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Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers

Maksymilian Kuna, Mateusz Raczyński, Julia Kucharek, Takashi Taniguchi, Kenji Watanabe, Tomasz Kazimierczuk, Wojciech Pacuski, Piotr Kossacki

Abstract

We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe$_2$) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically increases the effectiveness of the optical orientation of the emission associated with defect-bound localized excitons. We compare the obtained results with the earlier studies on the reference exfoliated monolayers, discussing both qualitative similarity as well as quantitative differences. Our observations are further supplemented by the results of time-resolved PL measurements, which confirm the pseudospin relaxation time of approximately 25 ps, a value significantly shorter than the $\approx$100 ps previously reported for mechanically exfoliated samples.

Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers

Abstract

We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically increases the effectiveness of the optical orientation of the emission associated with defect-bound localized excitons. We compare the obtained results with the earlier studies on the reference exfoliated monolayers, discussing both qualitative similarity as well as quantitative differences. Our observations are further supplemented by the results of time-resolved PL measurements, which confirm the pseudospin relaxation time of approximately 25 ps, a value significantly shorter than the 100 ps previously reported for mechanically exfoliated samples.

Paper Structure

This paper contains 7 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Sample and experimental setup
  3. Results and discussion
  4. Conclusion
  5. CRediT authorship contribution statement
  6. Declaration of competing interest
  7. Acknowledgements

Figures (3)

  • Figure 1: (a) Characteristic emission spectra at T=5 K under 700 nm pulsed excitation, highlighting the Local Exciton (LE) band Charged Exciton (CX) peaks (note the distortion of the CX peak due to the long-pass filter's spectral proximity). (b) Map of the polarization degree, with the profiling region delimited by solid black lines. (c) Mean polarization degree calculated from the marked region. (d) Plot of the $B_0$ (HWHM) of the observed dip versus detected photon energy, demonstrating a stable value of $B_0$ that indicates the FIPE effect is independent of the spectral region within the LE band. (e) Relative depth of the dip as a function of photon energy, showing a clear decrease to zero at the energy corresponding to the cessation of the FIPE effect.
  • Figure 2: (a) Time-resolved photoluminescence (PL), acquired using a streak camera at $B = 0 \, T$ in the two circular polarization detection configurations (co- and cross-polarized). The panel (b) shows the time-resolved polarization degree.
  • Figure 3: (a) Degree of polarization profiles for different temperatures, averaged over the detection energy range of $1570 \pm 3 \, \unit{meV}$. (b) Relative depth and (c)$B_0$ (HWHM) extracted from the profiles shown in (a).