Experimental measurement of plasmonic nanostructures embedded in silicon waveguide gaps

Light Guidance Aided by the Toroidal Dipole and the Magnetic Quadrupole in Silicon Slotted-Disk Chains

Far-field scattering of high-index nanoparticles can be hugely reduced via interference of multipolar moments giving rise to the so-called anapole states. It has been suggested that this reduced scattering can contribute to efficient transmission along periodic chains of such nanoparticles. In this work, we analyze via numerical simulation and experiments the transmission of light along chains of regular and slotted silicon disks in the frequency region over the light cone. We do not observe transmission at wavelengths corresponding to the excitation of the first electric anapole for regular disks. However, large transmission along straight and curved chains is observed for slotted disks due to the simultaneous excitation of the toroidal dipole and magnetic quadrupole modes in the disks. Photonic band calculations unveil that such large transmission can be ascribed to leaky resonances, though bound states in the continuum do not appear in the structures under analysis. Experiments at telecom wavelengths using silicon disk chains confirm the numerical results for straight and bent chains. Our results provide new insights into the role of radiationless states in light guidance along nanoparticle chains and offer new avenues to utilize Mie resonances of simple nanophotonic structures for on-chip dielectric photonics.

Plasmonic-Induced Transparencies in an Integrated Metaphotonic System

In this contribution, we numerically demonstrate the generation of plasmonic transparency windows in the transmission spectrum of an integrated metaphotonic device. The hybrid photonic–plasmonic structure consists of two rectangular-shaped gold nanoparticles fully embedded in the core of a multimode dielectric optical waveguide, with their major axis aligned to the electric field lines of transverse electric guided modes. We show that these transparencies arise from different phenomena depending on the symmetry of the guided modes. For the TE0 mode, the quadrupolar and dipolar plasmonic resonances of the nanoparticles are weakly coupled, and the transparency window is due to the plasmonic analogue of electromagnetically induced transparency. For the TE1 mode, the quadrupolar and dipolar resonances of the nanoparticles are strongly coupled, and the transparency is originated from the classical analogue of the Autler–Townes effect. This analysis contributes to the understanding of plasmonic transparency windows, opening new perspectives in the design of on-chip devices for optical communications, sensing, and signal filtering applications.

Enhanced excitation and readout of plasmonic cavity modes in NPoM via SiN waveguides for on-chip SERS

Metallic nanoparticle-on-a-mirror (NPoM) cavities enable extreme field confinement in sub-nm gaps, leading to unrivaled performance for nonlinear processes such as surface-enhanced Raman scattering (SERS). So far, prevailing experimental approaches based on NPoMs have been performed by means of free-space light excitation and collection under oblique incidence, since the fundamental radiatively-coupled NPoM mode does not scatter in the normal direction. Retaining this working principle, here we numerically show that plasmonic cavity modes in NPoM configurations can be efficiently excited in an integrated SERS approach through TM guided modes of silicon nitride (SiN) waveguides. Intensity enhancements beyond 10⁵ can be achieved for gap spacings around 1 nm. So as to reduce unwanted SiN Raman background, the output Stokes signals are transferred to transversely placed waveguides, reaching coupling efficiencies of up to 10%. Geometrical parameters such as the gap thickness as well as the radius and position of the nanoparticle provide full control over the main spectral features, thereby enabling us to engineer and drive the optical response of NPoMs for high-performance SERS in Si-based photonic integrated platforms.

Radiationless anapole states in on-chip photonics

High-index nanoparticles are known to support radiationless states called anapoles, where dipolar and toroidal moments interfere to inhibit scattering to the far field. In order to exploit the striking properties arising from these interference conditions in photonic integrated circuits, the particles must be driven in-plane via integrated waveguides. Here, we address the excitation of electric anapole states in silicon disks when excited on-chip at telecom wavelengths. In contrast to normal illumination, we find that the anapole condition-identified by a strong reduction of the scattering-does not overlap with the near-field energy maximum, an observation attributed to retardation effects. We experimentally verify the two distinct spectral regions in individual disks illuminated in-plane from closely placed waveguide terminations via far-field and near-field measurements. Our finding has important consequences concerning the use of anapole states and interference effects of other Mie-type resonances in high-index nanoparticles for building complex photonic integrated circuitry.

Plasmonic nanocavity for obtaining bound state in the continuum in silicon waveguides

Bound states in the continuum (BICs) have become a new trend in the area of metaoptics and nanophotonics. Strong interactions in electromagnetic fields are analogous to electron transitions in atoms, giving rise to BICs with vanishing radiative losses. However, it is still a great challenge to realize BICs in the lossy plasmonic systems. For this problem, we propose a supercavity-like plasmonic nanocavity consisting of an Au nanorod deposited inside an Au symmetric split ring, and explore the possibility of exciting quasi-BICs that own finite but high quality (Q) factors. In such hybrid configuration, the excited resonances can be easily engineered by modifying the rotation angle or the length of the Au nanorod. With the integration of such nanocavity in silicon (Si) waveguides, sharp transmission spectra could be achieved with fiber-chip in-parallel excitations and detections. Besides, the ultracompact geometry of this plasmonic nanocavity provides a route to boost enhanced electric fields, thus improving sensing performances significantly. Our study not only offers a novel platform for the realization of chip-scale quasi-BICs, but extends functionalities of photonic-plasmonic hybrid circuits.

Waveguide-integrated mid-infrared plasmonics with high-efficiency coupling for ultracompact surface-enhanced infrared absorption spectroscopy

Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. Built atop a silicon-on-insulator (SOI) waveguide platform, two key plasmonic structures useful for SEIRA are examined using computational modeling: gold nanorods and coaxial nanoapertures. We find resonance dips of 90% in near diffraction-limited areas due to arrays of our structures and up to 50% from a single resonator. Each of the structures is evaluated using the simulated SEIRA signal from poly(methyl methacrylate) and an octadecanethiol self-assembled monolayer. The platforms we present allow for a compact, on-chip SEIRA sensing system with highly efficient waveguide coupling in the mid-IR.

On-chip surface-enhanced Raman spectroscopy using nanosphere-lithography patterned antennas on silicon nitride waveguides

A hybrid integration of nanoplasmonic antennas with silicon nitride waveguides enables miniaturized chips for surface-enhanced Raman spectroscopy at visible and near-infrared wavelengths. This integration can result in high-throughput SERS assays on low sampling volumes. However, current fabrication methods are complex and rely on electron-beam lithography, thereby obstructing the full use of an integrated photonics platform. Here, we demonstrate the electron-beam-free fabrication of gold nanotriangles on deep-UV patterned silicon nitride waveguides using nanosphere lithography. The localized surface-plasmon resonance of these nanotriangles is optimized for Raman excitation at 785 nm, resulting in a SERS substrate enhancement factor of 2.5 × 10⁵. Furthermore, the SERS signal excited and collected through the waveguide is as strong as the free-space excited and collected signal through a high NA objective.

Integrated metaphotonics: symmetries and confined excitation of LSP resonances in a single metallic nanoparticle

Using numerical simulations, we demonstrate that the dipolar plasmonic resonance of a single metallic nanoparticle inserted in the core of a dielectric waveguide can be excited with higher order photonic modes of the waveguide only if their symmetry is compatible with the charge distribution of the plasmonic mode. For the case of a symmetric waveguide, we demonstrate that this condition is only achieved if the particle is shifted from the center of the core. The simple and comprehensive analysis presented in this contribution will serve as basis for applications in integrated nanophotonic/metamaterials devices, such as optical filters, modulators and mode converters.