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1. THz Induced Nonlinear Photocarrier Dynamics in Multilayer WSe2

   https://doi.org/10.1364/FIO.2023.FW5C.3  

   Guay, Viela; Johnson, Nathan; Zhang, Yue; van_der_Zande, Arend M; Thorp, Spencer G; Graham, Matthew; Lee, Yun-Shik (January 2023, Optica Publishing Group)

   We demonstrate that strong THz pulses enhance nonlinear THz absorption in optically excited WSe₂. The increase in THz conductivity indicates that the intense THz fields drive the photocarriers into sidebands of higher mobility. 

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2. Nonlinear photocarrier dynamics in multilayer WSe2 induced by intense terahertz pulses

   Guay, Viela; Johnson, Nathan; Zhang, Yue; van_der_Zande, Arend M; Thorp, Spencer G; Graham, Mattew; Lee, Yun-Shik (December 2023, Journal of the Optical Society of America B Online)

   Non-equilibrium photocarriers in multilayer WSe2 injected by femtosecond laser pulses exhibit extraordinary nonlinear dynamics in the presence of intense THz fields. The THz absorption in optically excited WSe2 rises rapidly in the low THz field regime and gradually ramps up at high intensities. The strong THz pulses drive the photocarriers into sidebands of higher mobility and release trapped charge carriers, which consequently enhance the transient conductivity of WSe2. The spectrally analyzed conductivity reveals distinctive features, indicating that the photocarriers undergo resonant interactions such as carrier-photon scattering. 

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3. Strong-field terahertz control of plasmon induced opacity in photoexcited metamaterial

   https://doi.org/10.1364/JOSAB.409224  

   Mousavian, Ali; Thompson, Zachary_J; Lee, Byounghwak; Bradley, Alden_N; Sprague, Milo_X; Lee, Yun-Shik (March 2021, Journal of the Optical Society of America B)

   A terahertz (THz) metamaterial consisting of radiative slot antennas and subradiant complementary split-ring resonators exhibits plasmon induced opacity in a narrow spectral range due to the destructive interference between the bright and dark modes of the coupled oscillators. Femtosecond optical excitations instantly quench the mode coupling and plasmon oscillations, injecting photocarriers into the metamaterial. The plasmon resonances in the coupled metamaterial are restored by intense THz pulses in a subpicoseond time scale. The strong THz fields induce intervalley scattering and interband tunneling of the photocarriers and achieve significant reduction of the photocarrier mobility. The ultrafast dynamics of the nonlinear THz interactions reveals intricate interplay between photocarriers and plasmon oscillations. The high-field THz control of the plasmon oscillations implies potential applications to ultrahigh-speed plasmonics. 

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4. Ultrafast Shift Current in SnS ₂ Single Crystals: Structure Considerations, Modeling, and THz Emission Spectroscopy

   https://doi.org/10.1002/adom.202400244  

   Kushnir_Friedman, Kateryna; Khanmohammadi, Sepideh; Morissette, Erin_M; Doiron, Curtis_W; Stoflet, Roy; Koski, Kristie_J; Grimm, Ronald_L; Ramasubramaniam, Ashwin; Titova, Lyubov_V (April 2024, Advanced Optical Materials)

   Abstract Above‐band gap optical excitation of non‐centrosymmetric semiconductors can lead to the spatial shift of the center of electron charge in a process known as shift current. Shift current is investigated in single‐crystal SnS₂, a layered semiconductor with the band gap of ≈2.3 eV, by THz emission spectroscopy and first principles density functional theory (DFT). It is observed that normal incidence excitation with above gap (400 nm; 3.1 eV) pulses results in THz emission from 2H SnS₂() polytype, where such emission is nominally forbidden by symmetry. It is argued that the underlying symmetry breaking arises due to the presence of stacking faults that are known to be ubiquitous in SnS₂single crystals and construct a possible structural model of a stacking fault with symmetry properties consistent with the experimental observations. In addition to shift current, it is observed THz emission by optical rectification excited by below band gap (800 nm; 1.55 eV) pulses but it requires excitation fluence more than two orders of magnitude higher to produce same signal amplitude. These results suggest that ultrafast shift current in which can be excited with visible light in blue–green portion of the spectrum makes SnS₂a promising source material for THz photonics. 

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5. Evidence for Topological States and a Lifshitz Transition in Metastable 2 M ‐WSe ₂

   https://doi.org/10.1002/adfm.202420356  

   He, Yangchen; Strasser, Alex; Hagopian, Nicholas; Bierman, Brenna; Ma, Hongrui; Fox, Carter; Li, Zizhong; Pederson, Nicholas; Taniguchi, Takashi; Watanabe, Kenji; et al (March 2025, Advanced Functional Materials)

   Abstract In recent years,T_dtransition metal dichalcogenides have been heavily explored for their type‐II Weyl topology, gate‐tunable superconductivity, and nontrivial edge states in the monolayer limit. Here, the Fermi surface characteristics and fundamental transport properties of similarly structured 2M‐WSe₂bulk single crystals are investigated. The measurements of the angular dependent Shubnikov–de Haas oscillations, with support from first‐principles calculations, reveal multiple three‐ and two‐dimensional Fermi pockets, one of which exhibits a nontrivial Berry's phase. In addition, it is shown that the electronic properties of 2M‐WSe₂are similar to those of orthorhombic MoTe₂and WTe₂, having a single dominant carrier type at high temperatures that evolves into coexisting electron and hole pockets with near compensation at temperatures below 100 K, suggesting the existence of a Lifshitz transition. Altogether, the observations provide evidence towards the topologically nontrivial electronic properties of 2M‐WSe₂and motivate further investigation on the topological properties of 2Mtransition metal dichalcogenides in the atomically thin limit. 

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