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1. Nonlinear photocarrier dynamics in multilayer W S e ₂ induced by intense terahertz pulses

   https://doi.org/10.1364/JOSAB.504204  

   Guay, Viela; Johnson, Nathan; Zhang, Yue; van_der_Zande, Arend_M; Thorp, Spencer_G; Graham, Matthew; Lee, Yun-Shik (November 2023, Journal of the Optical Society of America B)

   Non-equilibrium photocarriers in multilayer WSe₂injected by femtosecond laser pulses exhibit extraordinary nonlinear dynamics in the presence of intense THz fields. The THz absorption in optically excited WSe₂rises 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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2. 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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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. Cavity-Assisted Coherent Phonon Generation and Control in a WSe ₂ /Au Structure

   https://doi.org/10.1021/acs.jpclett.5c01190  

   Brennan, Christian; Jo, Kiyoung; Wang, Chih-Feng; Joly, Alan G; O’Callahan, Brian T; Farias, William Edward; Bielicki, Alexander Donald; Teklu, Alem; Shi, Leilei; Zhang, Qian; et al (June 2025, The Journal of Physical Chemistry Letters)

   Coherent phonons in the Terahertz (THz) regime have gained attention as potential candidates for next-generation high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. Nevertheless, achieving efficient control of the phonon generation dynamics over THz coherent phonons continues to pose a considerable challenge. In this work, we explore THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of WSe2 on Au (WSe2/Au) and Si (WSe2/Si) by using time-resolved pump–probe spectroscopy. The generation of THz coherent phonons was studied as a function of the WSe2 layer thickness and laser wavelength. Notably, a significant enhancement in THz coherent phonon generation was observed in the WSe2/Au structure, but only within a specific range of WSe2 thicknesses and laser wavelengths. The results from numerical simulations, which consider a self-hybridized optical cavity depending on WSe2 thickness and optical reflectance and Raman spectroscopy measurements, all align well with the time-domain observations of THz coherent phonon generation. We propose that the observed enhancement in THz coherent phonon generation is strongly influenced by light–matter interactions in the WSe2 cavity, a mechanism that may be applicable to a broader range of vdW materials. These findings offer promising insights for the development of THz phononic or phonon-integrated devices. 

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5. Tailoring ultrafast carrier dynamics in GeS and GeSe via Cu intercalation

   https://doi.org/10.1109/IRMMW-THz57677.2023.10299227  

   Khanmohammadi, Sepideh; Friedman, Kateryna Kushnir; Tran, Catherine; Kastuar, Srihari; Colin-Ulloa, Erika; Ekuma, Chinedu; Koski, Kristie J; Titova, Lyubov V (September 2023, IEEE)

   Germanium sulfide (GeS) and germanium selenide (GeSe) are layered 2D van der Waals materials that belong to a family of group-IV monochalcogenides. These semiconductors have high carrier mobilities and moderate band gaps in the near infrared. Additionally, we have demonstrated that above gap photoexcitation results in ultrafast surface photocurrents and emission of THz pulses due to a spontaneous ferroelectric polarization that breaks inversion symmetry in the monolayer. Beyond the sub-picosecond time scales of shift currents, photoexcited carriers in both materials result in long-lived transient conductivity. We find that 800 nm excitation results in longer lived free photocarriers, persisting for hundreds of picoseconds to several nanoseconds, compared to tens to hundreds of picoseconds lifetimes for 400 nm excitation. Here, we report on tailoring the free photoexcited carrier lifetimes by intercalation of zero-valent Cu into the van der Waals gaps of GeS and GeSe. Density functional theory calculations predict that Cu atoms introduce mid-gap states. We demonstrate that intercalating only ∼3 atomic % of zero-valent Cu reduces the carrier lifetime by as much as two-to-four-fold, raising the prospects of these materials being used for high-speed optoelectronics. 

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