Abstract-Interband transitions in narrow-gap carbon nanotubes and graphene nanoribbons

R. R. Hartmann, V. A. Saroka, M. E. Portnoi

https://arxiv.org/abs/1903.10544


We use the robust nearest-neighbour tight-binding approximation to study on the same footing interband dipole transitions in narrow-bandgap carbon nanotubes and graphene nanoribbons. It is demonstrated that curvature effects in metallic single-walled carbon nanotubes and edge effects in gapless graphene nanoribbons not only open up bang gaps, which typically correspond to THz frequencies, but also result in a giant enhancement of the probability of optical transitions across these gaps. Moreover, the matrix element of the velocity operator for these transitions has a universal value (equal to the Fermi velocity in graphene) when the photon energy coincides with the band-gap energy. Upon increasing the excitation energy, the transition matrix element first rapidly decreases (for photon energies remaining in the THz range but exceeding two band gap energies it is reduced by three orders of magnitude), and thereafter it starts to increase proportionally to the photon frequency. A similar effect occurs in an armchair carbon nanotube with a band gap opened and controlled by a magnetic field applied along the nanotube axis. There is a direct correspondence between armchair graphene nanoribbons and single-walled zigzag carbon nanotubes. The described sharp photon-energy dependence of the transition matrix element together with the van Hove singularity at the band gap edge of the considered quasi-one-dimensional systems make them promising candidates for active elements of coherent THz radiation emitters. The effect of Pauli blocking of low-energy interband transitions caused by residual doping can be suppressed by creating a population inversion using high-frequency (optical) excitation.
https://arxiv.org/abs/1903.10544

Abstract-The detection of sub-terahertz radiation using graphene-layer and graphene-nanoribbon FETs with asymmetric contacts

Igor A. Maxim, V.Moskotin, Yakov E. Matyushkin, Maxim G. Rybin,  Elena D. Obraztsova, Victor I.Ryzhii, Gregory N. Goltsman, Georgy E. Fedorov, 

https://www.sciencedirect.com/science/article/pii/S2214785318321321

We report on the detection of sub-terahertz radiation using single layer graphene and graphene-nanoribbon FETs with asymmetric contacts (one is the Schottky contact and one – the Ohmic contact). We found that cutting graphene into ribbons a hundred nanometers wide leads to a decrease of the response to sub-THz radiation. We show that suppression of the response in the graphene nanoribbons devices can be explained by unusual properties of the Schottky barrier on graphene-vanadium interface

Abstract-Open-end discontinuity model of graphene nanoribbon transmission line in the terahertz band

Amin Ghahremani, Gholamreza Moradi, Reza Sarraf Shirazi,

https://www.sciencedirect.com/science/article/pii/S0030402617315516


Recently, attention has been focused on the design of GNR (Graphene Nanoribbon) based devices in the Terahertz band. Considering the effects of discontinuity in the structure of these devices is necessary as an inevitable factor in order to adjust the device response in the desired specifications. In this paper, open-end GNR transmission line, as one of the most important discontinuities in waveguide structures, is studied and modeled in detailed for the first time. This discontinuity effect is analysed using fullwave simulation in the form of an additional length added to the end of the line. Then it is expressed by simple mathematical expressions for a broad parameter range and its lumped element model is extracted. The obtained formula with a relative error of less than 10% is consistent with the results of fullwave simulation and it is also consistent with behavioral analysis of the structure.Finally, a bandstop filter is designed and simulated using quarter-wavelength open-end resonators. Then, discontinuity effect of the filter open-end lines is considered and completely confirmed. The frequency response of filter in the absence of discontinuity effect shows the cutoff frequency of 4.55 THz while by applying it, cutoff frequency is shifted to 5THz.

Abstract-First-principles study of the terahertz third-order nonlinear response of metallic armchair graphene nanoribbons

Yichao Wang and David R. Andersen

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.235430

We compute the terahertz third-order nonlinear conductance of metallic armchair graphene nanoribbons using time-dependent perturbation theory. Significant enhancement of the intrinsic third-order conductance over the result for instrinsic 2D single-layer graphene is observed over a wide range of temperatures. We also investigate the nonlinear response of extrinsic metallic acGNR with |EF|≪200meV. We find that the third-order conductance exhibits a strong Fermi level dependence at low temperatures. A third-order critical field strength of between ∼1 and 5kV/m is computed for the Kerr conductance as a function of temperature. For the third-harmonic conductance, the minimum critical field is computed to be ∼5kV/m.

