(Invited) Engineering Multipolar Resonances in Nonlocal Metasurfaces
Authors: Karimi Vahid , Bosomtwi Dominic , Liu Li , Babicheva Viktoriia 
Published: December 2024

When arranged in a metasurface, enhancing field interactions within scattering elements enables precise control over incident light phase and amplitude. This arrangement induces scattering waves in each element, which then interact with neighboring elements, leading to lattice resonances. The scatterer materials can be categorized into lossy ones, including transition metal dichalcogenides and titanium, and high-refractive-index ones, such as silicon. Periodic lattice arrays support strong localized resonances through the collective response of individual antennas [1]. High-refractive-index antennas exhibit strong resonances and field enhancement, enabling photonic devices, emitters, and absorbers. By manipulating metasurface parameters such as antenna geometry and periodicity, we can engineer multipolar resonances in high-refractive-index metasurfaces.
We engineer multipolar resonances across the visible to mid-infrared wavelength spectrum, and the excitation of these resonances is inherent in the scattering elements of the metasurface. Our approach employs disk nanoantennas specifically engineered to exhibit electric dipole and electric quadrupole resonances. We analyze multipolar resonances in high-refractive-index materials, particularly in truncated-cone iron pyrite antennas in the mid-infrared region [2]. Our study reveals the excitation of electric dipole and magnetic dipole lattice resonances (Figure). The in-phase and equal amplitude relationship between these polarizabilities leads to a higher forward-to-backward scattering ratio in an isolated truncated-cone iron pyrite antenna, known as the Kerker effect. Varying periodicity leads to electric-magnetic lattice resonance coupling, known as Rabi splitting. In iron pyrite, multipolar resonances shift with oblique incidence, revealing an out-of-plane magnetic dipole resonance. We also enhance resonances in the proximity to symmetry-protected bound states in a continuum at normal light incidence on truncated-cone iron pyrite antenna metasurfaces. Breaking antenna symmetry causes an additional out-of-plane magnetic dipole resonance shift to shorter wavelengths. Introducing oblique incidences with transverse electric or magnetic polarization induces distinct shifts in lattice resonances and Rayleigh anomalies. The deviations from normal incidence result in the shift of the electric quadrupole lattice resonance when subjected to oblique incidence.
Electron-beam deposition techniques emerge as a pivotal approach for the accurate and controlled realization of thin coatings covering a wide range of materials easily undergoing vaporization. The realm of high-refractive-index nanoantennas and metasurface Mie resonances is growing, covering periodic arrays, metalenses, and beam-steering applications, with a preference for high-refractive-index materials, such as silicon, for confined modes [3]. Nevertheless, silicon, a frequently used photonic material, is prone to oxidation during the deposition process due to moisture and oxygen within the chamber. To overcome this obstacle, we utilize a systematic approach that involves regulating deposition conditions, specifically, the base pressure within the chamber and the rate of deposition. We effectively overcome silicon oxidation through parameter adjustment, yielding accomplished refractive index values similar to those achieved via alternative deposition methods for amorphous silicon. Furthermore, our study illustrates the potential of controlling deposition conditions to finely adjust the refractive index, providing flexibility in achieving desired optical properties. We study a high-refractive-index metasurface with the potential for nanoscale light manipulation. Our analysis unveiled multimode coupling and bound states in the continuum, leading to narrow Fano resonances. Through reflection and transmission spectrum analysis from experiments, we observed various mode excitations and the generalized Kerker effect within the nanoantenna array.
Figure: Multipolar modes of the iron pyrite antenna array. The modes overlap and do not couple with each other. The periodicity px is chosen for the analysis of the modes right before their overlap, where their interaction is expected to be the strongest. The disk antenna has a height of 0.38 um, a diameter of 1.5 um, periodicity in the x-direction px changes, and periodicity in the y-direction is fixed at 1.6 um. The solid magenta line shows the Rayleigh anomaly wavelengths. Mode A1 is a predominantly electric dipole, and mode A2(e) is a predominantly magnetic dipole. Due to periodicity changes, magnetic dipole shifts towards the longer wavelength along with the Rayleigh anomaly. The modes A1 and A2(e) in the multipolar decomposition for the disk array agree well with the multipolar spectral profiles in panel b.
This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract 89233218CNA000001) and Sandia National Laboratories (Contract DE-NA-0003525).
[1] V. Karimi,V. Babicheva, “MXene-antenna electrode with collective multipole resonances,” Nanoscale 16, 4656 (2024).
[2] M.S. Islam, V. Babicheva, “Lattice Mie resonances and emissivity enhancement in mid-infrared iron pyrite metasurfaces,” Optics Express 31, 40380 (2023).
[3] D. Bosomtwi, V. Babicheva, “Beyond conventional sensing: Hybrid plasmonic metasurfaces and bound states in the continuum,” Nanomaterials 13, 1261 (2023).

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