Anapole Excitation Research Articles

Advances in hybrid strategies for enhanced photocatalytic water splitting: Bridging conventional and emerging methods

Significant efforts have been dedicated to hydrogen production through photocatalytic water splitting (PWS) over the past five decades. However, achieving commercially viable solar-to-hydrogen conversion efficiency in PWS systems remains elusive. These systems face intrinsic and extrinsic challenges, such as inadequate light absorption, insufficient charge separation, limited redox active sites, low surface area, and scalability issues in practical designs. To address these issues, conventional strategies including heterojunction engineering, plasmonics, hybridization, lattice defects, sensitization, and upconversion processes have been extensively employed. More recently, innovative hybrid strategies like photonic crystal-assisted and polarization field-assisted PWS have emerged, which improve light absorption and charge separation by harnessing the slow photon effect, multiple light scattering, and the piezoelectric, pyroelectric, and ferroelectric properties of materials. This review article aims to provide a comprehensive examination and summary of these new synergistic hybrid approaches, integrating plasmonic effects, upconversion processes, and photonic crystal photocatalysis. It also explores the role of temperature in suppressing exciton recombination during photothermic photocatalysis. This article also highlights emerging strategies such as the effects of magnetic fields, periodic illumination, many-body large-hole polaron, and anapole excitations, which hold significant potential to advance PWS technology and facilitate renewable hydrogen generation.

Polarization‐Enabled Tuning of Anapole Resonances in Vertically Stacked Elliptical Silicon Nanodisks

This work presents the polarization‐dependent behavior of the anapole state in stacked amorphous silicon (a‐Si) nanodisks with elliptical geometries. Using SiO2 as a spacer layer between the a‐Si disks, the high index contrast between these materials can be used to significantly reduce the fabrication complexity of the system compared to traditional methods that require additional etching of the spacers. A novel way of continuous tuning of the electric dipole anapole excitation within elliptical stacked a‐Si nanoresonators is demonstrated. By rotating the incident electric field's polarization angle, the anapole state can be selectively excited at two distinct wavelength positions separated by 80 nm. Experimental results show characteristic dips in the reflectance of the fabricated elliptical a‐Si stacks with wavelength positions between 1135 and 1217 nm depending on the polarization angle of the incident field which is corroborated by FDTD simulations. Through simulating the internal electric field in the resonators and using multipole decomposition, it is shown that the reflectance dips are due to anapole excitation in the individual disks. The capability to excite anapoles at two distinct wavelengths in the same structure has promising implications for the development of tunable sensors, frequency converters, and quantum memory applications.

Wide angle anapole excitation in stacked resonators.

In the search for resonances with high localized field strengths in all-dielectric nanophotonics, novel states such as anapoles, hybrid anapoles and bound states in the continuum have been realized. Of these, the anapoles are the most readily achievable. Interaction between vertically stacked disks supporting anapole resonances increases the field localization further. When fabricated from materials with high non-linear coefficients, such stacked disk pillars can be used as non-linear antennas. The excitation of such 3D pillars often includes off normal incidence when using focusing optics. Therefore, it is important to evaluate the angular and polarization response of such pillars. In the paper we fabricate pillars with three AlGaAs disks in a stack separated by stems of GaAs. The angular and polarization responses are evaluated experimentally with integrating sphere measurements and numerically through simulation, multipole decomposition and quasi-normal modes. We find that the stacked geometry shows hybridized anapole excitation for a broad span of incidence angles, with tunability of the individual multipolar response up to octupoles, including an electric octupole anapole, and we show how the average enhanced confined energy varies under angled excitation. The results show that the vertical stacked geometry can be used with highly focusing optics for efficient in-coupling to the hybridized anapole.

Enhanced Thermo-optical Response by Means of Anapole Excitation.

