Browsing by Author "Zhou, Xu"

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    Extreme nonlinear strong-field photoemission from carbon nanotubes
    (NATURE PUBLISHING GROUP, 2019-12-01) Li, Chi; Chen, Ke; Guan, Mengxue; Wang, Xiaowei; Zhou, Xu; Zhai, Feng; Dai, Jiayu; Li, Zhenjun; Sun, Zhipei; Meng, Sheng; Liu, Kaihui; Dai, Qing; Department of Electronics and Nanoengineering; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; National Center for Nanoscience and Technology Beijing; Chinese Academy of Sciences; National University of Defense Technology; Peking University; Zhejiang Normal University
    Strong-field photoemission produces attosecond (10−18 s) electron pulses that are synchronized to the waveform of the incident light. This nonlinear photoemission lies at the heart of current attosecond technologies. Here we report a new nonlinear photoemission behaviour—the nonlinearity in strong-field regime sharply increases (approaching 40th power-law scaling), making use of sub-nanometric carbon nanotubes and 800 nm pulses. As a result, the carrier-envelope phase sensitive photoemission current shows a greatly improved modulation depth of up to 100% (with a total modulation current up to 2 nA). The calculations reveal that the behaviour is an interplay of valence band optical-field emission with charge interaction, and the nonlinear dynamics can be tunable by changing the bandgap of carbon nanotubes. The extreme nonlinear photoemission offers a new means of producing extreme temporal-spatial resolved electron pulses, and provides a new design philosophy for attosecond electronics and photonics.
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    Graphene photonic crystal fibre with strong and tunable light–matter interaction
    (Nature Publishing Group, 2019-08-12) Chen, Ke; Zhou, Xu; Cheng, Xu; Qiao, Ruixi; Cheng, Yi; Liu, Can; Xie, Yadian; Yu, Wentao; Yao, Fengrui; Sun, Zhipei; Wang, Feng; Liu, Kaihui; Liu, Zhongfan; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Peking University; Beijing Graphene Institute; University of California, Berkeley
    The integration of photonic crystal fibre (PCF) with various functional materials has greatly expanded the application regimes of optical fibre1–12. The emergence of graphene (Gr) has stimulated new opportunities when combined with PCF, allowing for electrical tunability, a broadband optical response and all-fibre integration ability13–18. However, previous demonstrations have typically been limited to micrometre-sized samples, far behind the requirements of real applications at the metre-scale level. Here, we demonstrate a new hybrid material, Gr–PCF, with length up to half a metre, produced using a chemical vapour deposition method. The Gr–PCF shows a strong light–matter interaction with ~8 dB cm−1 attenuation. In addition, the Gr–PCF-based electro-optic modulator demonstrates a broadband response (1,150–1,600 nm) and large modulation depth (~20 dB cm−1 at 1,550 nm) under a low gate voltage of ~2 V. Our results could enable industrial-level graphene applications based on this Gr–PCF and suggest an attractive platform for two-dimensional material-PCF.
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    Optical fibres with embedded two-dimensional materials for ultrahigh nonlinearity
    (Nature Publishing Group, 2020-12) Zuo, Yonggang; Yu, Wentao; Liu, Can; Cheng, Xu; Qiao, Ruixi; Liang, Jing; Zhou, Xu; Wang, Jinhuan; Wu, Muhong; Zhao, Yun; Gao, Peng; Wu, Shiwei; Sun, Zhipei; Liu, Kaihui; Bai, Xuedong; Liu, Zhongfan; Department of Electronics and Nanoengineering; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Chinese Academy of Sciences; Peking University; Beijing Institute of Technology; Fudan University
    Nonlinear optical fibres have been employed for a vast number of applications, including optical frequency conversion, ultrafast laser and optical communication1–4. In current manufacturing technologies, nonlinearity is realized by the injection of nonlinear materials into fibres5–7 or the fabrication of microstructured fibres8–10. Both strategies, however, suffer from either low optical nonlinearity or poor design flexibility. Here, we report the direct growth of MoS2, a highly nonlinear two-dimensional material11, onto the internal walls of a SiO2 optical fibre. This growth is realized via a two-step chemical vapour deposition method, where a solid precursor is pre-deposited to guarantee a homogeneous feedstock before achieving uniform two-dimensional material growth along the entire fibre walls. By using the as-fabricated 25-cm-long fibre, both second- and third-harmonic generation could be enhanced by ~300 times compared with monolayer MoS2/silica. Propagation losses remain at ~0.1 dB cm–1 for a wide frequency range. In addition, we demonstrate an all-fibre mode-locked laser (~6 mW output, ~500 fs pulse width and ~41 MHz repetition rate) by integrating the two-dimensional-material-embedded optical fibre as a saturable absorber. Initial tests show that our fabrication strategy is amenable to other transition metal dichalcogenides, making these embedded fibres versatile for several all-fibre nonlinear optics and optoelectronics applications.
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    Quiver-quenched optical-field-emission from carbon nanotubes
    (2017-09-25) Li, Chi; Zhou, Xu; Zhai, Feng; Li, Zhenjun; Yao, Fengrui; Qiao, Ruixi; Chen, Ke; Yu, Dapeng; Sun, Zhipei; Liu, Kaihui; Dai, Qing; Department of Micro and Nanosciences; Department of Electronics and Nanoengineering; Zhipei Sun Group; National Center for Nanoscience and Technology Beijing; Peking University; Zhejiang Normal University
    Carbon nanotubes (CNTs) enable large electric field enhancement for an extremely broad bandwidth spanning from the optical domain down to static fields. This is due to their high aspect ratio, small tip radius, and high structural stability. CNTs therefore represent an ideal model-system for the investigation of nonlinear and strong-field phenomena. In this paper, we extend the range of optical-field-emission materials from metal nanostructures to CNTs. Quiver-quenched optical-field-emission (i.e., the transition to a sub-cycle regime) is observed for CNTs tips in a short-wavelength laser field of 820 nm that requires a mid-infrared excitation field of conventional metal tips emitters. This special property relies on the ultrasmall tips radius (∼1 nm) and the high optical-field enhancement (∼21.6) properties of CNTs. This study suggests that CNTs are excellent candidates for optically driven ultrafast electron sources with both high spatial and high temporal coherence. They also provide more freedom for the manipulation and control of electron dynamics at the attosecond timescale, which extends the bandwidth of light-wave electronic devices.