Dipole Emission Research Articles

Longitudinal Piezoelectricity and Polarization-Insensitive Oxidation in Janus vdWs Nb3SeI7.

As a newly discovered Janus van der Waals (vdW) material, semiconducting Nb3SeI7 offers several notable advantages, including spontaneous out-of-plane polarization, facile exfoliation to the monolayer limit, and significant out-of-plane emission dipole in second harmonic generation. These properties make it a promising candidate for piezoelectric and piezophototronic applications in highly efficient energy conversion. However, Nb3SeI7 is prone to oxidation when exposed to oxygen, which can severely limit the exploration and utilization of these intriguing physical properties. Therefore, understanding the oxidation mechanism of pristine Nb3SeI7 and its correlation with electrical polarization-an area that remains largely unexplored-is highly significant. In this study, the out-of-plane piezoelectricity of Nb3SeI7 is experimentally demonstrated, with a piezoelectric coefficient (|d33 eff|) of 0.76nmV-1. Furthermore, by combining near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), Time-of-Flight secondary ion mass spectrometry (ToF-SIMS), and Density functional theory (DFT)calculations, it is revealed that the oxidation of Nb3SeI7 is self-limiting and independent of its electrical polarization, owing to the similar defect formation energies of Se and I atoms. This self-limiting and polarization-insensitive oxidation provides valuable insights into the stabilization mechanisms and expands the potential applications of out-of-plane piezoelectricity and other intriguing physical properties in Janus vdW Nb3SeI7.

Single-Molecule Spectral Fluctuation Originates from the Variation in Dipole Orientation Connected to Accessible Vibrational Modes.

Fluctuation in fluorescence emission of an immobilized single molecule is typically ascribed to the chromophore's intrinsic structural conformations and the influence of local environmental factors. Despite extensive research since its initial observation, a direct connection between these spectral fluctuations and the rearrangement of emission dipole orientations has remained elusive. Here, we elucidate this fundamental molecular behavior and its underlying mechanisms by employing unique single-molecule multidimensional tracking to simultaneously monitor both the emission spectrum and the three-dimensional dipole orientation of individual fluorophores. We present compelling evidence demonstrating a correlation between spectral fluctuations and dipolar rearrangements at room temperature. Our observations reveal that variations in the radiative relaxation probabilities among different vibronic emission bands, coupled with the interaction of associated vibrational modes, drive these spectral fluctuations. We identify significant out-of-plane dipole reorientations during pronounced spectral fluctuations, commonly known as spectral jumps, which primarily arise from transitions between dominant vibrational modes. Furthermore, we emphasize the potential for constructing vibrational spectra and optical nanoscopy with vibrational specificity, leveraging the vibronic emissions from single emitters.

Evaluating the Accuracy of the COMSOL-Based Finite-Element Method for Simulating Plasmon-Modified Fluorescence.

Accurately modeling plasmon-modified fluorescence is important for understanding and guiding the design of experimental nanostructures that reliably enhance fluorescence. They are of particular interest due to their potential to allow localized "hot spots" of high fluorescence enhancement in a reproducible manner. Given the increasingly prevalent use of the COMSOL Multiphysics software package for simulating these phenomena, we investigate its accuracy using an analytically tractable model consisting of a gold nanosphere interacting with either a plane wave or a radiating point dipole. COMSOL simulation results were compared with a formally exact analytical theory. It was found that simulation parameters commonly used for plane-wave scattering do not necessarily produce accurate results for the nanoparticle-plasmon-coupled dipole emission case. Instead, user-input adaptive meshing parameters were found to be helpful in achieving quantitative agreements between COMSOL and analytical theory results for plasmon-modified fluorescence. Our studies suggest convergence to analytically calculated values when a minimum of two additional user-input mesh elements separate the point-dipole position and the nanoparticle surface. This practical insight is expected to aid in the application of COMSOL simulations to planning and interpreting fluorescence modification experiments.

Fluorescence spatial anisotropy of emission dipoles in an orthogonal imaging system

In this study, we explore the relatively unexplored spatially anisotropic fluorescence emission induced by rotationally polarized excitation light in orthogonal imaging systems, a phenomenon that is particularly pronounced in orthogonal setups compared to coaxial configurations. Despite its significance, this aspect has been largely neglected, given the prevalent use of coaxial Fluorescence Polarization Microscope (FPM) setups. Our research endeavors to bridge this gap by formulating a physical model to investigate the polarization-dependent emission of dipoles and the corresponding fluorescence signal within an orthogonal imaging configuration. Additionally, we introduce the Fluorescence Spatial Anisotropy Index (FSAI) to quantify fluorescence spatial anisotropy.