• 
• 
• 
• 
• 
• 
• 

Terahertz Modulation of UV Light by Graphene Nano-ribbon

Simulating the feasibility of a terahertz radiation device

http://www.opli.net/opli_magazine/eo/2016/terahertz-modulation-of-uv-light-by-graphene-nano-ribbon-feb-news/

Yoshiyuki Miyamoto of Materials Interface Simulation Group, the Nanomaterials Research Institute, the National Institute of Advanced Industrial Science and Technology, in collaboration with Hong Zhang and Xinlu Cheng of Sichuan University and Angel Rubio, Max Planck Institute for Structure and Dynamics of Matters, theoretically presented terahertz (THz) modulation of UV light by a graphene nano-ribbon from computational simulation and proposed the application to a terahertz radiation device.

This simulation shows that the intensity of UV light passing through a graphene nano-ribbon is modulated with the frequency of terahertz. When such modulated UV light shines on a semiconductor which has a photo-conducting property, the semiconductor generates a photo-current whose intensity is modulated with a terahertz frequency. Therefore, such a photo-conducting semiconductor connected to an antenna is expected to be a source of terahertz radiation. This idea might lead to the production of compact terahertz-radiation devices which are useful for identification of organic compounds as well as observation of living matter.

The details of the current simulation have been published in Nanoscale, a journal published by the Royal Society of Chemistry (England).

Applications of graphene attract a lot of attention and graphene devices having highly conductive electron- and hole-carriers are being studied (AIST press release on December 11, 2012). However, for optical-devices, the highly conductive properties are not always beneficial. On the other hand, graphene nano-ribbons, which are obtained by cutting a graphene sheet into ribbons, have a band-gap and a semiconducting property, and light absorption/transmission properties of the graphene nano-ribbons have been studied.

Terahertz waves are known to be useful for identification of harmful substances and inspecting degradation of buildings. However, it is difficult to fabricate strong terahertz radiation devices with compact sizes and low cost.

Simulated total field (red) and optical field (blue) near the surface of the graphene nano-ribbon with photon energies of 6.20 eV and 6.53 eV, respectively

AIST is aiming at the acceleration of research and development of nano-scale materials by designing using computational simulations. By simulating dynamics of electrons and atoms in materials with first-principles calculations, electron dynamics of irradiated materials and optical responses of nano-scaled materials such as graphene were studied (AIST press release on March 18, 2015).

In this work, the application of graphene nano-ribbons was discussed by AIST and Sichuan University, and the usage of first-principles methods and data analysis were considered by the Max Planck Institute for Structure and Dynamics of Matters. Then AIST performed numerical calculations. This work was done with financial support by MEXT Grant-in-Aid for Scientific Research on Innovative Area, “Science of Atomic Layers (SATL),” (FY2013 - FY2017) and all computations were performed by using the Large-Scale Computer System in the Cybermedia Center of Osaka University.

In the present work, the researchers found terahertz modulation of the intensity of UV light passing through a graphene nano-ribbon by a simulation and proposed a terahertz radiation device using the discovered phenomenon. The graphene nano-ribbon, which is a one-dimensional material having a band-gap like semiconductors, is the current object. The edge of the graphene nano-ribbon was assumed to have an armchair structure with carbon atoms at the edge terminated by hydrogen atoms (Fig. 1). By performing a first-principles calculation based on the time-dependent density functional theory to simulate the irradiation of UV light with polarization vector shown in Fig. 1, oscillation of electrons running from one edge of the grapheme nano-ribbon to the other edge alternatively was computed. This suggests, that the oscillation of the electron cloud in the graphene nano-ribbon is following the oscillation of optical field of the UV light. If the eigen frequency of the electronic oscillation is close to that of the optical frequency, resonance is expected to occur. The first-principles simulation showed resonance of the optical field and the electron cloud with UV light irradiation (photon energy is around 6 eV), and periodic enhancement and decay in amplitude of the electron-cloud oscillation was computed.