High refractive index (HRI) dielectric nanostructures offer a versatile platform to control the light–matter interaction at the nanoscale as they can easily support electric and magnetic modes with low losses. An additional property that makes them extraordinary is that they can support low radiative modes, so-called anapole modes. In this work, we propose a spectrally tunable anapole nanoheater based on the use of a dielectric anapole resonator. We show that a gold ring nanostructure, a priori nonresonant, can be turned into a resonant unit by just filling its hole with an HRI material supporting anapole modes, resulting in a more efficient nanoheater able to amplify the photothermal response of the bare nanoring. As proof of concept, we perform a detailed study of the thermoplasmonic response of a gold nanoring used as heating source and a silicon disk, designed to support anapole modes, located in its center acting as an anapolar resonator. Furthermore, we utilize the anapole excitation to easily shift the thermal response of these structures from the shortwave infrared range to the near-infrared range.

Anapole due to a tangentially polarized dipole near a subwavelength sphere: Inhibition and enhancement of spontaneous emission

There exist oscillating current distributions that cannot generate radiation; these configurations are known as anapoles. Our goal is to theoretically demonstrate that an oscillating electric dipole, located in the vicinity of a subwavelength sphere and polarized tangentially, can yield an anapole. This paper complements our recent study [J. R. Zurita-S\'anchez, Phys. Rev. Research 1, 033064 (2019)], devoted to an anapole arising from an electric dipole with radial polarization. Our methodology relies on the multipolar expansion of the excitation and scattered electromagnetic fields with respect to the center of the spherical particle. We found that an anapole arises when the effective electric and magnetic dipoles, induced by the dipolar source, vanish. The aforementioned anapole condition considers that the lowest multipolar order contribution is responsible for the radiative response of the emitter. However, high-order multipoles yield an unavoidable residual radiated power which is much smaller still than the radiated power of the dipole in the absence of the scatterer. Particularly, for a nonabsorbing sphere, the geometrical and dielectromagnetic parameters that lead to an anapole state are determined. As a practical scenario, we exploit the anapole excitation for controlling the spontaneous emission decay rate of an excited atom (or a molecule). In comparison to the decay rate of an excited atom without the scatterer, the radiative decay in the presence of the particle can be inhibited by a factor of about three orders of magnitude; this factor is quite large for an open system like ours. Furthermore, we show that by varying the orientation of the atomic transition dipole moment from a tangential direction, where the anapole condition is satisfied, to a radial direction, the atom can change from a state in which the spontaneous emission rate is inhibited to one in which this rate is enhanced. Hence, the anapole setup enables the versatile control of the radiative properties of an atom, and it provides the possibility for implementing applications related to the sensing and localization of atoms.

Metasurface Photoelectrodes for Enhanced Solar Fuel Generation

AbstractTailoring optical properties in photocatalysts by nanostructuring them can help increase solar light harvesting efficiencies in a wide range of materials. Whereas plasmon resonances are widely employed in metallic catalysts for this purpose, latest advances of nonradiative, dielectric nanophotonics also enable light confinement and enhanced visible light absorption in semiconductors. Here, a design procedure for large‐scale nanofabrication of semiconductor photoelectrodes using imprint lithography is developed. Anapole excitations and metasurface lattice resonances are combined to enhance the absorption of the model material, amorphous gallium phosphide (a‐GaP), over the visible spectrum. It is shown that cost‐effective, high sample throughput is achieved while retaining the precise signature of the engineered photonic states. Photoelectrochemical measurements under hydrogen evolution reaction conditions and sunlight illumination reveal the contributions of the respective resonances and demonstrate an overall photocurrent enhancement of 5.7, compared to a planar film. These results are supported by optical and numerical analysis of single nanodisks and of the upscaled metasurface.

Engineering gallium phosphide nanostructures for efficient nonlinear photonics and enhanced spectroscopies