Mirror-coupled plasmonic nanostructures for enhanced in-plane magnetic dipole emission

Abstract Self-assembling of plasmonic nanoparticles into an oligomer can produce optical nanoantennas with resonant magnetic responses, which constitutes a promising platform to study magnetic light-matter interactions at the nanoscale. However, these self-assembled magnetic nanostructures typically feature a flat-lying geometric configuration, giving rise to out-of-plane magnetic dipole (MD) moments that cannot couple to the far-field light efficiently. Herein, we propose a new geometric configuration of plasmonic nanoantennas to realize in-plane MD responses. This is achieved by elegantly coupling a plasmonic nanoparticle oligomer to one or more adjacent metal mirrors, virtually forming a vertically standing nanoparticle tetramer or trimer, yet with in-plane MD moments. We verified this design strategy by numerically simulating the resonance responses of three typical mirror-coupled nanostructures, all exhibiting pronounced resonant MD characters. Furthermore, optical magnetic emitters can be readily coupled to these plasmonic nanoantennas and gain an emission enhancement of two orders of magnitude while simultaneously featuring a high emission directionality (collection efficiency up to ∼ 70 % evaluated with common microscopy optics). We believe these mirror-enabled magnetic nanoantennas could lead to novel nanophotonic devices fully exploiting the magnetic nature of light.

Stokes-based analysis for the estimation of 3D dipolar emission

We provide a general description of the measurement capabilities of systems that probe the 3D state of polarization of light emitted by a dipole or a collection of dipoles. This analysis is based on a generalization of the Stokes parameters for 3D polarization, and its goal is to provide insight into what constitutes a good measurement system under specific circumstances, through the definition of appropriate merit functions. Three cases are considered: the general case of arbitrary states of 3D polarization, the special case of 3D linear full or partial polarization states, and the even more specific case of linear dipoles that wobble with rotational symmetry around a central direction. Note that the latter two cases are of interest in fluorescence microscopy. The analysis presented here is illustrated by applying it to two different approaches used commonly in orientation microscopy: PSF engineering and ratiometric measurements.

Achieving Ideal Magnetic Light Emission with Electric-Type Emitters.

Optical magnetic dipole (MD) emission predominantly relies on emitters with significant MD transitions, which, however, rarely exist in nature. Here, we propose a strategy to transform electric dipole (ED) emission to a magnetic one by elegantly coupling an ED emitter to a silicon nanoparticle exhibiting a strong MD resonance. This emission mode transformation enables an artificially ideal magnetic dipole source with an MD purity factor of up to 99%. The far-field emission patterns of such artificial MD sources were experimentally measured, which unambiguously resolved their magnetic-type emission origin. This study opens the path to achieving ideal magnetic dipole emission with nonmagnetic emitters, largely extending the availability of magnetic light emitters conventionally limited by nature. Beyond the fundamental significance in science, we anticipate that this study will also facilitate the development of magnetic optical nanosource and enable potential photonic applications relying on magnetic light emission.

Circular Bragg grating single-photon source design exploiting extreme dielectric confinement

Abstract We theoretically propose a single-photon source design based on a circular Bragg grating microcavity incorporating two air triangles in a bowtie configuration. Our proposed design allows for extreme light confinement with a Purcell factor of approximately 166 with a bandwidth of ~9 nm for a triangle separation of 5 nm combined with a collection efficiency above 80 % . The Purcell factor is enhanced by a factor of 8 relative to the reference cavity thanks to the reduction of the mode volume. Furthermore, we demonstrate the critical role of the dipole emitter polarization orientation and spatial position in achieving optimal Purcell enhancement.

