Graphene on nanoscale gratings for THz electron-beam radiation and plasmonics

Terahertz (THz) technologies have numerous applications such as biological and medical imaging, security screening, remote sensing, and industrial process control. However, the lack of practical THz sources and detectors is still a significant problem limiting the impact of these applications. In this Thesis work, three novel THz radiation mechanisms are proposed and investigated, based on the distinctive electronic properties of charge carriers in 2D single-layer graphene and related 1D conductors (i.e., graphene nanoribbons and carbon nanotubes), combined with the use of nanoscale dielectric gratings. Numerical simulations as well as fabrication and characterization activities are carried out. The first proposed radiation mechanism is based on the mechanical corrugation of a single-layer sheet of graphene or 1D carbon conductor, deposited on a lithographically-defined sinusoidal grating. In the presence of a dc voltage, carriers will therefore undergo periodic angular motion and correspondingly radiate (similar to cyclotron emission but without the need for any external magnetic field). My numerical simulations indicate that technologically significant output power levels can correspondingly be obtained at geometrically tunable THz frequencies. Initial graphene samples on sinusoidal gratings were fabricated and found to undergo significant strain redistribution, which affects their structural quality. Charge carriers moving in a flat sheet of graphene or linear 1D carbon conductor parallel to a nanoscale grating can also produce THz radiation based on the Smith-Purcell effect. The role of the grating in this case is to diffract the evanescent electromagnetic fields produced by the moving electrons and holes so that THz light can be radiated. Once again, numerical simulations indicate that this approach is promising for the realization of ultra-compact THz sources capable of room-temperature operation. Initial experimental results with ultra-high-mobility graphene samples embedded in boron nitride films show promising THz electroluminescence spectra. The last approach considered in this Thesis involves graphene plasmons at THz frequencies, which can be excited through the decay of hot electrons injected with an applied bias voltage. A nearby grating can then be used to outcouple the guided electromagnetic fields associated with these collective charge oscillations into radiation. The excitation of these THz plasmonic resonances at geometrically tunable frequencies has been demonstrated experimentally via transmission spectroscopy measurements.2017-06-21T00:00:00

One-dimensional carbon nanostructures for terahertz electron-beam radiation

One-dimensional carbon nanostructures such as nanotubes and nanoribbons can feature near-ballistic electronic transport over micron-scale distances even at room temperature. As a result, these materials provide a uniquely suited solid-state platform for radiation mechanisms that so far have been the exclusive domain of electron beams in vacuum. Here we consider the generation of terahertz light based on two such mechanisms, namely, the emission of cyclotronlike radiation in a sinusoidally corrugated nanowire (where periodic angular motion is produced by the mechanical corrugation rather than an externally applied magnetic field), and the Smith-Purcell effect in a rectilinear nanowire over a dielectric grating. In both cases, the radiation properties of the individual charge carriers are investigated via full-wave electrodynamic simulations, including dephasing effects caused by carrier collisions. The overall light output is then computed with a standard model of charge transport for two particularly suitable types of carbon nanostructures, i.e., zigzag graphene nanoribbons and armchair single-wall nanotubes. Relatively sharp emission peaks at geometrically tunable terahertz frequencies are obtained in each case. The corresponding output powers are experimentally accessible even with individual nanowires, and can be scaled to technologically significant levels using array configurations. These radiation mechanisms therefore represent a promising paradigm for light emission in condensed matter, which may find important applications in nanoelectronics and terahertz photonics.DMR-1308659/National Science Foundationhttp://ultra.bu.edu/papers/Tantiwanichapan-2016-PRB-CNT-THz.pd

Herbicide/pesticide sensing with metamaterial absorber in THz regime

WOS:000702537600014Terahertz (THz) technology has been attracted great interest in many research areas over the years, especially THz plasmonics for sensing applications since intra- and inter-molecular vibrations are within the THz range. The sensitivity of free-space THz detection can be boosted up by use of metamaterials (MM), which are artificial structures in subwavelength scale of the incident light. These artificial materials present high electric field enhancement and high sensitivity to the change in their surroundings. In this article, residual herbicides/pesticides have been investigated both theoretically and experimentally with a polarization-insensitive THz metamaterial absorber (MMA) composed of dielectric-metal disk antennas. Our THz MMA can detect as small as 5 ppM of commonly used herbicide/pesticide, namely paraquat and glyphosate. The results show that sensitivity is greatly improved by using THz MMA, with the limit of detection (LOD) of these herbicides/pesticides reaching to 5 ppM. These results indicate that THz MMA platform could be a valuable method for highly sensitive THz applications in food quality and safety control. We believe, our sensor platform based on general c-mos technology fabrication could be a potential detection tool for herbicides/ pesticides residues in agriculture and food products

Improvement of response time and heat transfer capacity of metamaterial absorber for terahertz detector applications

WOS:000966786300001WOS:000968066000042We have introduced a THz metamaterial absorber system that consists of a metal disk antenna connected to successive dielectric and metal layers via a metal rod. The heat transfer speed and capacity of the proposed platform with a metal rod inserted through the dielectric layer have been numerically studied for the first time in this work. The proposed THz metamaterial absorber system can improve the heat transfer capacity 0.7 K/s compared to without a metal rod situation. The simulation proves that heat transfer can be achieved in the proposed absorber less than 10 μs times compared to without a metal rod case. The simulation results indicate the absorption strength of the metamaterial absorber is almost independent of the rod depth. This detector system will be remarkably beneficial for imaging applications where fast heat-signal conversion is necessary

