Exciting Bright and Dark Eigenmodes in Strongly Coupled Asymmetric Metallic Nanoparticle Arrays

The strong coupling between planar arrays of gold and silver nanoparticles mediated by a near-field interaction is investigated both theoretically and experimentally to provide an in-depth study of symmetry breaking in complex nanoparticle structures. The asymmetric composition allows to probe for bright and dark eigenmodes, in accordance with plasmon hybridization theory. The strong coupling could only be observed by separating the layers by a nanometric distance with monolayers of suitably chosen polymers. The bottom-up assembly of the nanoparticles as well as the stratified structures themselves gives rise to an extremely flexible system that, moreover, allows the facile variation of a number of important material parameters as well as the preparation of samples on large scales. This flexibility was used to modify the coupling distance between arrays, showing that both the positions and relative intensities of the resonances observed can be tuned with a high degree of precision. Our work renders research in the field of “plasmonic molecules” mature to the extent that it could be incorporated into functional optical devices

Identification of Dielectric, Plasmonic, and Hybrid Modes in Metal-Coated Whispering-Gallery-Mode Resonators

Making available and accessing in a controlled manner optical modes with largely disparate properties in a given system constitutes a prime challenge for different applications. Here, we propose, realize, and optically characterize a high-<i>Q</i> polymeric wedge-like whispering-gallery-mode resonator coated with a thin silver layer that supports pure surface plasmon polariton modes, pure dielectric modes, and hybrid photonic–plasmonic modes with <i>Q</i>-factors larger than 1000 and modal volumes as small as only a few cubic micrometers. We demonstrate both theoretically and experimentally that all three distinct kinds of cavity eigenmodes can be efficiently excited in the infrared via evanescent coupling to a tapered fiber. Performing finite-element simulations and coupled-mode theory, we develop an experimental procedure based on mode filtering to unambiguously identify the resonances observed in fiber transmission spectra. By controlling both the position of the tapered fiber with respect to the resonator and the input laser polarization, we successfully demonstrate that dielectric, plasmonic, and hybrid modes can be selectively excited, allowing for an explicit classification of the distinct cavity eigenmodes. Experimental results are in excellent agreement with the simulations

Supplementary document for Modeling four-dimensional metamaterials: A T-matrix approach to describe time-varying metasurfaces - 6068526.pdf

Computational detail

Bottom-Up Fabrication of Hybrid Plasmonic Sensors: Gold-Capped Hydrogel Microspheres Embedded in Periodic Metal Hole Arrays

The high potential of bottom-up fabrication strategies for realizing sophisticated optical sensors combining the high sensitivity of a surface plasmon resonance with the exceptional properties of stimuli-responsive hydrogel is demonstrated. The sensor is composed of a periodic hole array in a gold film whose holes are filled with gold-capped poly­(<i>N</i>-isoproyl-acrylamide) (polyNIPAM) microspheres. The production of this sensor relies on a pure chemical approach enabling simple, time-efficient, and cost-efficient preparation of sensor platforms covering areas of cm². The transmission spectrum of this plasmonic sensor shows a strong interaction between propagating surface plasmon polaritons at the metal film surface and localized surface plasmon resonance of the gold cap on top of the polyNIPAM microspheres. Computer simulations support this experimental observation. These interactions lead to distinct changes in the transmission spectrum, which allow for the simultaneous, sensitive optical detection of refractive index changes in the surrounding medium and the swelling state of the embedded polyNIPAM microsphere under the gold cap. The volume of the polyNIPAM microsphere located underneath the gold cap can be changed by certain stimuli such as temperature, pH, ionic strength, and distinct molecules bound to the hydrogel matrix facilitating the detection of analytes which do not change the refractive index of the surrounding medium significantly

3356827.pdf

Supplementary Materia

Plasmon Coupling in Self-Assembled Gold Nanoparticle-Based Honeycomb Islands

Metallic nanostructures that sustain plasmonic resonances are indispensable ingredients for many functional devices. Whereas structures fabricated with top-down methods entail the advantage of a nearly unlimited control over all plasmonic properties, they are in most cases unsuitable for a low cost fabrication on large surfaces; and eventually a truly nanometric size domain is difficult to reach due to limitations in the fabrication resolution. Although ordinary bottom-up techniques based on colloidal nanolithography promise to lift these limitations, they often suffer from their incapability to self-assemble nanoparticles at large surfaces and at a density necessary to observe effects that strongly deviate from those of isolated nanoparticles. Here, we rely on the application of sequential bottom-up fabrication steps to realize honeycomb structures from gold nanoparticles that show strong extinction bands in the near-infrared. The extraordinary properties are only facilitated by densely packing the nanoparticles into clusters with a finite size; causing the clusters to act as plasmonic macromolecules. These strongly interacting bottom-up materials with a deterministic geometry but fabricated by self-assembly might be of use in future sensing applications and in material platforms to mediate strong light–matter-interactions

