Mustafa Karabiyik

Highly sensitive wide bandwidth photodetector based on internal photoemission in CVD grown p-type MoS2/graphene Schottky junction.

Two dimensional (2D) Molybdenum disulfide (MoS2) has evolved as a promising material for next generation optoelectronic devices owing to its unique electrical and optical properties, such as band gap modulation, high optical absorption, and increased luminescence quantum yield. The 2D MoS2 photodetectors reported in the literature have presented low responsivity compared to silicon based photodetectors. In this study, we assembled atomically thin p-type MoS2 with graphene to form a MoS2/graphene Schottky photodetector where photo generated holes travel from graphene to MoS2 over the Schottky barrier under illumination. We found that the p-type MoS2 forms a Schottky junction with graphene with a barrier height of 139 meV, which results in high photocurrent and wide spectral range of detection with wavelength selectivity. The fabricated photodetector showed excellent photosensitivity with a maximum photo responsivity of 1.26 AW(-1) and a noise equivalent power of 7.8 × 10(-12) W/√Hz at 1440 nm.

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.

Tunable Room Temperature THz Sources Based on Nonlinear Mixing in a Hybrid Optical and THz Micro-Ring Resonator

We propose and systematically investigate a novel tunable, compact room temperature terahertz (THz) source based on difference frequency generation in a hybrid optical and THz micro-ring resonator. We describe detailed design steps of the source capable of generating THz wave in 0.5–10 THz with a tunability resolution of 0.05 THz by using high second order optical susceptibility (χ(2)) in crystals and polymers. In order to enhance THz generation compared to bulk nonlinear material, we employ a nonlinear optical micro-ring resonator with high-Q resonant modes for infrared input waves. Another ring oscillator with the same outer radius underneath the nonlinear ring with an insulation of SiO2 layer supports the generated THz with resonant modes and out-couples them into a THz waveguide. The phase matching condition is satisfied by engineering both the optical and THz resonators with appropriate effective indices. We analytically estimate THz output power of the device by using practical values of susceptibility in available crystals and polymers. The proposed source can enable tunable, compact THz emitters, on-chip integrated spectrometers, inspire a broader use of THz sources and motivate many important potential THz applications in different fields.

Fano Resonances in Complex Plasmonic Necklaces Composed of Gold Nanodisks Clusters for Enhanced LSPR Sensing

Plasmonic nanoparticles in complex clusters and in specific molecular orientations are able to support strong plasmon and Fano resonances in their nanoscale geometries. In this paper, we examine the spectral response of a symmetric necklace shape nanostructure composed of Au nanodisk heptamers that are located in a close proximity to each other. Determining the appropriate geometrical parameters for the proposed necklace, we analyzed the effect of geometrical variations on the Fano resonance position and quality by calculating the scattering cross-sectional profile, numerically. Considering the strong localization of surface plasmon resonances (LSPR) in the heptamer clusters, the LSPR sensitivity for the studied necklace has been determined. Moreover, we evaluated the performance of the structure for different medium conditions by plotting corresponding figure of merit (FoM). To this end, we measured the plasmon resonance energy differences (Δ E eV) over the refractive index (n) alterations, we quantified the corresponding FoM for the final necklace as 13.7, which can be utilized in designing precise and highly sensitive sensors. Ultimately, we proved that highly complex structures composed of metal nanoparticles clusters yield high sensitivity to the environmental perturbations.

Hot electron generation by aluminum oligomers in plasmonic ultraviolet photodetectors.

We report on an integrated plasmonic ultraviolet (UV) photodetector composed of aluminum Fano-resonant heptamer nanoantennas deposited on a Gallium Nitride (GaN) active layer which is grown on a sapphire substrate to generate significant photocurrent via formation of hot electrons by nanoclusters upon the decay of nonequilibrium plasmons. Using the plasmon hybridization theory and finite-difference time-domain (FDTD) method, it is shown that the generation of hot carriers by metallic clusters illuminated by UV beam leads to a large photocurrent. The induced Fano resonance (FR) minimum across the UV spectrum allows for noticeable enhancement in the absorption of optical power yielding a plasmonic UV photodetector with a high responsivity. It is also shown that varying the thickness of the oxide layer (Al2O3) around the nanodisks (tox) in a heptamer assembly adjusted the generated photocurrent and responsivity. The proposed plasmonic structure opens new horizons for designing and fabricating efficient opto-electronics devices with high gain and responsivity.