Abstract Optical resonances arising from quasi-bound states in the continuum (QBICs) have been recently identified in nanostructured dielectrics, showing ultrahigh quality factors accompanied by very large electromagnetic field enhancements. In this work, we design a periodic array of gallium phosphide (GaP) elliptical cylinders supporting, concurrently, three spectrally separated QBIC resonances with in-plane magnetic dipole, out-of-plane magnetic dipole, and electric quadrupole characters. We numerically explore this system for second-harmonic generation and degenerate four-wave mixing, demonstrating giant per unit cell conversion efficiencies of up to ∼ 2 W−1 and ∼ 60 W−2, respectively, when considering realistic introduced asymmetries in the metasurface, compatible with current fabrication limitations. We find that this configuration outperforms by up to more than four orders of magnitude the response of low-Q Mie or anapole resonances in individual GaP nanoantennas with engineered nonlinear mode-matching conditions. Benefiting from the straight-oriented electric field of one of the examined high-Q resonances, we further propose a novel nanocavity design for enhanced spectroscopies by slotting the meta-atoms of the periodic array. We discover that the optical cavity sustains high-intensity fields homogeneously distributed inside the slot, delivering its best performance when the elliptical cylinders are cut from end to end forming a gap, which represents a convenient model for experimental investigations. When placing an electric point dipole inside the added aperture, we find that the metasurface offers ultrahigh radiative enhancements, exceeding the previously reported slotted dielectric nanodisk at the anapole excitation by more than two orders of magnitude.

Polarization-independent anapole response of a trimer-based dielectric metasurface

Abstract The phenomenon of anapole has attracted considerable attention in the field of metamaterials as a possible realization of radiationless objects. We comprehensively study this phenomenon in the cluster-based systems of dielectric particles by considering conditions of anapole manifestation in both single trimers of disk-shaped particles and metamaterial composed on such trimers. Our analytical approach is based on the multipole decomposition method and the secondary multipole decomposition technique. They allow us to associate the anapole with the multipole moments of the trimer and the separate multipole moments of its constitutive particles. The manifestation of anapole in a two-dimensional metamaterial (metasurface) is confirmed by checking the resonant states in the reflected field as well as from the electromagnetic near-field patterns obtained from the full-wave numerical simulation. It is demonstrated that the anapole excitation in trimers results in the polarization-independent suppression of reflection with the resonant enhancement of local electromagnetic fields in the metasurface. Finally, experimental verification of the theoretical results is presented and discussed.

Terahertz Microfluidic Sensing with Dual‐Torus Toroidal Metasurfaces

AbstractToroidal excitation manifests as currents flowing on the surface of a torus along its meridians that are known as poloidal currents. Several intriguing phenomena such as nonradiating anapole excitation, dynamic Aharonov–Bohm effect, and vector potential in the absence of electromagnetic fields occur in the presence of toroidal excitations. Toroidal resonances in metamaterials also offer a platform to probe strong light–matter interactions in its proximity due to the nonradiating confinement of photons. Here, a dual‐torus toroidal flexible metasurface containing an array of overlapped split ring resonators is experimentally demonstrated, which results in strong field confinement for enhanced sensing of polar liquid analytes. Microfluidic integrated dual‐torus toroidal resonance experimentally shows significantly higher sensitivity of 124.3 GHz per refractive index unit compared to a single torus toroidal resonance. Flexible metasurfaces supporting dual torus toroidal resonance can find a wide range of applications in the development of ultrasensitive terahertz molecular sensors for probing real time monitoring of chemicals and biomolecules.

Anapole-Assisted Absorption Engineering in Arrays of Coupled Amorphous Gallium Phosphide Nanodisks

Broadband solar light harvesting plays a crucial role for efficient energy conversion. Anapole excitations and associated absorption engineering in dielectric nanoresonators are a focus of nanophot...

Measures of space-time nonseparability of electromagnetic pulses

Electromagnetic pulses are typically treated as space-time (or space-frequency) separable solutions of Maxwell's equations, where spatial and temporal (spectral) dependence can be treated separately. In contrast to this traditional viewpoint, recent advances in structured light and topological optics have highlighted the non-trivial wave-matter interactions of pulses with complex topology and space-time non-separable structure, as well as their potential for energy and information transfer. A characteristic example of such a pulse is the "Flying Doughnut" (FD), a space-time non-separable toroidal few-cycle pulse with links to toroidal and non-radiating (anapole) excitations in matter. Here, we propose a quantum-mechanics-inspired methodology for the characterization of space-time non-separability in structured pulses. In analogy to the non-separability of entangled quantum systems, we introduce the concept of space-spectrum entangled states to describe the space-time non-separability of classical electromagnetic pulses and develop a method to reconstruct the corresponding density matrix by state tomography. We apply our method to the FD pulse and obtain the corresponding fidelity, concurrence, and entanglement of formation. We demonstrate that such properties dug out from quantum mechanics quantitatively characterize the evolution of the general spatiotemporal structured pulse upon propagation.

