Dimer Axis Research Articles

Real-Time Simulation of Ultrafast Electronic Dynamics of Nanoscale Systems Involving an Organic Molecule and a Nanoparticle Dimer.

Understanding and predicting the behavior of nanomaterials composed of plasmons interacting with quantum emitters at ultrafast timescales is crucial for the better manipulation of light at the nanoscale and advancing technologies like ultrafast communication and computing. Here we perform a simulation of the "real-time" electronic dynamics of a coupled molecule-metal nanoparticle dimer interacting with an ultrashort resonant laser pulse by combining the real-time time-dependent density functional theory (RT-TDDFT) approach with the time-domain frequency-dependent fluctuating charge (TD-ωFQ) model, an atomistic electromagnetic (AEM) model for the dynamic plasmonic response of nanoparticles. It is shown that the induced dipoles evolve from an exponential decay pattern to a beat pattern with an increase in coupling strength, which is altered by changing the molecular orientation relative to the dimer axis. It is further shown that in the strong coupling regime, both the excited molecule and the plasmon relax rapidly due to the molecule-plasmon interaction, and the efficient coherent energy exchange between the interacting molecule and plasmon modes occurs on a femtosecond (fs) timescale. This work provides guidance on manipulating light-matter interaction and studying molecular plasmonics at extremely fast timescales.

Plasmonic couplings in Ag-Au heterodimers.

The plasmonic coupling between silver (Ag) and gold (Au) nanoparticles (NPs) under four polarization modes was examined: a longitudinal mode (L-mode), where the electric field of a linearly polarized incident light parallels the dimer axis, and three transverse modes (T-modes), where the electric field of the light is perpendicular to the dimer axis. The coupling was studied using the discrete dipole approximation followed by an in-house postprocessing code that determines the extinction (Qext), absorption (Qabs), and near-field (Qnf) spectra from the individual NPs as well as the whole system. In agreement with the literature results, the extinction/absorption spectra of the whole dimer have two peaks, one near the Ag localized surface plasmon resonance (LSPR) region and the other at the Au LSPR region, with the peak at Ag LSPR being reduced in all modes and the peak at Au LSPR being red-shifted and increased in the L-mode but not in the T-modes. It is further shown that the scattering at the Ag LSPR region is reduced and becomes less than the isolated Ag NPs, but the absorption at the Ag LSPR is increased and becomes greater than the isolated Ag NPs for the 50nm Ag-Au heterodimer. This suggests that the scattering from Ag NPs is being reabsorbed by the neighboring Au NPs due to the interband electronic transition in Au at that wavelength range. The Qext from the individual NP in the heterodimer shows the presence of the Fano profile on the Au NP but not on the Ag NP. This phenomenon was further investigated by using a dielectric particle (DP) placed near the Ag or Au NPs. The Fano profile appears in the absorbing DP spectra placed near either Ag or Au NPs. However, the Fano profile is masked upon further increases in the refractive index value of the DP particle. This explains the absence of a Fano profile on the Ag NPs in the Ag-Au heterodimer. The large near-field enhancement on both Ag and Au NPs at the Au plasmonic wavelength in the L-mode for large NPs was investigated through a DP-Au system. The large enhancement was shown to arise from a large imaginary component of the DP refractive index and a small real component. Through examination of both the near- and far-field properties of the individual NPs as well as the whole system and examinations of DP-Ag and DP-Au systems, our study provides a new understanding of the couplings between Ag and Au NPs.

Reversal of optical binding force on a plasmonic heterodimer with dipole–dipole type Fano resonance

In the present work, we theoretically and numerically investigate the near field optical binding force on Au–Ag nanorod heterodimers, which can support the electrical dipole–dipole type Fano resonance, under a normally plane wave or polarized laser beam. The numerical results show that the optical binding force between the heterodimer can be reversed near the position of the Fano dip when the incident wave is polarized along the dimer axis, indicating an attractive to repulsive transition. Compared with the Au–Ag heterodimer, the Au and Ag homodimers with the same material show no repulsive binding force. Using the multipole decomposition method, we find that the sign of optical binding force is completely determined by the phase difference of the separated electric dipole plasmon modes excited in the different particles, which can be strongly affected by the wavelength of the incident wave. In addition, we demonstrate the effects of three geometrical parameters (including the length, radius, and gap of the heterodimer) on the Fano-mediated optical binding force in detail. Finally, the numerical results indicate that the reversal of attractive and repulsive forces can also be obtained when the heterodimer is illuminated with a laser beam. Our findings are important for not only a deep understanding of plasmonic-mediated optical binding force but also stable optical manipulation of the plasmonic particles.

Nanoscale Hotspot-Induced Emitters in DNA Origami-Assisted Nanoantennas.

