Multipolar Decomposition Research Articles

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.

Anisotropic virtual gain and large tuning of particles’ scattering by complex-frequency excitations

Active tuning of the scattering of particles and metasurfaces is a highly sought-after property for a host of electromagnetic and photonic applications, but it normally requires challenging-to-control tunable (reconfigurable) or active (gain) media. Here, we introduce the concepts of anisotropic virtual gain and oblique Kerker effect, where a completely lossy anisotropic medium behaves exactly as its anisotropic gain counterpart upon excitation by a synthetic complex-frequency wave. The strategy allows one to largely tune the magnitude and angle of a particle’s scattering simply by changing the shape (envelope) of the incoming radiation, rather than by an involved medium-tuning mechanism. The so-attained anisotropic virtual gain enables directional super-scattering at an oblique direction with fine-management of the scattering angle. Our study is based on analytical techniques that allow multipolar decomposition of the scattered field in agreement with full-wave simulations, and lays the foundations for a light management method.

Metamaterial-induced-transparency engineering through quasi-bound states in the continuum by using dielectric cross-shaped trimers

This study presents a novel approach to activate a narrowband transparency line within a reflecting broadband window in all-dielectric metasurfaces, in analogy to the electromagnetically-induced transparency effect, by means of a quasi-bound state in the continuum (qBIC). We demonstrate that the resonance overlapping of a bright mode and a qBIC-based nearly-dark mode with distinct Q-factor can be fully governed by a silicon trimer-based unit cell with broken-inversion-symmetry cross shape, thus providing the required response under normal incidence of a linearly-polarized light. Our analysis that is derived from the far-field multipolar decomposition and near-field electromagnetic distributions uncovers the main contributions of different multipoles on the qBIC resonance, with governing magnetic dipole and electric quadrupole terms supplied by distinct parts of the dielectric “molecule”. The findings extracted from this research open up new avenues for the development of polarization-dependent technologies, with particular interest in its capabilities for sensing and biosensing.

Silicon nanoparticle-based near-infrared surface enhanced fluorescence without any “dielectric spacer”

For the first time, herein, we report the theoretical investigation on silicon (Si) nanoparticle-based near-infrared (NIR) surface-enhanced fluorescence (SEF). Initially, the scattering spectra of single spherical-shaped Si nanoparticles of different sizes are plotted and characterized all the observed modes using the multipolar decomposition method. Later, the electric field intensity enhancement (EFIE) distribution inside and outside Si nanoparticles is plotted at the wavelengths of electric and magnetic type modes. Finally, the SEF enhancement (χSEF) is estimated by varying the excitation wavelength (λex), fluorescence wavelength (λem), and separation (d) between the fluorophore and Si nanoparticle of different sizes. The χSEF is found to vary from 1 to 3 orders of magnitude when the λem falls in the NIR region. In contrast to the plasmonic or metal nanoparticle-based SEF, the maximum χSEF is observed when d = 0. This indicates that the thin dielectric spacers between the fluorophores and Si nanoparticles are not required to obtain the enhancement in the case of the Si nanoparticle-based SEF technique. This can be considered as a significant advantage over the conventional metal nanoparticle-based SEF, where dielectric spacers are mandatory. Finally, the average SEF enhancement ⟨χSEF⟩ is also estimated.

Multi-mode resonance of bound states in the continuum in dielectric metasurfaces

Bound states in the continuum (BIC) represent distinct non-radiative states endowed with infinite lifetime and vanishing resonance linewidth. Introducing asymmetric perturbation to the system can convert true BICs into high quality leaky modes which is useful in many photonic applications. Previously, such perturbation and resonance of interest is only limited to a single factor. However, different perturbations by unit cell gap, geometry and rotation angle result distinctive resonance modes. The combination of two perturbation factors can excite multi-mode resonance contributed from each asymmetric factor which coexist simultaneously; thus, the number of reflectance peaks can be controlled. In addition, we have carefully analyzed the electric field variations under different perturbation factors, followed by a multipolar decomposition of resonances to reveal underlying mechanisms of distinct resonance modes. Through simulations, we find that the introduction of multiple asymmetric perturbations also influences the metasurface sensitivity in refractive index sensing and compare the performance of different resonance modes. These observations provide structural design insights for achieving high quality resonance with multiple modes and ultra-sensitive sensing.

On-Axis Optical Trapping with Vortex Beams: The Role of the Multipolar Decomposition.