Efficient ultrafast all-optical modulation in a nonlinear crystalline gallium phosphide nanodisk at the anapole excitation.

High-refractive index nanostructured dielectrics have the ability to locally enhance electromagnetic fields with low losses while presenting high third-order nonlinearities. In this work, we exploit these characteristics to achieve efficient ultrafast all-optical modulation in a crystalline gallium phosphide (GaP) nanoantenna through the optical Kerr effect (OKE) and two-photon absorption (TPA) in the visible/near-infrared range. We show that an individual GaP nanodisk can yield differential reflectivity modulations of up to ~40%, with characteristic modulation times between 14 and 66 fs, when probed at the anapole excitation (AE). Numerical simulations reveal that the AE represents a unique condition where both the OKE and TPA contribute with the same modulation sign, maximizing the response. These findings highly outperform previous reports on sub-100-fs all-optical switching from resonant nanoscale dielectrics, which have demonstrated modulation depths no larger than 0.5%, placing GaP nanoantennas as a promising choice for ultrafast all-optical modulation at the nanometer scale.

Radiative Toroidal Dipole and Anapole Excitations in Collectively Responding Arrays of Atoms

A toroidal dipole represents an often overlooked electromagnetic excitation distinct from the standard electric and magnetic multipole expansion. We show how a simple arrangement of strongly radiatively coupled atoms can be used to synthesize a toroidal dipole where the toroidal topology is generated by radiative transitions forming an effective poloidal electric current wound around a torus. We extend the protocol for methods to prepare a delocalized collective excitation mode consisting of a synthetic lattice of such toroidal dipoles and a nonradiating, yet oscillating charge-current configuration, dynamic anapole, for which the far-field radiation of a toroidal dipole is identically canceled by an electric dipole.

Optical anapole mode in nanostructured lithium niobate for enhancing second harmonic generation

Abstract Second harmonic generation (SHG) with a material of large transparency is an attractive way of generating coherent light sources at exotic wavelength range such as VUV, UV and visible light. It is of critical importance to improve nonlinear conversion efficiency in order to find practical applications in quantum light source and high resolution nonlinear microscopy, etc. Here an enhanced SHG with conversion efficiency up to 10−2% at SH wavelength of 282.7 nm under 11 GW/cm2 pump intensity via the excitation of anapole in lithium niobite (LiNbO3, or LN) nanodisk through the dominating d 33 nonlinear coefficient is investigated. The anapole has advantages of strongly suppressing far-field scattering and well-confined internal field which helps to boost the nonlinear conversion. Anapoles in LN nanodisk is facilitated by high index contrast between LN and substrate with properties of near-zero-index via hyperbolic metamaterial structure design. By tailoring the multi-layers structure of hyperbolic metamaterials, the anapole excitation wavelength can be tuned at different wavelengths. It indicates that an enhanced SHG can be achieved at a wide range of pump light wavelengths via different design of the epsilon-near-zero (ENZ) hyperbolic metamaterials substrates. The proposed nanostructure in this work might hold significances for the enhanced light–matter interactions at the nanoscale such as integrated optics.

Dielectric slotted nanodisk laser with ultralow pump threshold by anapole excitation

Developing ultralow threshold nanolasers is a challenging task despite of their great potential for a variety of applications. This work proposes to combine the advantages of low loss of dielectric materials and tight confinement of field in slotted dielectric nanodisks with gain medium embedded to them and supported by metallic substrate, which generates anapole mode with a lifetime longer than for dielectric substrate. Since the dielectric nanocavity has intrinsically low loss, one can achieve lasing with ultralow threshold down to 0.02 pJ for a spot size 1.5 μm of pump, several orders-of-magnitude lower than the preceding records. Dielectric slotted disk nanolasers proposed here can find broad applications for biosensing, near-field spectroscopy, and many other fields.

Tunable Terahertz Metamaterials Based on Anapole Excitation with Graphene for Reconfigurable Sensors

Anapole mode is excited by the destructive interference between the toroidal- and electric-dipole moments, resulting in a high-quality (Q) factor. In this paper, we designed a high-Q-factor metamat...