We report the observation of hotspot-induced emitters and photoluminescence enhancement of up to 42-fold from DNA origami-assisted plasmonic dimer nanoantennas upon excess polarized laser illumination. The presence of DNA and laser polarization alignment along the dimer axis are critical for the generation of bright emitters responsible for the observed PL increase. The emission spectrum reveals characteristic Raman peaks of amorphous carbon, suggesting the formation of carbon-based emitters in the nanoantenna due to the plasmonic hotspots at the longitudinal antenna resonance.

Influence of non-uniform magnetic field on spatial orientation of Heisenberg dimers with spin [formula omitted

The paper presents exact results for the Heisenberg dimer with spin S=1 placed in a non-uniform magnetic field. It was assumed that the dimer has a degree of freedom of movement enabling the spatial arrangement of the dimer axis parallel or perpendicular to the field gradient. Based on the calculations of the Gibbs magnetic energy, the occurrence of magneto-spatial phase transitions of the 1st kind was found, in which the dimer changing its magnetization state simultaneously changes the spatial orientation relative to the field gradient. Ground state phase diagrams and temperature diagrams are presented, as well as calculations of the Gibbs energy, magnetization, entropy and dimer quadrupolar moment. These calculations form an internally consistent thermodynamic description and are presented in figures. The discussed model illustrates the possibility of a magnetic contribution to the self-assembly of dimers on the surface.

Probing the Fragmentation Pathways of an Argon Dimer in Slow Ion–Dimer Collisions

We report the development of a supersonic jet assembly to study electron transfer collisions with atoms, molecules, and van der Waals clusters. A comparative study of Ar monomer and dimer cations is presented for different capture-associated channels with a 2.5 keV/u O2+ projectile beam. For the Ar+ + Ar+ fragmentation channel, the interatomic relaxation channels are discussed. The vacancies of the dimer single site or double site show the dependence on capture mechanisms. In the Ar2+ + Ar+ fragmentation channel, double capture, in addition to the single ionization process, dominates. The orientation effect reflects the maximum yield at around 50 and 130 degrees, and angular distributions are nearly symmetric about the axis perpendicular to the dimer axis.

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters.

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Magnetic Purcell effect in all-dielectric periodic optical antennas

We investigate the magnetic dipole spontaneous emission in the periodic all-dielectric subwavelength spheres array by the magnetic dyadic Green function, together with Cartesian multipole decomposition approach. We find that magnetic Purcell factor exhibits energies splitting (forming quasi-normal modes) due to strong coupling effect between different part of all-dielectric optical antennas. The period plays an important role in the magnetic Purcell effect, and there is an optimized period for the optical antenna to maximize the magnetic Purcell factor. In addition, the magnetic dipole along the spheres’ dimer axis will significantly couple to the higher order multipolar Mie resonances with magnetic Purcell factor greater than 400. Our work will help to design all-dielectric nanoantennas to manipulate magnetic light emission originating from magnetic-dipole transitions and study the magnetic light–matter interactions in topological photonics.

Dimer of dielectric nanospheres as a generalized Huygens element

We study the conditions of suppressed backward scattering in optical oligomers. Optical oligomers are closely packed clusters of nanoparticles which support multiple resonances, allowing to achieve scattering phase rotation by 2π necessary for metalenses. In this paper we consider the simplest oligomer — a dimer, which is irradiated by a normally-incident plane wave with the magnetic field oriented along the dimer axis. Using the dipole approximation, we have derived analytically the generalized Kerker condition for this system. Similarly to a single sphere case, the electric and magnetic dipole have to be of the same amplitude to suppress the backward scattering. However, due to the inter-particle coupling, the full coupled-dipole problem must be solved to find the wavelength of the backscattering suppression. Using the coupled-dipole and coupled-multipole methods, we have found that the Kerker condition wavelength for the considered dimer is red-shifted compared to a single sphere.

Mutations in the stator protein PomA affect switching of rotational direction in bacterial flagellar motor

The flagellar motor rotates bi-directionally in counter-clockwise (CCW) and clockwise (CW) directions. The motor consists of a stator and a rotor. Recent structural studies have revealed that the stator is composed of a pentameric ring of A subunits and a dimer axis of B subunits. Highly conserved charged and neighboring residues of the A subunit interacts with the rotor, generating torque through a gear-like mechanism. The rotational direction is controlled by chemotaxis signaling transmitted to the rotor, with less evidence for the stator being involved. In this study, we report novel mutations that affect the switching of the rotational direction at the putative interaction site of the stator to generate rotational force. Our results highlight an aspect of flagellar motor function that appropriate switching of the interaction states between the stator and rotor is critical for controlling the rotational direction.