Optical trapping is a well-established, decades old technology with applications in several fields of research. The most common scenario deals with particles that tend to be centered on the brightest part of the optical trap. Consequently, the optical forces keep the particle away from the dark zones of the beam. However, this is not the case when a focused doughnut-shaped beam generates on-axis trapping. In this system, the particle is centered on the intensity minima of the laser beam and the bright annular part lies on the periphery of the particle. Researchers have shown great interest in this phenomenon due to its advantage of reducing light interaction with trapped particles and the intriguing increase in the trapping strength. This work presents experimental and theoretical results that extend the analysis of on-axis trapping with light vortex beams. Specifically, in our experiments, we trap micron-sized spherical silica (SiO2) particles in water and we measure, through the power spectrum density method, the trap stiffness constant κ generated by vortex beams with different topological charge orders. The optical forces are calculated from the exact solutions of the electromagnetic fields provided by the generalized Lorentz-Mie theory. We show a remarkable agreement between the theoretical prediction and the experimental measurements of κ. Moreover, our numerical model gives us information about the electromagnetic fields inside the particle, offering valuable insights into the influence of the electromagnetic fields present in the vortex beam trapping scenario.

Bound states in continuum domain of double resonant ring metal metasurfaces

Metasurfaces have found extensive applications in microwave, terahertz, and optical range, serving different purposes such as filters, sensors, slow light devices, and nonlinear devices due to their distinctive electromagnetic response characteristics. Recent development requires metasurface devices to exhibit enhanced monochromaticity and stronger light interaction. Consequently, there is a growing interest in designing metasurfaces with high-quality factor (<i>Q</i>-factor) resonances, considering their crucial role in achieving sharp resonances through constructing bound states in the continuum (BIC) mode. The utilization of BIC has emerged as a prominent method of designing metasurfaces with high <i>Q</i>-factor resonances. Due to the fact that the changes in the structural parameters of metasurfaces can simultaneously affect the resonance of two components of q-BICs, it is difficult to achieve on-demand design of operating frequency, bandwidth, and <i>Q</i>-factor. In this work, we investigate a novel THz metasurfaces supporting q-BIC resonance. We optimize the geometric parameters of two split ring resonators (SRRs) to tailor the operating frequencies of intrinsic resonance, and tune the coupling between different resonance modes to form the q-BIC mode resonance. The dominant modes are demonstrated by the results of multipolar decomposition calculations of the electromagnetic field distributions and scattered power at different resonant operating frequencies. In <i>x</i>-polarized and <i>y</i>-polarized incident electromagnetic wave, the normalized coupling strength ratio between the two modes are calculated by Jaynes-Cummings model to be 0.54% (<i>x-</i>polarized) and 4.42% (<i>y-</i>polarized) respectively, which explains the law that the resonant frequency of different modes changes with the structural parameters of SRRs device. In order to analyze the refractive index sensing capabilities of our designed metasurfaces under the incident electromagnetic waves with different polarizations, we investigate the variations of the transmitted spectrum of the metasurface with refractive index of matters. The calculated results show that the sensitivity of the metasurface is 151 GHz/RIU when the incident wave is <i>y</i>-polarized and 108 GHz/RIU when the incident wave is <i>x</i>-polarized. We realize the effective control of the operating frequency, bandwidth, and <i>Q</i>-factor of the q-BIC mode resonance in the transmission spectrum of the metasurface, which provides a new idea for the practical designing of terahertz metasurfaces with high <i>Q</i>-factor.

Anisotropic medium sensing controlled by bound states in the continuum in polarization-independent metasurfaces.

Bound states in the continuum (BICs) with infinite quality factor (Q-factor) and significant field enhancement pave the way for realizing highly sensitive optical sensors with enhanced light-matter interactions on the nanoscale. However, current optical sensing methods are difficult to discriminate between isotropic and anisotropic media from resonance spectral lines, resulting in optical sensing methods still being limited to isotropic media. In this work, we demonstrate that BICs can be realized by modulating the period of structural units to convert BICs to QBICs without changing their space group symmetry, and propose a polarization-independent metasurfaces-based realization of highly sensitive refractive index sensors for isotropic and anisotropic media as well as discrimination. We propose a metasurface of tetrameric silicon nanoboxes with C4 symmetry as structural units to achieve the conversion of BICs to QBICs by modulating the period of structural units without changing the geometry of the structure. Two QBICs modes dominated by electric toroidal dipole and magnetic toroidal dipole are identified by multipolar decomposition and electromagnetic distribution calculations. Meanwhile, we realize the refractive index detection and resolution of isotropic and anisotropic media based on polarization-independent metasurfaces combined with isotropic and anisotropic media layers. Our work provides what we believe to be a new method for realizing the fast resolution and refractive index optical sensing of isotropic and anisotropic media, and offers new ideas for the design and application of polarization-independent metasurfaces.