Electromagnetic scattering controlled all-dielectric cavity-antenna for bright, directional, and purely radiative single-photon emission

A deterministic, bright, room-temperature stable single-photon source (SPS) has been a major demand in the field of quantum photonics. Here, using computational and analytical techniques, we showed that the Mie-scattering moments of an all-dielectric cavity-cum-antenna help in shaping the spontaneous emission process of an embedded point-dipole emitter, the nanodiamond-based NV− and SiV color centers here. Our resonator-cum-antenna design comprises two top and bottom TiO2 cylinders with a sandwiched polyvinyl alcohol (PVA) layer enclosing the nanodiamond crystal. The Cartesian multi-polar decomposition of the Mie-scattering moments of the sandwiched PVA layer (enclosing the dipole emitter) with subwavelength scale thickness showed strong electric-dipole (ED) resonance. This resulted in significant field confinement, making the PVA layer to act as a cavity, providing a Purcell enhancement of more than an order of magnitude for all dipole orientations. The top and bottom TiO2 cylinders were observed to act as an antenna, and the far-field radiation pattern of the embedded dipole-emitter is controlled by the Mie-scattering moments of the TiO2 cylinders. The radiation directionality along the vertical directions was found to be maximum at the Kerker point (electric dipole moment, ED = magnetic dipole moment), the collection efficiency (CE) being about 80%. For dipole emission coupled to the antenna, the quantum efficiency was observed to increase to a high value of 0.98 for nanodiamond NV− center, very close to an ideal case of purely radiative emission. Our scheme is shown to be universal and can be applied to any solid-state-based quantum emitters, for generating on-demand SPS for quantum-photonic applications.

Single Quantum Emitters with Arbitrarily Polarized Dipole Moments Under Ambient Conditions

AbstractThe ability to control the spin of dipole moments of single quantum emitters is crucial for spin‐based quantum technologies. However, realizing circularly polarized (CP) dipole moments under ambient conditions has remained an outstanding challenge. Here, this work proposes an optical interface capable of converting a single quantum emitter into a deep‐subwavelength super‐emitter with an arbitrarily polarized dipole moment, including the long‐sought CP dipole. It is accomplished by coupling the emitter to dual dipolar modes of a plasmonic nanoantenna with precisely controlled amplitudes and phases, resulting in a coherent superposition of the modes that generates the desired dipole polarization. Meanwhile, the dipole moment amplitude is enhanced by over an order of magnitude. The CP super‐emitter behaves analogously to an ideal CP point dipole emitter, exhibiting characteristic spiraling field pattern and spin‐directional coupling to waveguide modes due to spin‐orbit locking. The results overcome a long‐standing challenge in constructing spin‐controlled quantum emitters under ambient conditions, opening new possibilities for tailoring light–matter interactions and designing novel quantum optical devices.

GRB 210610B: The Internal and External Plateau as Evidence for the Delayed Outflow of Magnetar

After launching a jet, outflows of magnetar were used to account for the achromatic plateau of afterglow and the early X-ray flux plateau known as “internal plateau”. The lack of detecting magnetic dipole emission together with the energy injection feature in a single observation poses confusion until the long gamma-ray burst (GRB) 210610B is detected. GRB 210610B is presented with an optical bump following an early X-ray plateau during the afterglow phase. The plateau followed by a steep decline flux overlays in the steadily decaying X-ray flux with index α X,1 ∼ 2.06, indicating an internal origin and that can be fitted by the spin-down luminosity law with the initial plateau luminosity log10LX∼48.29ergs−1 and the characteristic spin-down timescale T ∼ 2818 s. A subsequent bump begins at ∼4000 s in the R band with a rising index α R,1 ∼ − 0.30 and peaks at ∼14125 s, after which a decay index α R,2 ∼ 0.87 and finally transiting to a steep decay with α R,3 ∼ 1.77 achieve the closure relation of the external shock for the normal decay phase as well as the magnetar spin-down energy injection phase, provided that the average value of the photon index Γ γ = 1.80 derived from the spectral energy distributions (SEDs) between the X-ray and optical afterglow. The closure relation also works for the late X-ray flux. Akin to the traditional picture of GRB, the outflow powers the early X-ray plateau by dissipating energy internally and collides with the leading decelerating blast burst as time goes on, which could interpret the exotic feature of GRB 210610B. We carry out a Markov Chain Monte Carlo simulation and obtain a set of best parameters: ϵ B ≃ 4.7 × 10−5, ϵ e ≃ 0.15, E K,iso ≃ 4.6 × 1053erg, Γ0 ≃ 832, A * ≃ 0.10, L inj,0 ≃ 3.55 × 1050erg s−1. The artificial light curve can fit the afterglow data well. After that, we estimated the average Lorentz factor and the X-ray radiation efficiency of the later ejecta are 35% and 0.13%, respectively.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7.