Deep-Subwavelength Plasmonic Nanoresonators Exploiting Extreme Coupling

A metal–insulator–metal (MIM) waveguide is a canonical structure used in many functional plasmonic devices. Recently, research on nanoresonantors made from finite, that is, truncated, MIM waveguides attracted a considerable deal of interest motivated by the promise for many applications. However, most suggested nanoresonators do not reach a deep-subwavelength domain. With ordinary fabrication techniques the dielectric spacers usually remain fairly thick, that is, in the order of tens of nanometers. This prevents the wavevector of the guided surface plasmon polariton to strongly deviate from the light line. Here, we will show that the exploitation of an extreme coupling regime, which appears for only a few nanometers thick dielectric spacer, can lift this limitation. By taking advantage of atomic layer deposition we fabricated and characterized exemplarily deep-subwavelength perfect absorbers. Our results are fully supported by numerical simulations and analytical considerations. Our work provides impetus on many fields of nanoscience and will foster various applications in high-impact areas such as metamaterials, light harvesting, and sensing or the fabrication of quantum-plasmonic devices

Multipolar Coupling in Hybrid Metal–Dielectric Metasurfaces

We study functional hybrid metasurfaces consisting of metal–dielectric nanoantennas that direct light from an incident plane wave or from localized light sources into a preferential direction. The directionality is obtained by carefully balancing the multipolar contributions to the scattering response from the constituents of the metasurface. The hybrid nanoantennas are composed of a plasmonic gold nanorod acting as a feed element and a silicon nanodisk acting as a director element. In order to experimentally realize this design, we have developed a two-step electron-beam lithography process in combination with a precision alignment step. The optical response of the fabricated sample is measured and reveals distinct signatures of coupling between the plasmonic and the dielectric nanoantenna elements that ultimately leads to unidirectional radiation of light

A digital twin for a chiral sensing platform

Nanophotonic concepts can improve many measurement techniques by enhancing and tailoring the light-matter interaction. However, the optical response of devices that implement such techniques can be intricate, depending on the sample under investigation. That combination of a promise and a challenge makes nanophotonics a ripe field for applying the concept of a digital twin: a digital representation of an entire real-world device. In this work, we detail the concept of a digital twin with the example of a nanophotonically-enhanced chiral sensing platform. In that platform, helicity-preserving cavities with diffractive mirrors enhance the light-matter interaction between chiral molecules and circularly polarized light, allowing a faster measurement of the circular dichroism of the molecules. However, the sheer presence of the molecules affects the cavity's functionality, demanding a holistic treatment to understand the device's performance. In our digital twin, optical and quantum chemistry simulations are fused to provide a comprehensive description of the device with the molecules across all length scales and predict the circular dichroism spectrum of the device containing molecules to be sensed. Performing simulations in lockstep with the experiment will allow a clear interpretation of the results of complex measurements. We also demonstrate how to design a cavity-enhanced circular dichroism spectrometer by utilizing our digital twin. The digital twin can be used to guide experiments and analyze results, and its underlying concept can be translated to many other optical experiments

Enhanced Directional Emission from Monolayer WSe₂ Integrated onto a Multiresonant Silicon-Based Photonic Structure

Two-dimensional transition-metal dichalcogenides such as WSe₂ show great promise as versatile atomic-scale light sources for on-chip applications due to their advanced optoelectronic properties and compatibility with a silicon photonics platform. However, the sub-nanometer thickness of such active materials limits their emission efficiency. Hence, new approaches to simultaneously enhance the emission and control its directionality are required. Here, we demonstrate enhanced and directional emission from a WSe₂ monolayer integrated onto a silicon photonic structure. This is achieved by coupling of the WSe₂ layer to a multiresonant silicon grating-waveguide structure. The interaction with the multiple resonant modes supported by the structure provides simultaneous excitation and emission enhancement, while the dispersion of the modes further routes the emission into specified directions. In addition, our hybrid structure offers the opportunity for ultrafast emission modulation, owing to the reduced emission lifetime of WSe₂. Such a silicon-based hybrid platform is fully scalable and promising as efficient chip-integrated and spatially multiplexed light sources