Polymer-free transfer of large area CVD-grown graphene film for THz application

We propose a polymer-free graphene transfer technique for chemical vapor deposition-grown graphene to ensure the intrinsic electrical properties of graphene for reliable transistor applications. The use of a metal catalyst as a supporting layer avoids contamination from the polymer material and graphene films become free of polymer residue after the transfer process. Atomic force microscopy and Raman spectroscopy indicate that the polymer-free transferred graphene shows closer properties to intrinsic graphene properties. The reliability of graphene field-effect transistors (GFETs) was investigated through the analysis of the negative gate bias-stress-induced instability. This work reveals the effect of polymer residues on the reliability of GFETs, and that the developed new polymer-free transfer method enhances the reliability.

Synthesis of Large-Area Single-Crystal Graphene

Frank-van der Merwe growth in bilayer graphene

Photonic terahertz technology

Abstract

Potentiality of Impact Avalanche Transit Time (IMPATT) devices based on different semiconductor materials such as InP, 4H-SiC, and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz (THz) frequencies. Drift-diffusion model is used to design double-drift region (DDR) IMPATTs based on above mentioned materials at different millimeter-wave (mm-wave) and THz frequencies and the upper cut-off frequency limits of those devices are obtained from the avalanche response times at those mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of both InP and 4H-SiC DDR IMPATTs are 1.0 THz; whereas, the same is 5.0 THz in Wz-GaN DDR IMPATTs. The Wz-GaN DDR IMPATTs emerge as the most suitable devices for generation of THz frequencies due to their small avalanche response time, high DC to RF conversion ratio, and sufficiently high RF power output at THz frequencies. But, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs due to their higher output power densities. Thus, the wide bandgap semiconductors such as Wz-GaN and 4H-SiC are highly suitable materials for DDR IMPATTs at both mm-wave and THz frequency ranges.

In the aim to get high quality graphene films, with large domains and free from impurities, minimizing also the manufacturing costs, we investigate the graphene grown on copper (Cu) foil by chemical vapor deposition at ambient pressure conditions, by using methane (CH4) as carbon source, diluted in a suitable mixture of argon (Ar) and hydrogen (H2). Several graphene samples were synthesized, for variable exposure times to hydrocarbon precursor, in the range from 1 min to 1 hr. The quality of the graphene films and their structural, morphological, and electronic properties were evaluated by micro‐Raman spectroscopy and other techniques, including, scanning tunneling microscopy, atomic force microscopy, and scanning electronic microscopy. In particular, samples obtained with shorter growth time (less than 10 min) exhibit a non‐uniform coverage of the Cu surface, whereas those synthesized with exposure time between 10 and 30 min show a prevalence of well‐ordered monolayer graphene domains. For longer deposition, the amount of disordered domains increases, as revealed by Raman analysis, and the resulting film shows a nonself‐limiting growth behavior for chemical vapor deposition at atmospheric conditions. In addition, we observed 2 kinds of monolayer graphene, in terms of coupling with the Cu surface, for the samples synthesized between 10 and 30 min. To the best of our knowledge, “coupled” and “decoupled” graphene regions have never been reported at the same time on Cu surface. Furthermore, a Raman statistical analysis has been performed on the G and 2D bands measured in both the kinds of regions, gaining evidence of a bimodal behavior for the graphene spots, corresponding to “coupled” and “decoupled” configurations. This difference, which is appreciable also by the optical microscopy inspection, could be related to the local Cu oxidation and to oxygen intercalation after graphene growth.

Deposition of multi-layer graphene (MLG) film on glass slide by flame synthesis technique

