Burak Gerislioglu

Transition from capacitive coupling to direct charge transfer in asymmetric terahertz plasmonic assemblies

We experimentally and numerically analyze the charge transfer THz plasmons using an asymmetric plasmonic assembly of metallic V-shaped blocks. The asymmetric design of the blocks allows for the excitation of classical dipolar and multipolar modes due to the capacitive coupling. Introducing a conductive microdisk between the blocks, we facilitated the excitation of the charge transfer plasmons and studied their characteristics along with the capacitive coupling by varying the size of the disk.

Optical Switching Using Transition from Dipolar to Charge Transfer Plasmon Modes in Ge2Sb2Te5 Bridged Metallodielectric Dimers

Capacitive coupling and direct shuttling of charges in nanoscale plasmonic components across a dielectric spacer and through a conductive junction lead to excitation of significantly different dipolar and charge transfer plasmon (CTP) resonances, respectively. Here, we demonstrate the excitation of dipolar and CTP resonant modes in metallic nanodimers bridged by phase-change material (PCM) sections, material and electrical characteristics of which can be controlled by external stimuli. Ultrafast switching (in the range of a few nanoseconds) between amorphous and crystalline phases of the PCM section (here Ge2Sb2Te5 (GST)) allows for designing a tunable plasmonic switch for optical communication applications with significant modulation depth (up to 88%). Judiciously selecting the geometrical parameters and taking advantage of the electrical properties of the amorphous phase of the GST section we adjusted the extinction peak of the dipolar mode at the telecommunication band (λ~1.55 μm), which is considered as the OFF state. Changing the GST phase to crystalline via optical heating allows for direct transfer of charges through the junction between nanodisks and formation of a distinct CTP peak at longer wavelengths (λ~1.85 μm) far from the telecommunication wavelength, which constitutes the ON state.

Extreme sensitive metasensor for targeted biomarkers identification using colloidal nanoparticles-integrated plasmonic unit cells

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 in vivo and in vitro 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 Δω~0.35 cm-1) 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.

Rapid Detection of Infectious Envelope Proteins by Magnetoplasmonic Toroidal Metasensors

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.

Functional Quadrumer Clusters for Switching Between Fano and Charge Transfer Plasmons

The study of the highly tunable plasmonic spectral features has received growing interest in recent years because of broad range of potential applications in the next generation photonics technologies. Here, we developed a four-member nanoparticle cluster composed of gold nanodisks, where two of the nanodisks are connected by a metallodielectric bridge composed of Ge2Sb2Te5 and gold sections. We showed that the optothermal functionality of the Ge2Sb2Te5 allows for excitation of Fano resonances and charge transfer plasmons in a single system. This feature opens new paths for developing highly tunable nanophotonic devices.

Large-Modulation-Depth Polarization-Sensitive Plasmonic Toroidal Terahertz Metamaterial

In this letter, we propose a terahertz (THz) plasmonic metamaterial based on planar metamolecule arrays with strong polarization sensitivity and large-modulation-depth for potential switching applications. Using both numerical and experimental studies, we analyzed the spectral features of the plasmonic unit cells. Employing bimetallic resonators, we showed that the proposed metamolecules can be efficiently tailored to support strong toroidal dipole resonant mode across the THz spectrum with high quality factor. The photo-induced toroidal moment with ultrasharp linewidth is used to design a polarization sensitive structure. This approach is the first utilization of toroidal metamaterials for switching applications, possessing strong potential to be used for advanced integrated photonic devices.

Azimuthally and radially excited charge transfer plasmon and Fano lineshapes in conductive sublayer-mediated nanoassemblies

Here, the plasmon responses of both symmetric and antisymmetric oligomers on a conductive substrate under linear, azimuthal, and radial polarization excitations are analyzed numerically. By observing charge transfer plasmons under cylindrical vector beam (CVB) illumination for what we believe is the first time, we show that our studies open new horizons to induce significant charge transfer plasmons and antisymmetric Fano resonance lineshapes in metallic substrate-mediated plasmonic nanoclusters under both azimuthal and radial excitation as CVBs.

Sonochemical Synthesis of a Zinc Oxide Core–Shell Nanorod Radial p–n Homojunction Ultraviolet Photodetector

We report for the first time on the growth of a homogeneous radial p-n junction in the ZnO core-shell configuration with a p-doped ZnO nanoshell structure grown around a high-quality unintentionally n-doped ZnO nanorod using sonochemistry. The simultaneous decomposition of phosphorous (P), zinc (Zn), and oxygen (O) from their respective precursors during sonication allows for the successful incorporation of P atoms into the ZnO lattice. The as-formed p-n junction shows a rectifying current-voltage characteristic that is consistent with a p-n junction with a threshold voltage of 1.3 V and an ideality factor of 33. The concentration of doping was estimated to be NA = 6.7 × 1017 cm-3 on the p side from the capacitance-voltage measurements. The fabricated radial p-n junction demonstrated a record optical responsivity of 9.64 A/W and a noise equivalent power of 0.573 pW/√Hz under ultraviolet illumination, which is the highest for ZnO p-n junction devices.

Tunable terahertz response of plasmonic vee-shaped assemblies with a graphene monolayer

There have been extensive research to systematically tailor terahertz (THz) plasmonics structures to support strong resonant modes as platforms for designing practical nano and microscale devices with high efficiency and tunability. Here, using a symmetric four-member assembly of plasmonic vee-shaped structures, we introduce a quadratic, multiresonant plasmonic THz antenna to support significant resonant modes with high efficiancy. The effect of unique geometrical shape of the structure on the quality of the proposed structure is analyzed to provide ultrasharp resonant modes with high plasmon resonance confinement. Employing both numerical and experimental studies, we investigated and confirmed the tunable plasmonic response of the suggested antenna. In addition, by depositing an atomic graphene monolayer below the antenna, we enhanced the THz response of the proposed structure, including remarkable absorption cross-section correlating with the resonant frequencies. This understanding will contribute to a promising model for practical all-optical and optoelectronic devices operating in the THz spectrum.

Optothermally Controlled Charge Transfer Plasmons in Au-Ge 2 Sb 2 Te 5 Core-Shell Dimers

Functional and reversible plasmonic resonances across the visible and near-infrared spectrum have opened new avenues for developing advanced next-generation nanophotonic devices. In this study, by using optothermally controlled phase-change material (PCM) for plasmonic nanostructures, we successfully induced highly tunable charge transfer plasmon (CTP) resonance modes. To this end, we have chosen a two-member dimer assembly consisting of gold cores and Ge2Sb2Te5 (GST) shells in distant, touching, and overlapping regimes. We show that switching between amorphous (dielectric) and crystalline (conductive) phases of GST allows for achieving tunable dipolar and CTP resonances and enables an effective interplay between these modes along the near-infrared spectrum. By analyzing electromagnetically calculated spectral responses for the dimer antenna in tunneling and direct charge transfer regimes, we confirmed that the induced CTPs in touching and overlapping regimes are highly controllable and pronounced in comparison to the quantum tunneling regime. We also use the precise, fast, and controllable switching between dipolar and CTP resonant modes to develop a telecommunication switch based on a simple metallodielectric dimer. The proposed structures can help designing optothermally controlled devices without morphological variations in the geometry of the design, and having strong potential for advanced plasmon modulation and fast data routing.