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Paving the way for tunable graphene plasmonic THz amplifiers

Posted By Graphene Council, Wednesday, September 9, 2020
Tohoku University Professor Taiichi Otsuji has led a team of international researchers in successfully demonstrating a room-temperature coherent amplification of terahertz (THz) radiation in graphene, electrically driven by a dry cell battery.

Roughly 40 years ago, the arrival of plasma wave electronics opened up a wealth of new opportunities. Scientists were fascinated with the possibility that plasma waves could propagate faster than electrons, suggesting that so-called "plasmonic" devices could work at THz frequencies. However, experimental attempts to realize such amplifiers or emitters remained elusive.

"Our study explored THz light-plasmon coupling, light absorption, and amplification using a graphene-based system because of its excellent room-temperature electrical and optical properties," said Professor Otsuji who is based at the Ultra-Broadband Signal Processing Laboratory at Tohoku University's Research Institute of Electrical Communication (RIEC).

The research team, which consisted of members from Japanese, French, Polish and Russian institutions, designed a series of monolayer-graphene channel transistor structures. These featured an original dual-gathering gate that worked as a highly efficient antenna to couple the THz radiations and graphene plasmons.

Using these devices allowed the researchers to demonstrate tunable resonant plasmon absorption that, with an increase in current, results in THz radiation amplification. The amplification gain of up to 9% was observed in the monolayer graphene -- far beyond the well-known landmark level of 2.3% that is the maximum available when photons directly interact with electrons without excitation of graphene plasmons.

To interpret the results, the research team used a dissipative plasmonic crystal model, capturing the main trends and basic physics of the amplification phenomena. Specifically, the model predicts the increase in the channel dc current that drives the system into an amplification regime. This indicates that the plasma waves may transfer the dc energy into the incoming THz electromagnetic waves in a coherent fashion.

"Because all results were obtained at room temperature, our experimental results pave the way toward further THz plasmonic technology with a new generation of all-electronic, resonant, and voltage-controlled THz amplifiers," added Professor Otsuji.

Tags:  Battery  Graphene  Taiichi Otsuji  Tohoku University 

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Clarification of a new synthesis mechanism of semiconductor atomic sheet

Posted By Graphene Council, Tuesday, September 24, 2019
In Japan Science and Technology Agency's Strategic Basic Research Programs, Associate Professor Toshiaki Kato and Professor Toshiro Kaneko of the Department of Electronic Engineering, Graduate School of Engineering, Tohoku University succeeded in clarifying a new synthesis mechanism regarding transition metal dichalcogenides (TMD)1), which are semiconductor atomic sheets having thickness in atomic order.

Because it is difficult to directly observe the aspect of the growing process of TMD in a special environment, the initial growth process remained unclear, and it has been desirable to elucidate a detailed mechanism of synthesis to obtain high-quality TMD.

An in-situ observing synthesis method2) has been developed by our research group to examine the growth aspect of TMD as a real-time optical image in a special high temperature atmosphere of about 800°C in the presence of corrosive gases. In addition, a synthesis substrate, which is a mechanism to control diffusion during the crystal growth of a precursor3), has been developed in advance; further, it has been clarified that the growing precursor diffuses a distance about 100 times larger than in conventional semiconductor materials. 

It was also demonstrated that nucleation occurs due to the involvement of the precursor in a droplet state. Furthermore, by utilizing this method, a large-scale integration of more than 35,000 monolayer single crystal atomic sheets has been achieved on a substrate in a practical scale (Figure 1).

Utilizing the results of the present research, the large-scale integration of atomic-order thick4) semiconductor atomic sheets can be fabricated and is expected to be put into practical use in the field of next-generation flexible electronics.

Tags:  Electronics  Graphene  Semiconductor  Tohoku University  Toshiaki Kato  Toshiro Kaneko 

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