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Chiral and directional optical emission from a dipole source coupled to a helical plasmonic antenna

Plasmonic antennas with helical geometry can convert linearly polarized dipole radiation into purely circularly polarized far-fields, and vice versa. Besides large Purcell enhancements, they possess a wide tunability due to the geometry dependence of their resonant modes. Here, the coupling of a dipole emitter embedded in a thin film to plasmonic single and double helices is numerically studied. Using a higher-order finite element method (FEM), the wavelength dependent Purcell enhancement of a dipole with different positions and orientations is calculated and the far-fields with respect to their chirality and radiation patterns are analyzed. Both single and double helices demonstrate highly directional and circularly polarized far-fields for resonant excitation but with significantly improved directional radiation for the case of double helices.

Modified diabolo antennas for broadband enhancement of sub-terahertz oscillating magnetic fields.

Enhancement of the magnetic field components of electromagnetic waves is of particular interest across a wide range of applications such as enhanced magnetic dipole emission and increased sensitivity of magnetic resonance. Diabolo antennas are known to significantly enhance the local intensity of oscillating magnetic fields. Although the enhancement factor is sufficiently high for these purposes, the drawback is a narrow frequency band due to their resonant nature. Here we propose modified diabolo antennas, or nested U-shaped antennas, to expand the working frequency band in the sub-terahertz region. In this study, we investigated the geometrical dependence of nested U-shaped antennas and optimized their shape for practical applications in sub-terahertz electron spin resonance spectroscopy.

Dynamic control of spontaneous emission using magnetized InSb higher-order-mode antennas

We exploit InSb’s magnetic-induced optical properties to design THz sub-wavelength antennas that actively tune the radiative decay rates of dipole emitters at their proximity. The proposed designs include a spherical InSb antenna and a cylindrical Si-InSb hybrid antenna demonstrating distinct behaviors. The former dramatically enhances both radiative and non-radiative decay rates in the epsilon-near-zero region due to the dominant contribution of the Zeeman-splitting electric octupole mode. The latter realizes significant radiative decay rate enhancement via magnetic octupole mode, mitigating the quenching process and accelerating the photon production rate. A deep-learning-based optimization of emitter positioning further enhances the quantum efficiency of the proposed hybrid system. These novel mechanisms are promising for tunable THz single-photon sources in integrated quantum networks.

Reconfiguring the Optical Selection Rule in Ultramicrotome‐Crafted Vertically Aligned InSe Ribbons

AbstractDue to the high carrier mobility and direct bandgap character, indium selenide (InSe) exhibits great advantages for advanced optoelectronic applications. However, the unique optical section rule in InSe weakens the light‐matter interaction when the electric field of illuminated light is perpendicular to the c‐axis of planar InSe flake. To overcome this constraint, ultramicrotome technique is introduced to achieve vertically aligned InSe ribbons, aiming to reconfigure the optical paths for improved optical absorption and device performance. The well‐designed InSe ribbons are acquired with uniform morphology, showing an integrated structure free of cracks or fragments. First, the structural symmetry, crystal orientation, and optical anisotropy of InSe ribbon are carefully revealed by polarization‐dependent Raman spectrum, second harmonic generation (SHG), and photoluminescence measurements. Then, the back focal plane (BFP) imaging of photoluminescence distinguishes the in‐plane and out‐of‐plane dipole emission of InSe ribbons and planar InSe flakes, respectively. Notably, the ribbon exhibits new PL peaks with energy lower than the exciton peak of InSe at low temperature, which is attributed to Se vacancies. The work by introducing vertically aligned InSe nanoribbons provides a new strategy to manipulate the light‐matter interactions and defect engineering for novel optoelectronics, which can be also extended to other 2D semiconductors.

Solvent Effects on the Absorption and Emission Spectra of the [5-amino-1-bromoindolizin-3-yl](4-bromophenyl)methanone Molecule.