Polarization-independent bound state in the continuum without the help of rotational symmetry.

Recently, research about bound states in the continuum (BICs) has become more and more attractive. Nanostructures with rotational symmetry are usually utilized to realize polarization-independent quasi-BIC resonances. Here, we propose a new, to the best of our knowledge, scheme for a polarization-independent quasi-BIC without the help of rotational symmetry. With the rotation of the polarization direction of the incident light, a quasi-BIC resonance can be consistently observed in a dielectric cubic tetramer metasurface without rotational symmetry. Based on far-field multipolar decomposition and near-field electromagnetic distributions, it is found that different multipoles exhibit different dependences on the polarization direction, and the switch between electric and magnetic quadrupoles results in polarization-independent quasi-BIC resonance. Our findings provide an alternative scheme to design polarization-independent devices and promote wider potential applications.

All-dielectric asymmetric planar metasurface based dual-parametric sensor

All-dielectric metasurfaces are a rapidly growing area of research because they have lower ohmic losses and sharper resonances than their metallic equivalents. In this work, we have reported a novel all-dielectric metasurface design for dual parameter (refractive index and temperature) sensing applications. The design consists of a square GaAs block on the top of a quartz substrate with a square hole etched on it. An outer cut to the GaAs block is introduced to break the symmetry. The asymmetry enables a sharp Fano resonance mode with 90% spectral contrast and extremely high quality (Q) factor of value 2.8×104 in the near-infrared (NIR) window. The multipolar nature of the high-Q resonance is studied using the cartesian multipolar decomposition technique, which reveals the electric dipolar nature of the resonance. To examine the refractive index capability of the proposed device, we varied the refractive index of the background medium and found that the device is highly sensitive and results in a very high Figure of Merit (FoM) 5384 RIU−1, owing to the very narrow linewidth of the resonance. The same design is also used for temperature sensing, which shows a notable improvement in sensitivity (72.4 pm/∘C) compared to various existing methods available. The in-depth analysis of the resonance in an optimized all-dielectric metasurface and its potential for sensing presented through this paper can be significant in designing high-performance optical sensors.

Dual symmetry breaking tunable bound states in the continuum in all dielectric split ring metamaterials

Dual-symmetry breakings including permittivity asymmetry and geometry asymmetry have been studied in all dielectric split-ring metamaterials supported bound states in the continuum. For any single symmetry breaking, the proposed metamaterials can support simultaneously dual quasi-bound states in the continuum (quasi-BICs). Multipolar decomposition reveals that dual quasi-BICs induced by permittivity asymmetry are both governed by magnetic dipole and electric quadrupole, whereas dual quasi-BICs induced by geometry asymmetry are governed by magnetic dipoles. Under the combined effect of the two types of symmetry breaking, it is found that the quasi-BIC can be weaken and vanished, which is different from enhanced quasi-BIC effect induced by a single symmetry breaking. In addition, asymmetric magnetic and electric field distributions can be realized by selecting different type of symmetry breaking, providing a new way of indirectly manipulating the localized magnetic fields. We show that dual-symmetry breakings point to a unique routine to analyze the physical mechanism of quasi-BICs, which furthermore provide a useful insight into their tuning behavior.

Tunable terahertz bound state in the continuum in graphene metagrating

Recently, high quality (Q) factor resonances governed by quasi-bound state in the continuum (BIC) have attracted increasing attention due to the fascinating electromagnetic resonance properties that provide a new guideline for the design of metasurfaces. In this paper, we propose a graphene metagrating that can support quasi-BIC by introducing periodic perturbations. We discuss the transmission spectrum at normal and oblique incidence. Resonances are analyzed by the multipolar decomposition of radiative power and the electric field distribution to clarify the mechanism of quasi-BIC. The amplitude of quasi-BIC Fano resonance can be tuned by adjusting the chemical potential of graphene, and the modulation amplitude can reach 74%. Furthermore, the relationship between the Q factor of quasi-BIC and the graphene chemical potential is thoroughly investigated. The Q factor of quasi-BIC resonance can achieve linear tunability within a specific range of chemical potential. The further evolution and comparison of the proposed unit cell show that our method of introducing perturbation is universal to some extent. Altogether, the proposed metagrating can be used as an excellent active THz modulator and has potential applications in biochemical sensing, laser and nonlinear optics.

