Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene

Abstract

Twisted bilayer graphene (tBLG) has received substantial attention in various research fields due to its unconventional physical properties originating from Moiré superlattices. The electronic band structure in tBLG modified by interlayer interactions enables the emergence of low-energy van Hove singularities in the density of states, allowing the observation of intriguing features such as increased optical conductivity and photocurrent at visible or near-infrared wavelengths. Here, we show that the third-order optical nonlinearity can be considerably modified depending on the stacking angle in tBLG. The third-harmonic generation (THG) efficiency is found to significantly increase when the energy gap at the van Hove singularity matches the three-photon resonance of incident light. Further study on electrically tuneable optical nonlinearity reveals that the gate-controlled THG enhancement varies with the twist angle in tBLG, resulting in a THG enhanced up to 60 times compared to neutral monolayer graphene. Our results prove that the twist angle opens up a new way to control and increase the optical nonlinearity of tBLG, suggesting rotation-induced tuneable nonlinear optics in stacked two-dimensional material systems.

High-Speed Imaging of Second-Harmonic Generation in MoS2 Bilayer under Femtosecond Laser Ablation

“…We could not observe extraordinary signals, for example, higher than four times that of the monolayer even when there was a strong coupling between the layers, as confirmed by the PL measurements. This is contrary to the behavior of the twisted bilayer graphene, which exhibits a large enhancement in the third-harmonic generation [37]. In addition, the peak angle (ϕ s ) in the twisted region exhibits a value that follows the relation ϕ s = (ϕ 1ϕ 2 )/2, as indicated by the dashed line in Figure 3c.…”

Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

“…After the discovery of graphene, it has been widely studied and investigated in various scientific research fields because of its distinctive nonlinear optical properties [31,54,136]. The features of broadband optical absorption, ultrafast carrier recovery, gapless band structure, and high damage threshold make graphene one of the leading SAs in the application of fiber laser lasers.…”

“…All of these LD-based tunable devices could also be investigated for various in-line all-fiber devices, such as ultrafast all-optical tunable switchers, optical limiters, all-optical modulators, polarizers etc., based on electrically/optically controlled nonlinear optical properties (i.e., higher-order susceptibility/nonlinearity, multiphoton absorption, etc.). The twist-angle in Bi-layer LD materials also can be engineered along with the externally (electrically and/or optically) controlled scheme to find the enhanced higher-order optical nonlinearity and SA properties for high performance, ultrafast and precisely controlled fiber lasers, and other nonlinear optical devices [54]. LD SA-based fs mode-locked ultrafast fiber lasers have grown emerging interest in space-borne applications, as they can sustain more than the life span of satellites [136,208,209].…”

“…2D materials have solid covalent bonding in a layer and weak interlayer van der Waals force. The electrons' dynamic is limited in the 2D structure if there is no interference of interlayer interaction, which provides 2D materials with numerous novel optical and electrical characteristics [53][54][55]. Graphene is the earliest monoatomic 2D layer material revealed, with remarkable optical, electrical, mechanical, and thermal behaviors [40][41][42][43][44][45].…”

Theoretical studies on optical absorption in twisted bilayer graphene under vertical electric field

“…会改变原有石墨烯晶格的周期性和对称性，晶格结构出现 moiré图案 [8] ；层间周 期性势场得到调制，从而改变了载流子能态，费米面附近的电子能态密度出现范 霍夫奇点(van Hove singularities,简记为 VHS ) [9,10] ；原有能带可能发生杂化而形 成平带 [11] 。因而在扭转石墨烯材料中会出现霍夫斯坦德蝴蝶、超导、量子化反常 霍尔态等各种新奇的物理现象 [12][13][14][15] 。扭转石墨烯体系的物理性质成为当前科学研 究的前沿与热点问题之一。 扭转角度是对石墨烯光学性质宽波段可调谐的又一个新的参量。非扭转石墨 烯材料由于其中心对称性原本不具有偶数阶非线性光学效应，但 2020 年美国加 州大学伯克利分校 Jie Yao 课题组发现当扭转角度 6°时，扭转双层石墨烯 (twisted bilayer graphene，简记为 tBLG)的非线性光学极化率可以达到 2.8×10 5 pm 2 /V，其数值已经可以和单层 MoS 2 等具有很强光学非线性的材料相比拟 [16] ； Xia Fengnian 课题组研究了小角度下 tBLG 在 5-12 m 波段的光学响应，发现当 °时，光学响应可以达到 26 mA/W [17] ；北京大学 Liu Zhongfan 研究小组 测量了 tBLG 与等离子体集成器件中的光电流，发现可以通过调节扭转角度达到 跃迁共振，从而使光电流增强 80 倍 [18] ； 韩国 Yeom 团队测量了 tBLG 材料中三 次谐波效应的非线性光学极化率，发现比在单层石墨烯中增强了 60 倍，并且利 用调节扭转角度使三阶非线性光学效应得到增强 [19] 。理论方面，Moon、Nicol、 Yu 以及 Stauber 等人分别采用第一性原理、 连续近似模型或者紧束缚方法对 tBLG 的能带结构以及光电导、光吸收等性质进行了理论研究，去探究扭转角度对石墨 烯光学性质宽波段可调谐的物理机理 [20][21][22][23][24]…”