The effects of solvent on the absorption and emission spectra and dipole moments of the 5ABBM have been extensively studied in a series of solvents. The dipole moments in the excited state are observed to be greater than those in the ground-state in all the solvents studied for the chosen molecule. The dipole moment increase in the excited singlet state ranges from 2.42 to 24.14 D. The experimentally calculated ground state and excited state dipole moments were determined using the solvatochromatic shifts in the absorption and emission spectra as a function of dielectric constant (ɛ) and refractive index (n). These data are used to estimate the excited-state dipole moment using the experimentally determined ground-state dipole moment. A series of fifteen different organic solvents (toluene, methanol, n-butyl alcohol, ethyl acetate, DMS, acetonitrile, benzene, isopropyl alcohol, water, DMF, DCM, DIO, THF, ethanol, and octanol) were investigated at constant dye concentrations. Small changes in the fluorescence spectrum were observed for the different solvents; the highest fluorescence intensity was observed for DMS, and the lowest was observed for water. The Stokes shift in different solvents was studied for the 5ABBM molecule. This results in molecule being more polar in the excited state than in the ground state for the solvents used. The ground state dipole moments, HOMO-LUMO, and molecular electrostatic potential maps were also computed via ab initio calculations and evaluated via Gaussian 09W software.

Stimulated emission does not radiate in a pure dipole pattern

Stimulated emission (StE) remains relatively unused as an image-forming signal despite having potential advantages over fluorescence in speed, coherence, and ultimately resolution. Several ideas for the radiation pattern and directionality of StE remain prevalent, namely, whether a single molecule would radiate StE itself in a pure dipole pattern, or whether its emission direction depends on the driving field. Previous StE imaging has been carried out in transmission, which would collect signal either way. Here, we introduce the StE driving field (the probe) at an angle, using total internal reflection to avoid incident probe light and its specular reflections in our detection path. In this non-collinear detection configuration that also collects some fluorescence from the sample, we observe fluorescence depletion even in the spectral window where an increase in detected signal from StE would be expected if StE radiated like a simple classical dipole. Because simultaneous direct measurement of the fluorescence represents a calibration of the potential size of StE were it spatially patterned like a classical dipole emitter, our study clarifies a critical characteristic of StE for optimal microscope design, optical cooling, and applications using small arrays of emitters.

Hyperbolic phonon polaritons-induced photonic spin Hall effect in an α -MoO3 thin film

Photonic spin Hall effect (PSHE) can be achieved by using the patterned structures or metal/dielectric multilayers in the nanophotonic systems; however, the complicated structures of these devices hinder their further applications. Herein, we demonstrate that highly directional PSHE can be realized through the excitation of hyperbolic phonon polaritons (HPPs) in a comparably simple architecture based on an anisotropic α-MoO3 thin film. It is shown that the propagation of the HPP modes of α-MoO3 in the reststrahlen (RS) bands exhibits topological transitions between open hyperbola and closed ellipse in both real space and momentum space (k-space) due to the extreme in-plane anisotropy. Specifically, larger dispersion angle possesses larger figure of merit (FoM), and high k mode of HPPs exhibits robust propagation properties at the maximum dispersion angle. Spin-selected propagation with asymmetric ratio of intensity equal to ±0.94 can be realized by changing the handedness of the dipole emitters. By changing the incident wavelengths of the RS bands, the propagation angle of the HPP modes can be dynamically tuned in wide angular and wavelength ranges.

High-Q Tamm plasmon-like resonance in spherical Bragg microcavity resonators.

This work proposes what we believe to be a novel Tamm plasmon-like resonance supporting structure consisting of an Au/SiO2 core-shell metal nanosphere structure surrounded by a TiO2/SiO2 spherical Bragg resonator (SBR). The cavity formed between the core metal particle and the SBR supports a localized mode similar to Tamm plasmons in planar dielectric multilayers. Theoretical simulations reveal a sharp absorption peak in the SBR bandgap region, associated with this mode, together with strong local field enhancement. We studied the modification of a dipolar electric emitter's radiative and non-radiative decay rates in this resonant structure, resulting in a quantum efficiency of ∼90% for a dipole at a distance of r=60n m from the Au nanosphere surface. A 30-layer metal-SBR Tamm plasmon-like resonant supporting structure results in a Q up to ∼103. The Tamm plasmon-like mode is affected by the Bragg wavelength and the number of layers of the SBR, and the thickness of the spacer cavity layer. These results will open a new avenue for generating high-Q Tamm plasmon-like modes for switches, optical logic computing devices, and nonlinear applications.