Raju Sinha

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.

Rhodium Plasmonics for Deep-Ultraviolet Bio-Chemical Sensing

Rhodium (Rh) has been recently introduced as a perfect metal for ultraviolet (UV) applications with the advantages of its oxide-free nature and support of strong plasmon resonant modes at very short wavelengths. We report on a simple platform of nanoplasmonic structures to support strong plasmonic Fano resonances across the deep-UV spectrum for biochemical sensing applications. We investigate the plasmonic response of several types of Rh nanoparticles and designed dimer-type antennas using nanorings with geometrical tunability in both symmetric and antisymmetric assemblies. Using numerical and theoretical methods, it is shown that Rh-based dimer antennas with broken symmetry can be tailored to support strong plasmon resonant modes at the deep-UV region (E>6eV

Hot electron generation by aluminum oligomers in plasmonic ultraviolet photodetectors.

E>6\; eV

Bandgap engineering of single layer graphene by randomly distributed nanoparticles

). We also propose a complex infinity-shaped structure composed of a pair of split rings with a nanodisk in between with extra degree of tunability to push the plasmon resonant modes further in deep-UV spectrum. Plasmon hybridization theory is used to describe formation of plasmonic Fano-resonant dips in simple nanoscale assemblies. We calculate the corresponding figure of merit for the Rh-based nanostructure around 11.5 which shows an excellent sensitivity to the refractive index perturbations of the surrounding medium at very short wavelengths for sensing applications.

Resonance coupling in plasmonic nanomatryoshka homo- and heterodimers

In this research, we demonstrate cell uptake of magneto-electric nanoparticles (MENPs) through nanoelectroporation (NEP) using alternating current (ac)-magnetic field stimulation. Uptake of MENPs was confirmed using focused-ion-beam assisted transmission electron microscopy (FIB-TEM) and validated by a numerical simulation model. The NEP was performed in microglial (MG) brain cells, which are highly sensitive for neuro-viral infection and were selected as target for nano-neuro-therapeutics. When the ac-magnetic field optimized (60 Oe at 1 kHz), MENPs were taken up by MG cells without affecting cell health (viability > 92%). FIB-TEM analysis of porated MG cells confirmed the non-agglomerated distribution of MENPs inside the cell and no loss of their elemental and crystalline characteristics. The presented NEP method can be adopted as a part of future nanotherapeutics and nanoneurosurgery strategies where a high uptake of a nanomedicine is required for effective and timely treatment of brain diseases.