A Computational Study on Performance Improvement of THz Signal from a Grating Photoconductive Antenna

A diffractive grating is a well-known optical component and is extensively used in many applications. This research work explores application of the diffractive grating in a photoconductive antenna (PCA) of a terahertz time domain spectroscopy (THz-TDS) system, by utilizing benefits of a sub-wavelength grating structure. The grating PCA structure was modeled and simulated by COMSOL Multiphysics software (COMSOL, Inc., Burlington, MA, USA). Performance of the proposed PCA design is studied in terms of its induced photocurrent. The effects of geometrical parameters of the grating are also investigated and analyzed through its optical and electrical responses. Thanks to the increase in absorption of the incident laser’s electric field, the simulation results show a 63% increment of the induced photocurrent in the grating PCA, compared with the conventional planar PCA

Graphene Terahertz Plasmons: A Combined Transmission Spectroscopy and Raman Microscopy Study

Paiella, Roberto/0000-0002-7183-6249; Swan, Anna/0000-0002-3978-7993WOS: 000408077800018Graphene provides a promising materials platform for fundamental studies and device applications in plasmonics. Here we investigate the excitation of THz plasmon polaritons in large-area graphene samples on standard oxidized silicon substrates, via diffractive coupling from an overlying periodic array of metallic nanoparticles. Pronounced plasmonic absorption features are measured, whose frequencies can be tuned across a large portion of the THz spectrum by varying the array period. At the same time, the ability to tune these resonances actively via electrostatic doping is found to be strongly limited by the presence of large carrier density variations across the sample area induced by the underlying SiO2, which are measured directly by Raman microscopy. These results highlight the importance of minimizing charge "puddles" in graphene plasmonic devices, e.g., through the use of more inert substrates, in order to take full advantage of their expected dynamic tunability for applications in THz optoelectronics.National Science FoundationNational Science Foundation (NSF) [DMR-1308659]; Royal Thai Government Fellowship; Turkey Ministry of Education Fellowship; Division of Materials ResearchNational Science Foundation (NSF)NSF - Directorate for Mathematical & Physical Sciences (MPS) [1411008] Funding Source: National Science FoundationThis work was supported by the National Science Foundation under Grant DMR-1308659. K.T. and H.D. acknowledge partial support by a Royal Thai Government Fellowship and by a Turkey Ministry of Education Fellowship, respectively

Demonstration of cross reaction in hybrid graphene oxide/tantalum dioxide guided mode resonance sensor for selective volatile organic compound

Abstract This paper experimentally demonstrates a crossed reaction of pure and hybrid graphene oxide (GO)/tantalum dioxide (TaO2) as a volatile organic compound (VOC) absorber in a guided mode resonance (GMR) sensing platform. The proposed GMR platform has a porous TaO2 film as the main guiding layer, allowing for more molecular adsorption and enhanced sensitivity. GO is applied on top as an additional VOC absorber to increase the selectivity. The hybrid sensing mechanism is introduced by varying the concentration of the GO aqueous solution. The experimental results show that the pure TaO2-GMR has a high tendency to adsorb most of the tested VOC molecules, with the resonance wavelength shifting accordingly to the physical properties of the VOCs (molecular weight, vapor pressure, etc). The largest signal appears in the large molecule such as toluene, and its sensitivity is gradually reduced in the hybrid sensors. At the optimum GO concentration of 3 mg/mL, the hybrid GO/TaO2 -GMR is more sensitive to methanol, while the pure GO sensor coated with GO at 5 mg/mL is highly selective to ammonia. The sensing mechanisms are verified using the distribution function theory (DFT) to simulate the molecular absorption, along with the measured functional groups measured on the sensor surface by the Fourier transform infrared spectroscopy (FTIR). The crossed reaction of these sensors is further analyzed by means of machine learning, specifically the principal component analysis (PCA) method and decision tree algorithm. The results show that this sensor is a promising candidate for quantitative and qualitative VOCs detection in sensor array platform

The Simulation of a Surface Plasmon Resonance Metallic Grating for Maximizing THz Sensitivity in Refractive Index Sensor Application

Nowadays, the simplicity of both designing and fabrication process of a terahertz (THz) resonator-based sensing technique leads to its ongoing development. The consumable THz resonator needs to be easily integrated into an existing terahertz time domain spectroscopy (THz TDS) measurement system. It should also be able to be fabricated in a mass scale with a low production cost. In this work, a metal-coated surface plasmon resonance- (SPR-) based sensor is simulated and designed as a low-cost refractive index sensor utilizing rigorous coupled wave analysis (RCWA). To demonstrate our methodology, we design a gold-coated grating with a polydimethylsiloxane (PDMS) as a substrate, in order to perform quantitative analysis of gasoline-toluene mixture composition, which has a refraction index variation of 0.1 at THz frequency. The grating period is tuned such that its surface plasmon resonance (SPR) frequency matches with the peak frequency of the THz TDS system. Moreover, other grating parameters, i.e., a filling factor and a grating depth, are optimized to increase the sensor sensitivity and sharpen the resonance dip. High sensitivity up to 500 GHz/RIU with a refractive index resolution up to 0.01 is numerically revealed. The H-field of the designed grating is then evaluated to indicate a strong SPR excitation. The well-developed designed grating introduces a promising, low-cost, and easily fabricated THz refractive index sensor