Frequencies from 0.5 THz to 5 THz are practically useful region of terahertz radiation. Besides graphene nano-ribbons, further exploration will be made for new materials that can be used for radiation with frequencies from 0.5 THz to 5 THz. In the exploration, the wavelength of the incident light spanning UV, visible, and infrared regions will be the target.

Abstract-Quantum Size Effects in the Terahertz Nonlinear Response of Metallic Armchair Graphene Nanoribbons

Yichao Wang, David R. Andersen

(Submitted on 30 Mar 2016)

http://arxiv.org/abs/1603.09394

We use time dependent perturbation theory to study quantum size effects on the terahertz nonlinear response of metallic graphene armchair nanoribbons of finite length under an applied electric field. Our work shows that quantization due to the finite length of the nanoribbon, the applied field distribution, and the broadening of the graphene spectrum all play a significant role in the resulting nonlinear conductances. In certain cases, these effects can significantly enhance the nonlinearity over that for infinitely-long metallic armchair graphene nanoribbon.

Abstract-First-principles study of the terahertz third-order nonlinear response of metallic armchair graphene nanoribbons

Yichao Wang, David R. Andersen

(Submitted on 19 Mar 2016)

http://arxiv.org/abs/1603.06027

We compute the terahertz third-order nonlinear conductance of metallic armchair graphene nanoribbons using time-dependent perturbation theory. Significant enhancement of the intrinsic nonlinear third-order conductance over the result for intrinsic 2D single-layer graphene is observed over a wide range of temperatures and sample geometries. We also investigate the nonlinear response of extrinsic metallic acGNR with |Ef| much smaller than 200 meV. We find that the third-order conductance exhibits a strong Fermi level dependence at low temperatures. A third-order critical field strength of between roughly 1 and 5 kV/m is computed for the nonlinear Kerr terms as a function of temperature. For the third-harmonic terms, the minimum critical field is computed to be around 5 kV/m.

Abstract-Optical field terahertz amplitude modulation by graphene nanoribbons

Hong Zhang,*^a   Yoshiyuki Miyamoto,^b   Xinlu Cheng^c and  Angel Rubio^def  

http://pubs.rsc.org/en/content/articlelanding/2015/nr/c5nr05889a#!divAbstract

*

Corresponding authors

^a

College of Physical Science and Technology, Sichuan University, Chengdu 610065, China
E-mail: hongzhang@scu.edu.cn

^b

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba 305-8568, Japan

^c

Key Laboratory of High Energy Density Physics and Technology of Ministry of Education; Sichuan University, Chengdu, China

^d

Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany

^e

Nano-Bio Spectroscopy group, Universidad del País Vasco CFM CSIC-UPV/EHU-MPC DIPC, 20018 San Sebastian, Spain

^f

European Theoretical Spectroscopy Facility (ETSF)

Nanoscale, 2015, Advance Article

DOI: 10.1039/C5NR05889A






















In this study, first-principles time-dependent density functional theory calculations were used to demonstrate the possibility to modulate the amplitude of the optical electric field (E-field) near a semiconducting graphene nanoribbon. A significant enhancement of the optical E-field was observed 3.34 Å above the graphene nanoribbon sheet, with an amplitude modulation of approximately 100 fs, which corresponds to a frequency of 10 THz. In general, a six-fold E-field enhancement could be obtained, which means that the power of the obtained THz is about 36 times that of incident UV light. We suggest the use of semiconducting graphene nanoribbons for converting visible and UV light into a THz signal.

Abstract-Plasmonic bandpass filter based on graphene nanoribbon

Huawei Zhuang, Fanmin Kong, Kang Li, and Shiwei Sheng
https://www.osapublishing.org/ao/abstract.cfm?uri=ao-54-10-2558

A plasmonic bandpass filter based on graphene is proposed and numerically investigated using the finite-difference time-domain method. The proposed filter has a very simple structure, including two graphene nanoribbon waveguides laterally coupled to a graphene ribbon resonator. The transmission efficiency can be tuned by altering the coupling distance between the ribbons. At the same time, the variation of the transmission spectra is investigated by tuning the size of the graphene resonant ribbon. Notably, due to the unique electronic tunability of graphene, the transmission spectra can be freely tuned in a broad frequency range by choosing the chemical potential, which exhibits more flexible tunability than that used in conventional metallic devices. Attributed to the standing wave distribution of different modes excited in the graphene resonant ribbon, the proposed filter can be used for the plasmonic device with the capability of band selection or power splitting by locating the output waveguide ports in the suitable positions.

