Exploiting interband transitions in graphene for optical harmonic generation

Conventional electro-optical applications based on bulk material rely on strong external fields to control light properties. But in low-dimensional materials, doping can affect the free carrier response to change the material properties and enable operations at weak fields.

The challenge is that free carrier responses are often limited to long-wavelength operations and accompanied by strong dissipation. However, in 2D materials like graphene, the optical transmittance is inherently high and a relatively small number of additional carriers could appreciably change the chemical potential for optical tuning. Ying Li et al. reviewed how adding free carriers to graphene can significantly alter the interband transitions to produce an entirely different optical tuning mechanism.

In one study, an ion gel was added in lieu of a doping agent for the extra-charge transport in an electrochemical transistor made of graphene on fused silica. When the chemical potential gradually shifted away from the charge neutral point as a function of electrical gating, the researchers expected the third harmonic generation (THG) to diminish as the one- and two-photon resonances sequentially shut off.

Instead, the THG became stronger because the phases of the one-, two- and three-photon transition pathways were different from each other: positive, negative and positive, respectively. Based on this quantum interference between the multi-photon transition pathways, the researchers determined that the tuning mechanism of graphene’s nonlinear optical properties is intrinsically different from most other systems.

“Intriguingly, the strongest response occurs when the linear absorption is blocked, which contrasts conventional materials, but is highly desirable for novel device applications,” author Weitao Liu said. “The phenomenon opens up the study of nonlinear optical processes in other low-dimensional materials as well.”

Source: “Tuning the optical nonlinearity of graphene,” by Ying Li, Hui Li, Shiwei Wu, and Wei-Tao Liu, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0019563 .