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Unconventional characteristics of magnetic toroidal multipoles have triggered researchers to study these unique resonant phenomena by using both 3D and planar resonators under intense radiation. Here, going beyond conventional planar unit cells, we report on the observation of magnetic toroidal modes using artificially engineered multimetallic planar plasmonic resonators. The proposed microstructures consist of iron (Fe) and titanium (Ti) components acting as magnetic resonators and torus, respectively. Our numerical studies and following experimental verifications show that the proposed structures allow for excitation of toroidal dipoles in the terahertz (THz) domain with the experimental Q-factor of ∼18. Taking the advantage of high-Q toroidal line shape and its dependence on the environmental perturbations, we demonstrate that room-temperature toroidal metasurface is a reliable platform for immunosensing applications. As a proof of concept, we utilized our plasmonic metasurface to detect Zika-virus (ZIKV) envelope protein (with diameter of 40 nm) using a specific ZIKV antibody. The sharp toroidal resonant modes of the surface functionalized structures shift as a function of the ZIKV envelope protein for small concentrations (∼pM). The results of sensing experiments reveal rapid, accurate, and quantitative detection of envelope proteins with the limit of detection of ∼24.2 pg/mL and sensitivity of 6.47 GHz/log(pg/mL). We envision that the proposed toroidal metasurface opens new avenues for developing low-cost, and efficient THz plasmonic sensors for infection and targeted bioagent detection.

Graphene-enhanced optoelectronic terahertz (THz) signal processing offers an exquisite potential for tailoring extreme-subwavelength platforms to develop tunable and highly-responsive photonic tools.

Aluminum nanocrystals have emerged as an earth-abundant material for plasmonics applications. Al nanocrystals readily oxidize in aqueous-based solutions, however, transforming into highly stratified γ-AlOOH nanoparticles with a 700% increase in surface area in a matter of minutes. Here we show that by functionalizing Al nanocrystals with the bioinspired polymer polydopamine, their stability in aqueous media is dramatically increased, maintaining their integrity in aqueous solution for over 2 weeks with no discernible structural changes. Polydopamine functionalization also provides a molecular capture layer that enables the capture of polycyclic aromatic hydrocarbon pollutants in H2O samples and their detection by surface-enhanced Raman spectroscopy, when polydopamine-stabilized Al nanocrystal aggregates are used as substrates. This approach was used to detect a prime carcinogenic H2O pollutant, benzo­[a]­pyrene with a sensitivity in the sub part-per-billion range.

Localized surface plasmons (LSPs) in metal nanostructures have attracted much attentionfor their role in generating non-equilibrium hot carriers (HCs) for photochemistry 1-3 , photodetection 4,5 and photoluminescence 6 . In addition to optical excitation, LSPs and HC dynamics can be driven electrically via inelastic tunneling. LSP-mediated light emission in tunnel junctions 7-13 commonly features photon energies below the threshold set by the applied voltage bias. Recent work [14][15][16][17][18] has reported photon energies significantly above that threshold, while the underlying physical origin remains elusive. Proposed mechanisms include higher-order electron-plasmon and electron-electron interactions 15,17-20 , and blackbody radiation of hot electrons 16,21 . We report measurements of light emission in tunnel junctions of different plasmonic materials and reveal that HCs generated by nonradiative decay of electrically excited plasmons play a key role in above-threshold light emission. We observed the crossover from above-to below-threshold light emission regime by controlling the tunneling current. There is a giant material dependence of the photon yield, as much as four orders of magnitude, much greater than the plasmon-enhanced radiative efficiency of the tunneling gap. The spectral features of light emission are consistent with a proposed mechanism that incorporates the plasmonic field enhancement and a non-equilibrium HC distribution parametrized by a bias-dependent Boltzmann factor. The effective temperatures of HCs are found to correlate with the bias voltage, rather than the dissipated electrical power. Electrically driven HC generation (above 2000 K under modest voltage) and plasmon-enhanced light emission could open new strategies for chemistry, optoelectronics and quantum optics.Electrically driven light emission from tunnel junctions is of great interest for a variety of technologies requiring efficient optoelectronic integration and conversion at the nanoscale, such

Dimers, two closely spaced metallic nanostructures, are one of the primary nanoscale geometries in plasmonics, supporting high local field enhancements in their interparticle junction under excitation of their hybridized “bonding” plasmon. However, when a dimer is fabricated on a metallic substrate, its characteristics are changed profoundly. Here we examine the properties of a Au dimer on a Au substrate. This structure supports a bright “bonding” dimer plasmon, screened by the metal, and a lower energy magnetic charge transfer plasmon. Changing the dielectric environment of the dimer-on-film structure reveals a broad family of higher-order hybrid plasmons in the visible region of the spectrum. Both of the localized surface plasmons resonances (LSPR) of the individual dimer-on-film structures as well as their collective surface lattice resonances (SLR) show a highly sensitive refractive index sensing response. Implementation of such all-metal magnetic-resonant nanostructures offers a promising route to achieve higher-performance LSPR- and SLR-based plasmonic sensors.

Engineered terahertz (THz) plasmonic metamaterials have emerged as promising platforms for quick infection diagnosis, cost-effective and real-time pharmacology applications owing to their non-destructive and harmless interaction with biological tissues in both and assays. As a recent member of THz metamaterials family, toroidal metamaterials have been demonstrated to be supporting high-quality sharp resonance modes. Here we introduce a THz metasensor based on a plasmonic surface consisting of metamolecules that support ultra-narrow toroidal resonances excited by the incident radiation and demonstrate detection of an ultralow concertation targeted biomarker. The toroidal plasmonic metasurface was designed and optimized through extensive numerical studies and fabricated by standard microfabrication techniques. The surface then functionalized by immobilizing the antibody for virus-envelope proteins (ZIKV-EPs) for selective sensing. We sensed and quantified the ZIKV-EP in the assays by measuring the spectral shifts of the toroidal resonances while varying the concentration. In an improved protocol, we introduced gold nanoparticles (GNPs) decorated with the same antibodies onto the metamolecules and monitored the resonance shifts for the same concentrations. Our studies verified that the presence of GNPs enhances capturing of biomarker molecules in the surrounding medium of the metamaterial. By measuring the shift of the toroidal dipolar momentum (up to Δ~0.35 cm) for different concentrations of the biomarker proteins, we analyzed the sensitivity, repeatability, and limit of detection (LoD) of the proposed toroidal THz metasensor. The results show that up to 100-fold sensitivity enhancement can be obtained by utilizing plasmonic nanoparticles-integrated toroidal metamolecules in comparison to analogous devices. This approach allows for detection of low molecular-weight biomolecules (≈13 kDa) in diluted solutions using toroidal THz plasmonic unit cells.