Ultrafast carrier dynamics in 2D GeS in response to photoexcitation across the visible-NIR range

Sepideh Khanmohammadi; Catherine Tran; Erika Colin-Ulloa; Kateryna Kushnir; Kristie J. Koski; Lyubov V. Titova

doi:10.1117/12.2650672

15 March 2023 Ultrafast carrier dynamics in 2D GeS in response to photoexcitation across the visible-NIR range

Sepideh Khanmohammadi, Catherine Tran, Erika Colin-Ulloa, Kateryna Kushnir, Kristie J. Koski, Lyubov V. Titova

Proceedings Volume 12419, Ultrafast Phenomena and Nanophotonics XXVII; 124190M (2023) https://doi.org/10.1117/12.2650672
Event: SPIE OPTO, 2023, San Francisco, California, United States

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Abstract

Germanium sulfide (GeS) is a 2D semiconductor with high carrier mobility, a moderate band gap of about 1.6 eV, and highly anisotropic optical properties. In-plane anisotropy and a large in-plane spontaneous electric polarization in GeS monolayers have been predicted to result in significant second order nonlinear effects in response to above-the-gap excitation with photon energy < 2.5 eV¹. We have further confirmed it experimentally by demonstrating surface shift current generation in GeS using THz emission spectroscopy with 3.1 eV excitation.³ Here, we use time-resolved THz spectroscopy to investigate the dynamics and lifetimes of photoexcited carriers in GeS single crystals and nanoribbons in response to excitations with energies ranging from 1.5 eV, resonant with the bulk gap, to 3.1 eV. We find that resulting dynamics vary considerably. Lower energy (1.5 eV) excitation injects carriers directly into three low-lying valleys in the conduction band. Those carriers have long, which photoconductivity persisting for over 500 ps, as it can be seen in Fig. 1(a). On the other hand, injecting carriers high into the conduction band results in THz emission due to the shift current as well as into transient photoconductivity that recovers over <100 ps. Pronounced changes in the transient photoconductivity in response to optical excitation with photon energy across the visible-NIR range open intriguing possibilities for applications in ultrafast spectrally-sensitive photodetectors and solar energy conversion.

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Sepideh Khanmohammadi, Catherine Tran, Erika Colin-Ulloa, Kateryna Kushnir, Kristie J. Koski, and Lyubov V. Titova "Ultrafast carrier dynamics in 2D GeS in response to photoexcitation across the visible-NIR range", Proc. SPIE 12419, Ultrafast Phenomena and Nanophotonics XXVII, 124190M (15 March 2023); https://doi.org/10.1117/12.2650672

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