range interaction between the nanoparticles the trapping potential for each nanoparticle in the chain increases linearly with the system size making the formation of long chains more favourable. We propose that this effect can be observed with an optical nanofiber which is a versatile platform for achieving optical binding of atoms and nanoparticles. Our calculations show that binding energy for two nanoparticles is in the range of $9\div13~kT$ reaching the value of 100 $kT$ when the size of the chain is increased to 20 nanoparticles that makes the potential experimental observation of the effect possible. We also suggest the geometry of the two counter-propagating plane waves excitation, which will allow trapping the nanoparticles close to the optical nanofiber providing efficient interaction between the nanoparticles and the nanofiber.

Effect of the Calcination Temperature and Li(I) Doping on Ethanol Sensing Properties in p-Type CuO Thin Films

The gas response characteristic toward C2H5OH has been demonstrated in terms of copper-vacancy concentration, hole density, and microstructural factors for undoped/Li(I)-doped CuO thin films prepared by sol-gel method. For the films, both concentrations of intrinsic copper vacancies and electronic holes decrease with increasing calcination temperature from 400 to 500 to 600 oC. Li(I) doping into CuO leads to the reduction of copper-vacancy concentration and the enhancement of hole density. The increase of calcination temperature or Li(I) doping concentration in the film increases both optical band gap energy and Cu2p binding energy, which are characterized by UV-vis-NIR and X-ray photoelectron spectroscopy, respectively. The overall hole density of the film is determined by the offset effect of intrinsic and extrinsic hole densities, which depend on the calcination temperature and the Li(I) doping amount, respectively. The apparent resistance of the film is determined by the concentration of the structural defects such as copper vacancies, Li(I) dopants, and grain boundaries, as well as by the hole density. As a result, it is found that the gas response value of the film sensor is directly proportional to the apparent sensor resistance.

Optical binding of nanoparticles

Abstract Optical binding is a laser-induced inter-particle force that exists between two or more particles subjected to off-resonant light. It is one of the key tools in optical manipulation of particles. Distinct from the single-particle forces which operate in optical trapping and tweezing, it enables the light-induced self-assembly of non-contact multi-particle arrays and structures. Whilst optical binding at the microscale between microparticles is well-established, it is only within the last few years that the experimental difficulties of observing nanoscale optical binding between nanoparticles have been overcome. This hurdle surmounted, there has been a sudden proliferation in observations of nanoscale optical binding, where the corresponding theoretical understanding and predictions of the underlying nanophotonics have become ever more important. This article covers these new developments, giving an overview of the emergent field of nanoscale optical binding.

Finite-Field Approach to Solving the Bethe-Salpeter Equation.

We present a method to compute optical spectra and exciton binding energies of molecules and solids based on the solution of the Bethe-Salpeter equation and the calculation of the screened Coulomb interaction in a finite field. The method does not require either the explicit evaluation of dielectric matrices or of virtual electronic states, and can be easily applied without resorting to the random phase approximation. In addition, it utilizes localized orbitals obtained from Bloch states using bisection techniques, thus greatly reducing the complexity of the calculation and enabling the efficient use of hybrid functionals to obtain single particle wave functions. We report exciton binding energies of several molecules and absorption spectra of condensed systems of unprecedented size, including water and ice samples with hundreds of atoms.

Layered Ruddlesden–Popper Efficient Perovskite Solar Cells with Controlled Quantum and Dielectric Confinement Introduced via Doping

AbstractLayered Ruddlesden–Popper (RP) hybrid perovskite semiconductors have recently emerged as promising materials for photovoltaics application. However, the strong quantum and dielectric confinement of RP perovskite compounds increases their optical bandgap and binding energy of exciton, which limit their application in solar cells. Herein, the doping of RP‐based (BA)2(MA)3Pb4I13 perovskite materials by means of Li+ is reported for the first time, which can significantly help to reduce dielectric confinement and thus the exciton‐binding energy via reducing the dielectric constant difference between organic spacer cation and inorganic framework. Furthermore, the Li+ doping boosts the carrier mobility, reduces the trap density states, and thus allows to achieve power conversion efficiency of ≈15% via Li+‐(BA)2(MA)3Pb4I13‐based perovskite solar cell, which is the highest efficiency for layered perovskites (n = 4) so far. This work highlights the promising ionic doping engineering for further improvement of the layered perovskite materials.

Tuning Nanoparticle Electrodynamics by an Optical-Matter-Based Laser Beam Shaper.

Spatially modulated optical fields provide the perspective of tuning nanoparticle (NP) dynamics in a colloidal suspension. Here, it is shown that the lateral interferometric optical field created by a chain of optically bound Au NPs (i.e., optical matter) can tailor the electrodynamic interactions among more Au NPs. The free-standing NP chain, which is assembled and confined by an auxiliary optical line, shapes the main trapping beam and guides the self-organization of Au NPs under an optimized polarization direction. We find that the NP chain can largely enhance the anisotropic optical binding interaction of two nearby NPs but suppress the anisotropic interaction of multiple NPs, leading to isotropic self-organization. The dynamics and structural transitions of the NPs are well-reproduced in a simulation by using a coupled finite-difference time-domain (FDTD)-Langevin dynamics approach. Our work provides a new dual-beam optical trapping and in situ laser beam shaping approach to study and control interparticle electrodynamic interactions among colloidal NPs.