Millimeter-Wave Measurements and Applications of Dipole Modes in Spherical Alumina Dimer

Dielectric dimers composed of spherical Alumina ceramic resonators are studied. The electromagnetic scattering of dimer configurations is analyzed through spherical multipole decomposition. Different configurations of dimer operation are addressed with respect to propagation direction and polarization of the incident wave. In context to forward scattering caused by dipole interaction, measurements are performed in V-band (50–75 GHz) to quantify the corresponding electromagnetic signature of the dielectric dimer. Based on the fundamental dipole modes excited in the subwavelength free-space gap along the dimer axis, electric and magnetic hotspots are proposed, which can be exploited for near-field enhancement-based applications at millimeter-wave frequency. A dedicated frequency-scaled measurement setup is designed to measure the field-enhancement factor between 25 and 30 GHz in the free-space dimer gap, corresponding well to simulated results.

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.

Coulomb-explosion imaging of carbon monoxide dimers

We report on experimental results obtained from collisions of slow highly charged Ar9+ ions with a carbon monoxide dimer (CO)2 target. A COLTRIMS setup and a Coulomb explosion imaging approach are used to reconstruct the structure of the CO dimers. The three dimensional structure is deduced from the 2-body and 3-body dissociation channels from which both the intermolecular bond length and the relative orientation of the two molecules are determined. For the 3-body channels, the experimental data are interpreted with the help of a classical model in which the trajectories of the three emitted fragments are numerically integrated. We measured the equilibrium intermolecular distance to be Re = 4.2 A. The orientation of both CO molecules with respect to the dimer axis is found to be quasi-isotropic due to the large vibrational temperature of the gas jet.

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.

Plasmon-induced Circular Dichroism and Asymmetric Hot-electrons Generation Triggered by a Chiral Molecule for Polarization-dependent Chemical Reactions

Performing chiral photodetection, photocatalysis or photochemical reactions at the molecular level has always been a nearly impossible task, due to the very low efficiency of the generated optical circular dichroism signals. On the contrary, chiral colloidal nanocrystals have been shown recently to offer a very large differential response to circularly polarized light. Such a response is able to generate hot-electrons with a very strong asymmetry, thus potentially able to perform the aforementioned tasks. In this paper, an intermediate picture is chosen, for which an achiral small assembly of identical particles triggered by a chiral molecule is able to generate large plasmon-induced circular dichroism (PICD), in turn able to generate the required asymmetry in the generation rates of hot-electrons. By performing Finite Difference Time Domain simulations based on the combination of a classical model of PICD generation and a quantum-based model of hot-electrons generation, the simple design of an achiral gold NPs’ dimer triggered by a chiral molecule located in the center and oriented with its transition electric dipole moment parallel to the dimer axis is shown to be able to generate a strong asymmetry in its HEs’ generation response. The PICDs and related hot-electrons generation rates increase as a function of volume, surface, respectively, of the considered systems, thereby providing a way to trigger chemical reactions.

Second harmonic generation from metal nanoparticle dimer: an analytical approach in dipole approximation

In this theoretical study, the problem of Second Harmonic Generation (SHG) in the interaction of laser beam with a Metallic Nanoparticle (MNP) dimer is considered. Using a classical electrodynamics approach, the nonlinear interaction of laser beam fields with Nanoparticles (NPs) is considered taking into account the dipole-dipole interparticle interactions. Analytical formulae are derived for the effect of dipole-dipole interaction on the Second Harmonic (SH) radiation power for two different polarizations of laser beam. It is found that the interaction causes the substantial enhancement of the SH radiation power while for the case when the laser beam field is parallel to the dimer axis, this enhancement is larger. Additionally, the dipole-dipole interaction of NPs leads to the redshift of the peak value with respect to the individual NP radiation. The resonance frequency displacement of the parallel case is more than that of the perpendicular one. The effect of particles size and interparticle separation on the SHG is studied.

Adsorption and dissociation of dinitrogen on mono-metallic and bimetallic dimers

This paper deals with Density Functions Theory (DFT) based theoretical investigation of the dissociation of strong N–N triple bond of dinitrogen (N2). Mono-metallic and bimetallic dimers of selected transition metals (Zr, Nb, Hf, and Ta) are used as adsorbent. The dissociation of N–N bond is found to be strongly dependent on orientation of N–N axis with respect to the axis of the adsorbent dimer. N–N axis perpendicular to dimer axis has been found to be suitable for dissociative adsorption of dinitrogen. Apart from orientation the combination of two different transition metal atoms in the dimer also has significant effect in the elongation and dissociation of N–N bond. Our study shows even 2-atom clusters (dimers) of these transition metals are capable of breaking N–N triple bond.

