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Anapole states are fascinating for providing the seemingly contradictory properties of near-field enhancement and negligible scattering. These states are desirable to reduce the cross-coupling between meta-atoms in metamaterials. In dielectrics, anapoles are obtainable in simple but relatively large nanostructures. In metals, they are reported in composite designs that may not be appropriate as metamaterials' building blocks. Here, whether the anapole effect is obtainable in single and compact plasmonic nanostructures is investigated. Planar designs of plasmonic meta-atoms are presented and the anapole formation is explained using various multipole decomposition techniques. Due to the small nanostructure sizes compared to the wavelength, the toroidal dipole is small and does not play a principal role in the anapole formation. Therefore, the anapole can be qualitatively explained via destructive interference between quasi-static electric dipoles associated with complementary subvolumes of the nanostructure. The anapole meta-atom concept is tested by numerically demonstrating nearly transparent metasurfaces and metamaterials with high densities of near-field hot-spots. Moreover, strategies are discussed to tune the anapole state via polarization rotation, modification of the hosting domain, or re-design of the meta-atoms. Anapole plasmonic meta-atoms can enable metamaterials with combined local field concentration and controllable transparency for applications in nonlinear and tunable nanophotonics.

This article presents a density-based topology optimization scheme for locally optimizing the electric power dissipation in nanostructures made of lossy dispersive materials. We use the complex-conjugate pole-residue (CCPR) model, which can accurately model any linear materials’ dispersion without limiting them to specific material classes. Based on the CCPR model, we introduce a time-domain measure of the electric power dissipation in arbitrary dispersive media. The CCPR model is incorporated via auxiliary differential equations (ADE) into Maxwell’s equations in the time domain, and we formulate a gradient-based topology optimization problem to optimize the dissipation over a broad frequency spectrum. To estimate the objective function gradient, we use the adjoint field method, and explain the discretization and integration of the adjoint system into the finite-difference time-domain (FDTD) framework. Our method is demonstrated using the example of topology-optimized spherical nanoparticles made of Gold and Silicon with an enhanced absorption efficiency in the visible-ultraviolet spectral range. In this context, a detailed analysis of the challenges of topology optimization of plasmonic materials associated with a density-based approach is given. Our method offers efficient broadband optimization of power dissipation in dispersive media.

We demonstrate a computational inverse design method for optimizing broadband-absorbing metasurfaces made of arbitrary dispersive media. Our figure of merit is the time-averaged instantaneous power dissipation in a single unit cell within a periodic array. Its time-domain formulation allows capturing the response of arbitrary dispersive media over any desired spectral range. Employing the time-domain adjoint method within a topology optimization framework enables the design of complex metasurface structures exhibiting unprecedented broadband absorption.We applied the method to a thin-film Silicon-on-insulator configuration and explored the impact of structural and (time-domain inherent) excitation parameters on performance over the visible–ultraviolet. We provide a physical insight into the dissipation mechanism of the optimized structures. Since our incorporated material model can represent any linear material, the method can also be applied to other all-dielectric, plasmonic, or hybrid configurations.

Near-field and radiation coupling between nearby radiating elements is unavoidable, and it is considered a limiting factor for applications in wireless communications and active sensing. This article proposes a density-based topology optimization approach to design decoupling networks for such systems. The decoupling network is designed by formulating an optimization problem that considers both energy transmission and reflection at the network ports. We replace the radiating elements by their time-domain impulse response for efficient computations and to enable the solution of the design problem using gradient-based optimization methods. We use the adjoint-field method to compute the gradients of the optimization objectives. Additionally, nonlinear filters are applied during the optimization procedure to impose minimum-size control on the optimized designs. We demonstrate the concept by designing the decoupling network for a two-element planar antenna array; the antenna is designed in a separate optimization problem. The optimized decoupling networks provide a signal path that destructively interferes with the coupling between the radiating elements while preserving their individual matching to the feeding ports. Compact decoupling networks capable of suppressing the mutual coupling by more than 10 dB between two closely separated planar antennas operating around 2.45 GHz are presented and validated experimentally.

We introduce a novel adjoint scheme for topology optimization to enhance the absorption in nanostructures made of lossy dispersive media. The method employs the complex-conjugate pole-residue (CCPR) model to account for the optical dispersion of arbitrary (linear) materials. Its integration within a parallel time-domain Maxwell’s equations solver enables an efficient optimization of dispersive nanostructures over a broad frequency range. Our method is demonstrated by designing Gold and Silicon nanoparticles with enhanced absorption efficiency and a broadband absorbing Silicon metasurface in the visible-ultraviolet regime.

We use a density-based topology optimization approach to design dualband planar metallic antennas. The design problem is formulated based on the time-domain Maxwell's equations, solved using the finite-difference timedomain (FDTD) method. The antenna design is formulated as an optimization problem where the received and reflected energy by the antenna in two frequency bands, centered around 2.5 GHz and 5.5 GHz, are optimized. Two design examples that exhibit outstanding performance are presented. In one design case, we employ a nonlinear filtering scheme to impose size control on the optimized design and ensure manufacturability.