© 2015 Optical Society of America

Full Article  |  PDF Article

Abstract- Plasmonic bandpass filter based on graphene nanoribbon

Plasmonic bandpass filter based on graphene nanoribbon Huawei Zhuang, Fanmin Kong, Kang Li, and Shiwei Sheng  »View Author Affiliations

http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-54-10-2558

Applied Optics, Vol. 54, Issue 10, pp. 2558-2564 (2015)
http://dx.doi.org/10.1364/AO.54.002558

View Full Text Article

Enhanced HTML    Acrobat PDF (616 KB)

A plasmonic bandpass filter based on graphene is proposed and numerically investigated using the finite-difference time-domain method. The proposed filter has a very simple structure, including two graphene nanoribbon waveguides laterally coupled to a graphene ribbon resonator. The transmission efficiency can be tuned by altering the coupling distance between the ribbons. At the same time, the variation of the transmission spectra is investigated by tuning the size of the graphene resonant ribbon. Notably, due to the unique electronic tunability of graphene, the transmission spectra can be freely tuned in a broad frequency range by choosing the chemical potential, which exhibits more flexible tunability than that used in conventional metallic devices. Attributed to the standing wave distribution of different modes excited in the graphene resonant ribbon, the proposed filter can be used for the plasmonic device with the capability of band selection or power splitting by locating the output waveguide ports in the suitable positions.

© 2015 Optical Society of America

Bottom-Up Self Assembly of Graphene Holds Promise for Spintronics

                                    Image: Patrick Han
http://spectrum.ieee.org/nanoclast/semiconductors/materials/bottomup-self-assembly-of-graphene-holds-promise-for-spintronic-applications
By Dexter Johnson 

Not all graphene is alike. The way in which graphene is produced determines in large measure how it can be applied. The aim, of course, has been to produce the best quality graphene in large quantities.

However, these bulk production methods come at a price, which usually involves compromising those astounding electronic properties that make graphene so attractive in the first place.

Now researchers at the University of California Los Angeles (UCLA) and Tohoku University in Japan may have found a way around these limitations by abandoning “top-down” manufacturing techniques like lithography for a bottom-up approach in which the graphene nanoribbons self assemble exactly into the desired form.

The researchers were looking for a way to produce graphene nanoribbons that have the zigzag edges that give the material a strong magnetic property, making it attractive for spintronics. Spintronics exploits the way in which the spin of particles respond to magnetic fields so that the spin is either parallel or antiparallel to the magnetic field. These two possibilities make it useful for creating a digital signal that can be used in computing.

“To make devices out of graphene, we need to control its geometric and electronic structures,” said Paul Weiss of UCLA in a press release. “Making zigzag edges does both of these simultaneously, as there are some special properties of graphene nanoribbons with zigzag edges. Having these in hand will enable us to test theoretical predictions about them, such as magnetic properties.”

The typical lithographic method for producing graphene nanoribbons with these zigzag edges resulted in too many defects in the final product for the material to be useful.

In the research, which was published in the journal ACS Nano, the team exploited the properties of a copper substrate to alter the way the graphene precursor molecules reacted to each other as they assembled into graphene nanoribbons. With this method, the researchers were able to control the length, edge configuration, and location of the nanoribbons on the substrate.

This isn’t the first time that graphene nanoribbons were produced by self-assembly, but in earlier efforts the end results were bundles of ribbons that needed to go through another process to untangle them and position them in a device.

“Previous strategies in bottom-up molecular assemblies used inert substrates, such as gold or silver, to give molecules a lot of freedom to diffuse and react on the surface,” said Patrick Han of Tohoku University in the press release. “But this also means that the way these molecules assemble is completely determined by the intermolecular forces and by the molecular chemistry. Our method opens the possibility for self-assembling single-graphene devices at desired locations, because of the length and the direction control.”