Browsing by Author "Hong, Hao"

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  • Anomalous photovoltaics in Janus MoSSe monolayers
    (2025-01-09) Liu, Chang; Liang, Tianyu; Sui, Xin; Du, Lena; Guo, Quanlin; Xue, Guodong; Huang, Chen; You, Yilong; Yao, Guangjie; Zhao, Mengze; Yin, Jianbo; Sun, Zhipei; Hong, Hao; Wang, Enge; Liu, Kaihui
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The anomalous photovoltaic effect (APE) in polar crystals is a promising avenue for overcoming the energy conversion efficiency limits of conventional photoelectric devices utilizing p-n junction architectures. To facilitate effective photocarrier separation and enhance the APE, polar materials need to be thinned down to maximize the depolarization field. Here, we demonstrate Janus MoSSe monolayers (~0.67 nm thick) with strong spontaneous photocurrent generation. A photoresponsivity up to 3 mA/W, with ~ 1% external quantum efficiency and ultrafast photoresponse (~50 ps) were observed in the MoSSe device. Moreover, unlike conventional 2D materials that require careful twist alignment, the photovoltage can be further scaled up by simply stacking the MoSSe layers without the need for specific control on interlayer twist angles. Our work paves the way for the development of high-performance, flexible, and compact photovoltaics and optoelectronics with atomically engineered Janus polar materials.
  • Evidence of abnormal hot carrier thermalization at van Hove singularity of twisted bilayer graphene
    (2024-08-30) Shang, Nianze; Huang, Chen; Chen, Qing; Liu, Chang; Yao, Guangjie; Sun, Zhipei; Meng, Sheng; Liu, Kaihui; Hong, Hao
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Interlayer twist evokes revolutionary changes to the optical and electronic properties of twisted bilayer graphene (TBG) for electronics, photonics and optoelectronics. Although the ground state responses in TBG have been vastly and clearly studied, the dynamic process of its photoexcited carrier states mainly remains elusive. Here, we unveil the photoexcited hot carrier dynamics in TBG by time-resolved ultrafast photoluminescence (PL) autocorrelation spectroscopy. We demonstrate the unconventional ultrafast PL emission between the van Hove singularities (VHSs) with a ∼4 times prolonged relaxation lifetime. This intriguing photoexcited carrier behavior is ascribed to the abnormal hot carrier thermalization brought by bottleneck effects at VHSs and interlayer charge distribution process. Our study on hot carrier dynamics in TBG offers new insights into the excited states and correlated physics of graphene twistronics systems.
  • Giant enhancement of optical nonlinearity in two-dimensional materials by multiphoton-excitation resonance energy transfer from quantum dots
    (2021-07) Hong, Hao; Wu, Chunchun; Zhao, Zixun; Zuo, Yonggang; Wang, Jinhuan; Liu, Can; Zhang, Jin; Wang, Fangfang; Feng, Jiangang; Shen, Huaibin; Yin, Jianbo; Wu, Yuchen; Zhao, Yun; Liu, Kehai; Gao, Peng; Meng, Sheng; Wu, Shiwei; Sun, Zhipei; Liu, Kaihui; Xiong, Jie
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Colloidal quantum dots are promising photoactive materials that enable plentiful photonic and optoelectronic applications ranging from lasers, displays and photodetectors to solar cells1–9. However, these applications mainly utilize the linear optical properties of quantum dots, and their great potential in the broad nonlinear optical regime is still waiting for full exploration10–12. Here, we demonstrate that a simple coating of a sub-200-nm-thick quantum dot film on two-dimensional materials can significantly enhance their nonlinear optical responses (second, third and fourth harmonic generation) by more than three orders of magnitude. Systematic experimental results indicate that this enhancement is driven by a non-trivial mechanism of multiphoton-excitation resonance energy transfer, where the quantum dots directly deliver their strongly absorbed multiphoton energy to the adjacent two-dimensional materials by a remote dipole–dipole coupling. Our findings could expand the applications of quantum dots in many exciting areas beyond linear optics, such as nonlinear optical signal processing, multiphoton imaging and ultracompact nonlinear optical elements.
  • Nanoscale thickness Octave-spanning coherent supercontinuum light generation
    (2025) Das, Susobhan; Uddin, Md Gius; Li, Diao; Wang, Yadong; Dai, Yunyun; Toivonen, Juha; Hong, Hao; Liu, Kaihui; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Coherent broadband light generation has attracted massive attention due to its numerous applications ranging from metrology, sensing, and imaging to communication. In general, spectral broadening is realized via third-order and higher-order nonlinear optical processes (e.g., self-phase modulation, Raman transition, four-wave mixing, multiwave mixing), which are typically weak and thus require a long interaction length and the phase matching condition to enhance the efficient nonlinear light-matter interaction for broad-spectrum generation. Here, for the first time, we report octave-spanning coherent light generation at the nanometer scale enabled by a phase-matching-free frequency down-conversion process. Up to octave-spanning coherent light generation with a −40dB spectral width covering from ~565 to 1906 nm is demonstrated in discreate manner via difference-frequency generation, a second-order nonlinear process in gallium selenide and niobium oxide diiodide crystals at the 100-nanometer scale. Compared with conventional coherent broadband light sources based on bulk materials, our demonstration is ~5 orders of magnitude thinner and requires ~3 orders of magnitude lower excitation power. Our results open a new way to possibly create compact, versatile and integrated ultra-broadband light sources.
  • Twist Phase Matching in Two-Dimensional Materials
    (2023-12-08) Hong, Hao; Huang, Chen; Ma, Chenjun; Qi, Jiajie; Shi, Xuping; Liu, Can; Wu, Shiwei; Sun, Zhipei; Wang, Enge; Liu, Kaihui
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Optical phase matching involves establishing a proper phase relationship between the fundamental excitation and generated waves to enable efficient optical parametric processes. It is typically achieved through birefringence or periodic polarization. Here, we report that the interlayer twist angle in two-dimensional (2D) materials creates a nonlinear geometric phase that can compensate for the phase mismatch, and the vertical assembly of the 2D layers with a proper twist sequence generates a nontrivial "twist-phase-matching"(twist-PM) regime. The twist-PM model provides superior flexibility in the design of optical crystals, which can be applied for twisted layers with either periodic or random thickness distributions. The designed crystal from the twisted rhombohedral boron nitride films within a thickness of only 3.2 μm is capable of producing a second-harmonic generation with conversion efficiency of ∼8% and facile polarization controllability that is absent in conventional crystals. Our methodology establishes a platform for the rational design and atomic manufacturing of nonlinear optical crystals based on abundant 2D materials.