Engineering the field scattered by an object is an important problem across the entire electromagnetic spectrum. For example, directional scattering achieved by means of nanoantennas is sought for applications in integrated optics, nanophotonics, sensing, single photon sources, and quantum information processing. Since a scattered field can be decomposed into a superposition of multipolar fields, the multipole decomposition technique provides an ideal platform for scattering engineering. In this paper, we present a topology optimization method for the inverse design of nanostructures to achieve specific multipoles with amplitude and phase control at a given wavelength. Our technique is formulated based on the discrete dipole approximation (DDA), and the optimization objective is specified as the current density associated with each multipole. Our approach operates on near-field quantities and is computationally lighter than similar methods targeting the far-field. Moreover, we can enforce a desired size/shape of the design volume, e.g., to meet fabrication or diffractionless constraints. We demonstrate our method by optimizing dielectric and metallic nanoantennas to achieve directional scattering based on the Kerker effect, using different excitation sources, including a plane wave and a dipole emitter. However, the generality of our approach makes it suitable for engineering nanoantennas with arbitrary scattering properties under various illumination conditions.

Anapole mechanisms have been broadly investigated in nanophotonics for the possibility to obtain invisible electromagnetic devices [1]. In the context of nanostructures with dipolar response, such states are typically understood as the result of destructive interference between quasi-static electric dipole and toroidal dipole. However, the richness of the physics associated with higher order multipoles open several possibilities to achieve anapole effects in other exotic ways. In this talk, two anapole mechanisms in dielectric and plasmonic meta-optics will be presented. First, an anapole mechanism in dielectric metasurfaces due to the interplay between high order multipoles will be discussed [2]. In a dielectric metasurface with inversion symmetry, multipoles of odd [e.g., electric dipole (ED), magnetic quadrupole (MQ)] and even parity [e.g., magnetic dipole (MD), electric quadrupole (EQ)] are decoupled [3]. Multipoles of each parity can exhibit an anapole state based on the destructive interference between multipoles of the same parity. This lattice anapole mechanism explains the origin of accidental bound states in the continuum (BICs) in symmetric dielectric metasurfaces. Specifically, we demonstrate that these states are nonradiating eigenmodes of the system. We exploit such modes for the realization of quasi-BIC with high-quality factor resonances. Contrary to existing techniques, our system allows us to achieve the transformation of a nonradiating BIC into a radiating quasi-BIC under normal radiation incidence and only due to a change in the period of the metasurface [2]. In general, such transformation can happen for other rather simple perturbations of the system, such as by introducing a variable superstrate, which is particularly suitable for sensing applications. Second, the talk includes a focus on anapole states in plasmonic meta-atoms, and a close-toideal plasmonic meta-atom will be discussed. This meta-atom is characterized by low absorption, high near-field enhancement, and negligible scattering due to the anapole effect [4]. We show that the anapole state of this metaatom can be qualitatively explained via destructive interference between quasi-static electric dipoles associated with complementary subvolumes of the nanostructure. We numerically demonstrate the use of this meta-atom as a building block for transparent metasurfaces and metamaterials.

Gradient-based topology optimization via the adjoint method has been successfully used in nanophotonics to uncover shapes with superior performances compared to what would be possible with traditional design methods. Here, we have extended this technique to optimize nanostructures to engineer their induced multipole moments. As an example, we demonstrate the method's application to realize the first Kerker effect in a silicon nanoparticle. The optimization results show a complex shape with highly suppressed backscattering due to the excitation of in-phase electric and magnetic dipoles with the same amplitude. This promising approach can pave the way for the inverse design of photonic structures based on a set of desired multipole moments, which can exhibit a variety of complex photonic phenomena.

The adjoint method is an efficient technique for the topology optimization of complex nanophotonic systems, including nanostructures, metasurfaces and integrated optical circuits. While such method has been traditionally used in the frequency domain, its extension to the time domain opens new opportunities for wideband optimization of dispersive materials for applications ranging from broadband absorbers to enhanced quantum emitters in dispersive environments. We propose a topology optimization technique for the inverse design of linear optical materials with arbitrary dispersion and anisotropy. We introduce a general adjoint scheme in the time-domain based on the complex-conjugate pole-residue pair (CCPR) model. This approach has the advantage of treating dispersive media and broadband response naturally in a single simulation run. We implement this framework within the finite-difference time-domain (FDTD) method and investigate the method for optimizing metallic and dielectric nanoantennas over the optical spectral range of 350-1000 nm. The combination of the method with parallel computing enables the large-scale inverse design of nanostructures in 3D with extreme field confinement. Nanostructures found via inverse design and featuring the intriguing anapole effect are also discussed. This effect enables nanostructures that show field enhancement, negligible scattering, and low losses. The possibility of reducing losses in plasmonic nanostructures via inverse design is an interesting possibility offered by the method and may open new avenues towards the realization of transparent plasmonic metamaterials for applications in linear and nonlinear nanophotonics.