Browsing by Author "Sakai, Yuji"

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    Giant Terahertz-Wave Absorption by Monolayer Graphene in a Total Internal Reflection Geometry
    (American Chemical Society, 2017) Harada, Yoichi; Ukhtary, Muhammad Shoufie; Wang, Minjie; Srinivasan, Sanjay K.; Hasdeo, Eddwi H.; Nugraha, Ahmad R.T.; Noe, G. Timothy II; Sakai, Yuji; Vajtai, Robert; Ajayan, Pulickel M.; Saito, Riichiro; Kono, Junichiro
    We experimentally demonstrated significant enhancement of terahertz-wave absorption in monolayer graphene by simply sandwiching monolayer graphene between two dielectric media in a total internal reflection geometry. In going through this structure, the evanescent wave of the incident terahertz beam interacts with the sandwiched graphene layer multiple (up to four) times at varying incidence angles. We observed extremely large attenuation (up to ∼70% per reflection), especially for s-polarized radiation. The experimental results are quantitatively consistent with our calculations, where we modeled the experiment as an electromagnetic wave reflection process in monolayer graphene. We also derived analytical expressions for the absorptance, showing that the absorptance is proportional to the amount of Joule heating on the graphene surface induced by the terahertz radiation.
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    K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2
    (Springer Nature, 2022) Bae, Soungmin; Matsumoto, Kana; Raebiger, Hannes; Shudo, Ken-ichi; Kim, Yong-Hoon; Handegård, Ørjan Sele; Nagao, Tadaaki; Kitajima, Masahiro; Sakai, Yuji; Zhang, Xiang; Vajtai, Robert; Ajayan, Pulickel; Kono, Junichiro; Takeda, Jun; Katayama, Ikufumi
    In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.