Polarization-selective narrow band dual-toroidal-dipole resonances in a symmetry-broken dielectric tetramer metamaterial.

Here we propose a metasurface consisting of symmetry-broken dielectric tetramer arrays, which can generate polarization-selective dual-band toroidal dipole resonances (TDR) with ultra-narrow linewidth in the near-infrared region. We found, by breaking the C4v symmetry of the tetramer arrays, two narrow-band TDRs can be created with the linewidth reaching ∼ 1.5 nm. Multipolar decomposition of scattering power and electromagnetic field distribution calculations confirm the nature of TDRs. A 100% modulation depth in light absorption and selective field confinement has been demonstrated theoretically by simply changing the polarization orientation of the exciting light. Intriguingly, it is also found that absorption responses of TDRs on polarization angle follow the equation of Malus' law in this metasurface. Furthermore, the dual-band toroidal resonances are proposed to sense the birefringence of an anisotropic medium. Such polarization-tunable dual toroidal dipole resonances with ultra-narrow bandwidth offered by this structure may find potential applications in optical switching, storage, polarization detection, and light emitting devices.

Employing Topological Properties of Phase Singularities for Designing Supercavity Systems with Tailored Excitation

AbstractTrapping electromagnetic waves in high‐index resonators with extreme quality factors (Q‐factor) can be achieved by exciting supercavity (quasi‐bound states in the continuum, quasi‐BIC) modes with minimal radiation loss. Conventionally, the quasi‐BICs are achieved by tuning the resonator dimensions to promote destructive interference between two modes in the cavity. In this work, an enhanced Q‐factor is achieved by manipulating the structure of a near‐field excitation source and tailoring the excitation strength of individual multipolar modes. A pair of concentric rings with strong mutual coupling is used in contrast to a simple single‐ring case. We show that adding the second passive ring enables additional degrees of freedom to manipulate the multipolar content exited in the cavity. The multipole decomposition of the cavity modes is performed using full‐wave numerical simulations to support these claims. Experimental validation with a double‐ring fed supercavity demonstrates the highest measured Q‐factor of ≈310 or ≈2× the quality factor of a design without supercavity, which is consistent with a numerical model predicting up to 3× enhanced Q‐factor. We show that, analyses of polarization singularities and multipolar decomposition can be used to understand, characterize, and design such systems, employing their topological properties not broadly explored before.

Ultra-high Q resonances governed by quasi-bound states in the continuum in all-dielectric THz metamaterials

Bound states in the continuum (BICs) have received increasing attention due to their ultra-sharp resonance in the field of ultra-sensitive sensing and trace amount detection in the terahertz (THz) regime. In this paper, we propose a new asymmetric all-dielectric THz metamaterial and show that quasi-BICs result in four ultra-high quality (Q) factor resonances observed in the transmission spectrum. The Q factors of resonances can be achieved up to 252248. Electromagnetic field distribution and multipolar decomposition of scattering power are performed to clarify the physical mechanism of quasi-BICs resonances. In addition, the spectra response to the analytes exhibited the ability of the proposed device as an ultra-sensitive sensor, and the maximum sensitivity is 482 GHz/RIU. We further find that the sensitivity of the metamaterial is proportional to the negative quadratic of the gap between structures and explain it by the area of the field enhancement in the metamaterial. Our work paves a novel way to design ultra-high Q resonance metamaterials and ultra-sensitive sensors at THz frequencies.

Tailoring bound states in the continuum in symmetric photonic crystal slabs by coupling strengths.

In this work, we investigate polarization-insensitive dual bound states in the continuum (BICs) at Γ point in symmetric photonic crystal (PhC) slabs. Especially, BICs are tailored by tuning intra- and intercellular optical coupling strengths of PhC slabs. Based on four different approaches, we realize the transition from BIC to quasi-BIC resonances with various dispersion behaviors while maintaining the symmetry of slabs. Also, we show the two resonances are lowest-order even and odd eigenmodes that can match the symmetry of the incident plane wave, and their quality (Q) factors follow the inverse quadratic law except for cases with larger perturbations. Furthermore, multipolar decomposition reveals that even quasi-BICs are dominated by the toroidal dipole and magnetic quadrupole, while odd quasi-BICs are governed by the magnetic dipole and electric quadrupole. Interestingly, an anomalous increase of the Q factor is observed in one case, which is attributed to the mode transformation. Finally, anisotropic coupling adjustment is discussed, which enriches the degrees of freedom to manipulate BICs. This work introduces a novel perspective to tailor BICs at Γ point in PhC slabs and has potential planar photonic applications for nonlinear enhancement and sensing.