Magnetic dimer at a surface: Influence of gravity and external magnetic fields.

The interaction of two dipolar hard spheres near a surface and under the influence of gravity and external perpendicular magnetic fields is investigated theoretically. The full ground-state phase diagram as a function of gravity and magnetic field strengths is established. A dimer (i.e., two touching beads) can only exist when the gravity and magnetic field strengths are simultaneously not too large. Thereby, upon increasing the magnetic field strength, three dimeric states emerge: a lying state (dimer axis parallel to the substrate), an inclined state (intermediate state between the lying and standing ones) and a standing state (dimer axis normal to the substrate). It is found that the orientation angles of the dimer axis and the dipole moment in the newly discovered inclined phase are related by a strikingly simple Snell-Descartes-like law. We argue that our findings can be experimentally verified in colloidal and granular systems.

Structural Basis of Lipid Scrambling and Ion Conduction by TMEM16 Scramblases

The plasma membrane of eukaryotic cells is asymmetric, with polar and charged lipids sequestered to the inner leaflet when the cell is at rest. Activation of phospholipid scramblases rapidly collapses this asymmetry and externalizes negatively charged phosphatidylserine lipids, activating extracellular signaling networks that control processes such as apoptosis, blood coagulation, membrane fusion and repair. The TMEM16 family is comprised of Ca2+‐dependent Cl− channels and phospholipid scramblases with non‐selective ion channel activity. Structural and functional analyses of TMEM16 scramblases identified a membrane‐exposed hydrophilic groove that serves as a dual translocation pathway for ions and lipids. The mechanisms underlying Ca2+‐dependent gating of TMEM16 scramblases/non‐selective channels, lipid permeation and the emergence of their ion conduction pore remain poorly understood. We used cryo‐electron microscopy to determine the structures of a fungal scramblase/non‐selective channel from Aspergillus fumigatus, afTMEM16, reconstituted in lipid nanodiscs in the presence and absence of Ca2+. These structures reveal that Ca2+ binding induces a global rearrangement of the transmembrane and cytosolic regions, resulting in opening of the lipid permeation pathway by way of rearrangements of TM4 and TM6. The structures of the scramblase/nanodisc complex in the presence and absence of Ca2+ show that the scramblases induce a pronounced distortion of the surrounding membrane: the outer leaflet is bent along the dimer axis of the protein and the membrane is thinned near the opened lipid translocation pathway thereby facilitating transbilayer lipid movement.Molecular dynamics simulations of a second closely related fungal TMEM16 scramblase, nhTMEM16, revealed that a hydrophobic lock between TM3 and TM4 is essential to maintain the open, lipid‐conductive conformation of the scramblase. MD simulations suggest that disruption of this lock favors the positioning of TM4 giving rise to a Ca2+‐bound conformation that is closed to the membrane and thus non‐conductive for lipids. Indeed, mutations at the TM3/TM4 lock severely impair lipid scrambling. Notably, these mutants retain ion channel activity, indicating that the dual activity scramblase/channel was converted into a channel only protein. The cryoEM structure of a channel‐only mutant reveal a continuous, protein‐delimited pore of sufficient size to allow ion permeation. Our results show that TM6 and TM4 are the gating elements of TMEM16 scramblases where TM6 movement is directly controlled by Ca2+‐binding and TM4 rearrangements are controlled by a hydrophobic lock between TM3 and TM4.Support or Funding InformationNIGMS 5R01 GM106717

Isosbestic light absorption by metallic dimers: effect of interparticle electromagnetic coupling.

Isosbestic plasmonic nanostructures, which feature an invariance of optical absorption and heat generation upon varying the incident light polarization, have broad application in many fields such as nanochemistry, optical nanoantennas, and microbubble formation. In this study, we focus on the isosbestic optical absorption by metallic dimers and systematically investigate the coupling between two interacting particles by using both the superposition T-matrix method and dipole approximation model. We observe that the interparticle coupling effects on particle absorption can be both positive and negative, compared to an isolated particle. Meanwhile, the optical absorption properties of spheres with small size parameters can realize more flexible control through changing the sphere size, interparticle distance, and incident light wavelength. For illuminations with incident light propagating perpendicularly to the line joining the centers of the two spheres, isosbestic conditions will be satisfied as long as the absorption efficiencies for transverse and longitudinal illuminations are equal. For transverse illuminations along the dimer axis, the ratio of absorption efficiency of the two metallic spheres presents the fluctuation change with the interparticle distance. Owing to the strong interparticle coupling effects, it even leads to the absorption efficiency of the far sphere being higher than that of the near sphere. Our results are aimed at expanding our understanding of the interparticle electromagnetic coupling effects on isosbestic light absorption in plasmonic nanoparticle systems.