Deep Sub-Wavelength Multimode Tunable In-Plane Plasmonic Lenses Operating at Terahertz Frequencies

We report on focusing of terahertz (THz) plasmons in two-dimensional electron gas (2DEG) at III-N heterostructures and in graphene by using planar circular grating lenses. Propagation of a broadband pulse of EM waves in 0.5-10 THz is studied by using finite difference time-domain (FDTD) approach. The results show that plasmonic modes excited by incident THz radiation are concentrated into λ/180 area localized under the central disc. Electric field intensity under the central point is found to be orders of magnitude larger than the outer grating area. Optimal geometries for enhanced radiation coupling and plasmon focusing are investigated. Plasmonic lens modes supported by system has the advantage of tunability by an applied voltage to gratings. The large field enhancement by plasmonic confinement presents the potential for sub-wavelength imaging.

Fano-Like Resonances in Split Concentric Nanoshell Dimers in Designing Negative-Index Metamaterials for Biological-Chemical Sensing and Spectroscopic Purposes

In this study, we investigated numerically the plasmon response of a dimer configuration composed of a couple of split and concentric Au nanoshells in a complex orientation. We showed that an isolated composition of two concentric split nanoshells could be tailored to support strong plasmon resonant modes in the visible wavelengths. After determining the accurate geometric dimensions for the presented antisymmetric nanostructure, we designed a dimer array that shows complex behavior during exposure to different incident polarizations. We verified that the examined dimer was able to support destructive interference between dark and bright plasmon modes, which resulted in a pronounced Fano-like dip. Observation of a Fano minimum in such a simple molecular orientation of subwavelength particles opens new avenues for employing this structure in designing various practical plasmonic devices. Depositing the final dimer in a strong coupling condition on a semiconductor metasurface and measuring the effective refractive index at certain wavelengths, we demonstrate that each one of dimer units can be considered a meta-atom due to the high aspect ratio in the geometric parameters. Using this method, by extending the number of dimers periodically and illuminating the structure, we examined the isotropic, polarization-dependent, and transmission behavior of the metamaterial configuration. Using numerical methods and calculating the effective refractive indices, we computed and sketched corresponding figure of merit over the transmission window, where the maximum value obtained was 42.3 for Si and 54.6 for gallium phosphide (GaP) substrates.

Inducing multiple Fano resonant modes in split concentric nanoring resonator dimers for ultraprecise sensing

We report on finite-difference time domain analysis of silicon split concentric ring resonator dimers (SCRRDs) with nanoscale dimensions deposited on a dielectric substrate to design all-dielectric polarization-dependent metamaterial (MM) structures. Investigating the optical response of an SCRRD on a quartz (SiO2) substrate, we verified that it can be considered as a meta-atom to design a MM. The proposed structure is able to support double sharp Fano resonances in the visible spectrum under transverse electric polarization excitation. Immersing the all-dielectric MM in various chemical liquids with different refractive indices and calculating the transmission spectra, we estimated the sensitivity of the presented MM as 497 nm/RIU and the corresponding figure of merit as 118.5. This study could help designing ultra-sensitive, low-cost and efficient bio-chemical sensors based on all-dielectric components.

Graphene-based field-effect transistor structures for terahertz applications

We propose Terahertz (THz) plasmonic devices based on linearly integrated FETs (LFETs) on Graphene. LFET structures are advantageous for (THz) detection since the coupling between the THz radiation and the plasma wave is strongly enhanced over the single gate devices and accordingly higher-order plasma resonances become possible. AlGaN/GaN heterostructure LFETs with their high sheet carrier concentration and high electron mobility are promising for plasmonic THz detection. Nevertheless, our numerical studies show that room temperature resonant absorption of THz radiation by the plasmons in AlGaN/GaN LFETs is very weak even if the integration density is sufficiently large. Our simulations also demonstrate that similar LFETs on Graphene, which has very large electron mobility, can resonantly absorb THz radiation up to 5th harmonic at room temperature. Additionally, we investigated LFETs with integrated cavities on Graphene. Such Periodic Cavity LFETs substantially enhance the quality factor of the resonant modes.