Dual-band polarization-insensitive toroidal dipole quasi-bound states in the continuum in a permittivity-asymmetric all-dielectric meta-surface.

Ultra-high quality (Q) factor resonances derived from the bound states in the continuum (BICs) have drawn much attention in optics and photonics. Especially in meta-surfaces, they can enable ultrasensitive sensors, spectral filtering, and lasers because of their enhanced light-matter interactions and rare superiority of scalability. In this paper, we propose a permittivity-asymmetric all-dielectric meta-surface, comprising high-index cuboid tetramer clusters with symmetric structural parameters and configuring periodically on a glass substrate. Simulation results offer dual-band quasi-BICs with high Q values of 4447 and 11391, respectively. Multipolar decomposition in cartesian and electromagnetic distributions are engaged to analyze the physical mechanism of dual quasi-BIC modes, which reveals that they are both governed by magnetic quadrupole (MQ) and in-plane toroidal dipole (TD). The polarization-insensitive and scalable characteristics are also investigated. Additionally, we appraise the sensing performances of the proposed structure. As an example, our work supports an uncommon route to design dual-band polarization-insensitive TD quasi-BICs resonators and facilitates their applications in optic and photonics, such as low-threshold lasers and sensing.

Total angular momenta quantization of dielectric sphere modes

Spherical particles both dielectric and metallic are essential building blocks in nanophotonics. During the recent rapid development of Mie-tronic — nanophotonics devices heavily using various features of the Mie-resonances — the deep fundamental investigation of the eigenmodes of such particles by using the novel tools is still relevant and currently important. Moreover, eigenmodes of a sphere are closely related to the Vector Spherical Harmonics (VSH) which are widely used in the multipolar decomposition to analyze less symmetric structures. In this work, we study in detail the canonical spin and angular momenta (AM), helicity, and other properties of the eigenmodes of dielectric (nondispersive) and metallic (dispersive) spheres. We show that the canonical momentum density of the AM is quantized and has a close relation to the quantum picture of a single photon. Our work provides a solid platform for future studies and applications of the AM transfer from near fields of spherical particles to the matter in its vicinity.

Symmetric metasurface with dual band polarization-independent high-Q resonances governed by symmetry-protected BIC.

In this Letter, we propose a symmetric metasurface composed of single-sized amorphous-silicon (a-Si) cuboids tetramer clusters that support two resonances with opposite symmetry, i.e., in-plane magnetic dipole (MD) resonance and in-plane toroidal dipole (TD) resonance governed by symmetry-protected bound states in the continuum (SP-BIC) in the near-infrared region. Since the cuboids tetramer of the metasurface retains C4v symmetry and mirror symmetry, both resonances are polarization independent. Multipolar decomposition of scattering power and electromagnetic distribution are performed to clarify the physical mechanism of dual quasi-BIC resonances. The effects of geometric parameters on both high-quality (Q) resonances are also studied. Additionally, the sensing performance of the designed metasurface is evaluated. The effects of the material's loss on both resonances are also studied. Our work provides a new route to designing dual mode polarization- independent resonators without multi-sized complex structures that may facilitate designing high-performance sensing applications.

Giant chiro-optical responses in multipolar-resonances-based single-layer dielectric metasurfaces

Chiro-optical effects offer a wide range of potential applications in nanophotonics, such as advanced imaging and molecular sensing and separation. Flat single-layer metasurfaces composed of subwavelength meta-atoms have gained significant attention due to their exceptional characteristics in light–matter interactions. Although metasurface-based devices have manipulated electromagnetic waves, the compact on-chip realization of giant chiro-optical effects remains a challenge at optical frequencies. In this work, we experimentally and numerically demonstrate an all-dielectric metasurface to realize large chiro-optical effects in the visible regime. Notably, the proposed strategy of utilizing achiral nanofins instead of conventional chiral structures provides an extra degree of design freedom. The mutual coupling between carefully engineered nanofins produces constructive and destructive interference, leading to the asymmetric transmission of 70% and average circular dichroism exceeding 60%. We investigate the underlying mechanism behind the chiro-optical effects using the theory of multipolar decomposition. The proposed design mechanism maximizes the chiro-optical response through a single-layer metasurface with potential applications in high-efficiency integrated ultrathin polarization rotators and shapers, chiral polarizers for optical displays, chiral beam splitters, and chiral sensors.