The "Three Monks" Effect of Graphene

Professor Wushiwei of the Department of Physics of Fudan University realized the electrical regulation of third-order nonlinear effects in graphene and revealed its mechanism. Their research results, "Electric Regulation of the Non-Quality Dirac Fermion Third Order Nonlinear Optical Response in Graphene," was published online on May 21, London time in "Natural Photonics." Liuweitao, a professor of Fudan Physics, is the co-author of this article, and Huangdi, a doctoral student of the subject group, is the co-author of this article.

Graphene is a honeycomb material consisting only of a single layer of carbon atoms. Due to its unique lattice and band structure, the carriers in graphene have no stationary mass, and the movement speed is fixed and can only change direction. Their behavior follows Dirac's relativistic quantum mechanical equation and is therefore also called massless Dirac fermions. Since its discovery, the special properties of graphene and the many novel and interesting phenomena it causes have been the focus of frontier scientific research. The nonlinear effect of graphene is an important part of it. Prior to this, the scientific community has noticed the strong third-order nonlinear effects of graphene, which has made graphene have great application potential in the fields of micronano photonics, laser industry, optical communication, quantum information and computing, and biological imaging. However, past experimental reports have failed to form a unified view of graphene third-order nonlinear coefficients. Different experimental results even have up to 6 orders of magnitude differences.

Wushiwei and others realized that the third-order nonlinear response in graphene is the overall effect formed by the collaborative competition of multiple quantum resonance transitions, and proposed the use of ion-gel technology to prepare graphene Field-effect transistor devices. The method of controlling the relevant quantum resonance transition channels by controlling the carrier density and chemical potential in graphene on a large scale. The researchers found that the chemical potential in graphene strongly affects its third-order nonlinear optical response, and that different third-order nonlinear effects have completely different dependence on the chemical potential. For third-order nonlinear effects of additive types, such as third-order harmonic and additive four-wave mixing, when the chemical potential is adjusted to close single photon and two-photon resonance channels, the third-order nonlinear effects will be greatly enhanced. In contrast, for the third-order nonlinear effects of subtraction types, such as the subtraction four-wave mixed frequency and the Guangxuekeer effect, the same switch on single-photon and two-photon channels will cause the third-order nonlinear effect. The intensity decreases dramatically. The microquantum theoretical calculation of graphene nonlinear optical coefficients further supports the above conclusions. Wushiwei explained: "The interference effect between these different quantum resonance transitions is similar to the well-known story of the Three Monks. In the case of low-doping graphene, there are three monks A, B, and C in the temple of the 'graphene addition third-order nonlinear effect'. The three men not only do not cooperate in fetching water, but also push each other out of laziness. The result is no water(the third-order nonlinear effect is very weak); Until one day, Monk A went out to the clouds(to increase the doping concentration), the stalemate between the three monks was broken, and the remaining two monks, although still dissatisfied with each other, could talk about it. Persuaded the monk B to carry water(the third-order nonlinear effect was enhanced); Later, the monk B also went out to travel(further increase the doping concentration), and the monk C was quiet, and he alone drank water(the third-order nonlinear effect was further enhanced). Interestingly, the three monks D, E, and F in the "Graphene Subtraction Third Order Nonlinear Effect" Temple are completely different situations. These three people are very emotional and compete for water. Therefore, it is completely easy to drink water(the third-order nonlinear effect is very strong), but later Heshang D and E gradually descended the edge(increasing the doping concentration), so the water carrying capacity has decreased(the third-order nonlinear effect has weakened). In a nutshell, there is a competition/cooperation mechanism between the quantum resonance transitions of graphene third-order nonlinear effects. As the chemical potential is regulated, the forces that dominate competition and cooperation will disappear, which eventually lead to the observation of research results. Interesting phenomenon. "

The research team revealed the dominant role of interference between transition channels in the nonlinear optical effects of graphene and clarified the source of differences in third-order nonlinear optical coefficients between different groups in the world. The way of controlling graphene nonlinear optical effects on a large scale by chemical potential is also pointed out. At the same time, the first electronic optical device based on the third-order nonlinear optical effect of graphene has been prepared. In view of the major breakthrough in graphene's large-scale low-cost growth and preparation in recent years, this electro-optic regulation mechanism is expected to provide a high-end application for the graphene industry.

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