Excitation of Nonradiating Anapoles in Dielectric Nanospheres.

Although the study of nonradiating anapoles has long been part of fundamental physics, the dynamic anapole at optical frequencies was only recently experimentally demonstrated in a specialized silicon nanodisk structure. We report excitation of the electrodynamic anapole state in isotropic silicon nanospheres using radially polarized beam illumination. The superposition of equal and out-of-phase amplitudes of the Cartesian electric and toroidal dipoles produces a pronounced dip in the scattering spectra with the scattering intensity almost reaching zero-a signature of anapole excitation. The total scattering intensity associated with the anapole excitation is found to be more than 10 times weaker for illumination with radially vs linearly polarized beams. Our approach provides a simple, straightforward alternative path to realizing nonradiating anapole states at the optical frequencies.

Toroidal metasurfaces in a 2D flatland

A toroidal dipole is a new class of electromagnetic excitations and are different from traditional electric and magnetic dipoles. Toroidal dipoles are described by the poloidal currents flowing on the surface of torus and have opened a new route to control radiative losses via near field coupling mechanism or radiation cancellation approach in the unit cell of metasurface. Radiative loss engineering in metamaterials is one of the most fundamental requirements to gauge the suitability of a metaphotonic device for a specific on-demand application. Here, we discuss strategies to excite toroidal dipolar modes in a planar metasurface which were initially thought to exist only in three-dimensional (3D) arrangements. Two dimensional (2D) toroidal metasurfaces are conceptual simplification of 3D toroid configurations, which pose fabrication challenges at micro-nanoscales. We further discuss the destructive interference between electric and toroidal dipoles to realize non-radiating modes in the form of an anapole excitation that fulfills the requirement for the excitation of extremely large quality factor resonances. Overall, the intriguing features of a toroidal dipole could have significant implications on the design of resonant metamaterials that are important for the development of low-loss sensors, modulators, filters, and efficient cavities for strong light matter interactions.

Anapole Excitations in Oxygen-Vacancy-Rich TiO2-x Nanoresonators: Tuning the Absorption for Photocatalysis in the Visible Spectrum.

Research on optically resonant dielectric nanostructures has accelerated the development of photonic applications, driven by their ability to strongly confine light on the nanoscale. However, as dielectric resonators are typically operated below their band gap to minimize optical losses, the usage of dielectric nanoantenna concepts for absorption enhancement has largely remained unexplored. In this work, we realize engineered nanoantennas composed of photocatalytic dielectrics and demonstrate increased light-harvesting capabilities in otherwise weakly absorptive spectral regions. In particular, we employ anapole excitations, which are known for their strong light confinement, in nanodisks of oxygen-vacancy-rich TiO2-x, a prominent photocatalyst that provides a powerful platform for exploring concepts in absorption enhancement in tunable nanostructures. The arising photocatalytic effect is monitored on the single particle level using the well-established photocatalytic silver reduction reaction on TiO2. With the freedom of changing the optical properties of TiO2 through tuning the abundance of VO states, we discuss the interplay between cavity damping and the anapole-assisted field confinement for absorption enhancement. This concept is general and can be extended to other catalytic materials with higher refractive indices.

Broken symmetry theta-shaped dielectric arrays for a high Q-factor Fano resonance with anapole excitation and magnetic field tunability

We have designed and numerically analyzed the theta-shaped dielectric arrays based on a single resonator per unit cell and generated non-radiative anapole resonance with an enhanced Q-factor. Relying on breaking the symmetry, it is shown that instead of leading to additional radiation losses, the Q-factor in theta-shaped Si arrays is enlarged about one order larger than that of the perfect disk arrays. And the magnetic near-field enhancements can be extended outside of the structures and are extremely enlarged. Further, the asymmetrirc magnetic field distributions can be observed by changing the nanorod position, providing a new way of indirectly manipulating the localized magnetic fields. The high Q-factor and strong magnetic near-field enhancements outside of the structures can be easily tailored by adjusting different geometric parameters and are achieved simultaneously, which furthermore provides a useful insight into their tuning behavior.