Wireless communication at the terahertz (THz) frequency bands (0.1–10 THz) is viewed as one of the cornerstones of tomorrow’s 6G wireless systems. Owing to the large amount of available bandwidth, if properly deployed, THz frequencies can potentially provide significant wireless capacity performance gains and enable high-resolution environment sensing. However, operating a wireless system at high-frequency bands such as THz is limited by a highly uncertain and dynamic channel. Effectively, these channel limitations lead to unreliable intermittent links as a result of an inherently short communication range, and a high susceptibility to blockage and molecular absorption. Consequently, such impediments could disrupt the THz band’s promise of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">high-rate communications and high-resolution sensing</i> capabilities. In this context, this paper panoramically examines the steps needed to efficiently and reliably deploy and operate next-generation THz wireless systems that will synergistically support a fellowship of communication and sensing services. For this purpose, we first set the stage by describing the fundamentals of the THz frequency band. Based on these fundamentals, we characterize and comprehensively investigate <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">seven unique defining features of THz wireless systems</i> : 1) Quasi-opticality of the band, 2) THz-tailored wireless architectures, 3) Synergy with lower frequency bands, 4) Joint sensing and communication systems, 5) PHY-layer procedures, 6) Spectrum access techniques, and 7) Real-time network optimization. These seven defining features allow us to shed light on how to re-engineer wireless systems as we know them today so as to make them ready to support THz bands and their unique environments. On the one hand, THz systems benefit from their quasi-opticality and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">can turn every communication challenge into a sensing opportunity</i> , thus contributing to a new generation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">versatile wireless systems</i> that can perform multiple functions beyond basic communications. On the other hand, THz systems can capitalize on the role of intelligent surfaces, lower frequency bands, and machine learning (ML) tools to guarantee a robust system performance. We conclude our exposition by presenting the key THz 6G use cases along with their associated major challenges and open problems. Ultimately, the goal of this article is to chart a forward-looking roadmap that exposes the necessary solutions and milestones for enabling THz frequencies to realize their potential as a game changer for next-generation wireless systems.

We have demonstrated a high-energy and broadly tunable monochromatic terahertz (THz) source via difference frequency generation (DFG) in DAST crystal. The THz frequency is tuned randomly in the range of 0.3-19.6 THz, which is much wider than the THz source based on the inorganic crystal and the photoconductive antenna. The highest energy of 2.53μJ/pulse is obtained at 18.9 THz corresponding to the optical-to-optical conversion efficiency of 1.31×10-4. The THz output spectroscopy is theoretically and experimentally explained by DFG process and Raman spectroscopy. Meanwhile, a phenomenon of blue light from the KTP-OPO with tunable and multiple wavelengths was firstly observed and explained. Based on our THz source, an ultra-wideband THz frequency domain system (THz-FDS) with transmission mode is realized to measure the ultra-wideband THz spectroscopies of typical materials in solid and liquid states, such as Si, SiC, White PE, water, isopropyl myristate, simethicone, atonlein and oleic acid, etc.. Furthermore, we have studied the THz spectral characteristic of biomedical tissue in the ultra-wideband THz frequency range of 0.3-15THz to study the biomedical response in the entire THz frequency range, which contains more abundant spectral information and was rarely focused with the limit of the THz source.

High input intensities are usually required to efficiently excite optical nonlinear effects in ultrathin structures. This problem is particularly critical at terahertz (THz) frequencies because high input power THz sources are not available. The demonstration of enhanced nonlinear effects at THz frequencies is particularly important since these nonlinear mechanisms promise to play a significant role in the development and design of new reconfigurable planar THz nonlinear devices. In this work, we present a novel class of ultrathin nonlinear hybrid planar THz devices based on graphene-covered plasmonic gratings exhibiting very large nonlinear response. The robust localization and enhancement of the electric field along the graphene monolayer, combined with the large nonlinear conductivity of graphene, can lead to boosted third harmonic generation (THG) and four-wave mixing (FWM) nonlinear processes at THz frequencies. These interesting nonlinear effects exhibit very high nonlinear conversion efficiencies and are triggered by realistic input intensities with relative low values. In addition, the THG and FWM processes can be significantly tuned by the dimensions of the proposed hybrid structures, the doping level of graphene, or the input intensity values, whereas the nonlinear radiated power remains relatively insensitive to the incident angle of the excitation source. The presented nonlinear hybrid graphene-covered plasmonic gratings have a relative simple geometry and can be used to realize efficient third-order THz effects with a limited fabrication complexity. Several new nonlinear THz devices are envisioned based on the proposed hybrid nonlinear structures, such as frequency generators, all-optical signal processors, and wave mixers. These devices are expected to be useful for nonlinear THz spectroscopy, noninvasive THz subwavelength imaging, and THz communication applications.

