Anisotropic Screening of Excitons in Van der Waals Materials

Ting-Hsuan Wu^1*, Chih-En Hsu^2,3, Rajesh Kumar Ulaganathan⁴, Raman Sankar¹, Zhenglu Li³, Chi-Cheng Lee², Chia-Seng Chang¹, Kung-Hsuan Lin¹

¹Institute of Physics, Academia Sinica, Taipei, Taiwan
²Department of Physics, Tamkang University, New Taipei, Taiwan
³Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, California, USA
⁴Centre for Nanotechnology, Indian Institute of Technology, Roorkee, India

* Presenter:Ting-Hsuan Wu, email:wthse0423@gate.sinica.edu.tw

Van der Waals (vdW) materials demonstrate optical anisotropy due to the fundamental differences between in-plane (IP) covalent bonds and out-of-plane (OP) vdW bonds. Specifically, the layered vdW material indium selenide (InSe) provides a unique platform for studying the optical properties of anisotropic excitons, as the optical transitions at the band edges involve both IP and OP dipoles. By using polarization-resolved absorption spectra, the energy difference between excitons with IP and OP responses in InSe is ~0.4 meV. Based on many-body perturbation theory, we reveal that excitons with IP and OP responses originate from distinct composite electronic states. Under excitation by femtosecond pulses, photocarriers screen the excitons and increase the energy difference between the IP and OP optical gap up to ~8 meV. This energy difference results from varying screening lengths in IP and OP directions and persists over 100 ps at 77 K, while it recovers faster within 100 ps at room temperature (RT). We also explored the role of carrier density in the anisotropic screening of excitons. As this anisotropic exciton screening arises from the anisotropy of dielectric constants, it is anticipated to be a universal phenomenon in vdW and other 2D materials.

Keywords: indium selenide, 2D layered materials, exciton dynamics, exciton binding energy reduction, ultrafast pump-probe spectroscopy