Graphene based microstrip antenna for triple and quad band operation at terahertz frequencies

Scattering-type scanning near-field microscopy (s-SNOM) at terahertz (THz) frequencies could become a highly valuable tool for studying a variety of phenomena of both fundamental and applied interest, including mobile carrier excitations or phase transitions in 2D materials or exotic conductors. Applications, however, are strongly challenged by the limited signal to noise ratio. One major reason is that standard atomic force microscope (AFM) tips, which have made s-SNOM a highly practical and rapidly emerging tool, provide weak scattering efficiencies at THz frequencies. Here we report a combined experimental and theoretical study of commercial and custom-made AFM tips of different apex diameter and length, in order to understand signal formation in THz s-SNOM and to provide insights for tip optimization. Contrary to common beliefs, we find that AFM tips with large (micrometer-scale) apex diameter can enhance s-SNOM signals by more than one order of magnitude, while still offering a spatial resolution of about 100 nm at a wavelength of 119 micron. On the other hand, exploiting the increase of s-SNOM signals with tip length, we succeeded in sub-15 nm resolved THz imaging employing a tungsten tip with 6 nm apex radius. We explain our findings and provide novel insights into s-SNOM via rigorous numerical modeling of the near-field scattering process. Our findings will be of critical importance for pushing THz nanoscopy to its ultimate limits regarding sensitivity and spatial resolution.

Graphene has received great interest because of its peculiar band structure and excellent physical properties. But today, the development of graphene is limited to its size and quality. In this paper, single- and multilayer graphene films were synthesized on copper foils by chemical vapor deposition(CVD) using methane at ambient pressure. Experiment results find the high temperature, low concentration of methane gas, shorter growth time and suitable gas flow are the key to get high-quality and large-scale graphene films. Raman spectra, scanning electron microscope(SEM) and transmission electron microscope(TEM) characterization indicate the graphene films are mostly single-layer, only with rare area having multilayer around copper boundaries. Further electrical tests show the graphene films grown by CVD method represent semiconductor behaviors under low temperature and the sheet resistance of graphene films is decreasing with the external magnetic field increasing.

The generation and the detection of the THz radiation in the frequency range of 0.1 to 10 THz have attracted much research interest, which was mainly driven by the particular applications of THz time-domain spectroscopy (TDS) in physics, chemistry, nano and life sciences. THz TDS has shown many advantages due to its non-destructive analysis feature in characterizing complex dielectric properties and understanding the carrier-dynamics in various semiconductor materials. Extensive studies have been reported on the THz dielectric and optical properties such as electro-optical effects, optical rectification, and birefringence. In addition, the ultrafast relaxation and recombination dynamics of photogenerated carriers were investigated for new materials such as carbon nanotube, graphene, and perovskites using optical pump THz-probe spectroscopy. Recently, the terahertz (THz) quantum cascade lasers and intersubband transition devices has attracted considerable attention based on AlGaN/GaN quantum well structure for far-infrared applications. It is necessary to investigate the optical and dielectric properties of GaN and sapphire crystals in the far IR as well as THz frequency regions. In the previous report, nonpolar and semipolar GaN crystals in the hexagonal crystal system are found to show multi-phonon mode behavior in the far-infrared frequency region. In addition, the polarized IR reflectance response of sapphire is reported to exhibit the dependence on the crystal orientation. In this paper, we investigated the optical properties and dielectric response of nonpolar a-plane GaN (a-GaN) films in the terahertz (THz) frequency using polarized femto-second THz TDS. The phase separation and amplitude ratio were measured by changing the crystal orientation with respect to the polarization of the incident THz pulse. In sharp contrast with polar c-plane GaN, nonpolar a-GaN films exhibited the angle dependence of the THz absorptions at room temperature, presumably due to the birefringence in nonpolar a-GaN films. The transmitted THz signal intensity, the time delay, and the Fourier-transformed amplitude were investigated by rotating the azimuth angle of the crystal orientation. Strong THz field absorptions around 1 THz frequency were observed at the azimuth angle of 0°, 90°, and 180° for a-GaN films. We believe that the birefringence in the THz frequency region originates from the difference of the transverse-optical and the longitudinal optical phonon splitting between the optical phonon branches.