Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in $2\mathrm{H}\text{\ensuremath{-}}{\mathrm{WSe}}_{2}$

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Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in $2 H − {WSe}_{2}$

Mauro Fanciulli, David Bresteau, Jérôme Gaudin, Shuo Dong, Romain Géneaux, Thierry Ruchon, Olivier Tcherbakoff, Ján Minár, Olivier Heckmann, Maria Christine Richter, Karol Hricovini, and Samuel Beaulieu

Phys. Rev. Lett. 131, 066402 – Published 10 August 2023

Abstract

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy of excitons prepared by photoexcitation of inversion-symmetric $2 H − {WSe}_{2}$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most ${WSe}_{2}$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons’ hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent $K − K^{'}$ valleys leads to a decay of bright excitons’ hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

Received 17 May 2023
Accepted 6 July 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.066402

Physics Subject Headings (PhySH)

Phonons Photoinduced effect Spin-orbit coupling Spintronics Valley degrees of freedom Valleytronics

Angle-resolved photoemission spectroscopy Spin-resolved photoemission spectroscopy Time & angle resolved photoemission spectroscopy Ultrafast femtosecond pump probe

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Mauro Fanciulli ^1,2,*, David Bresteau ², Jérôme Gaudin ³, Shuo Dong⁴, Romain Géneaux², Thierry Ruchon ², Olivier Tcherbakoff², Ján Minár⁵, Olivier Heckmann^1,2, Maria Christine Richter ^1,2, Karol Hricovini^1,2,†, and Samuel Beaulieu ^3,‡

¹Laboratoire de Physique des Matériaux et Surfaces, CY Cergy Paris Université, 95031 Cergy-Pontoise, France
²Université Paris-Saclay, CEA, CNRS, LIDYL, Gif-sur-Yvette, 91191, France
³Université de Bordeaux—CNRS—CEA, CELIA, UMR5107, F33405 Talence, France
⁴Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁵University of West Bohemia, New Technologies Research Centre, 301 00 Plzeň, Czech Republic

^*mauro.fanciulli@u-cergy.fr
^†karol.hricovini@u-cergy.fr
^‡samuel.beaulieu@u-bordeaux.fr

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Vol. 131, Iss. 6 — 11 August 2023

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Figure 1
Hidden spin texture in $2 H − {WSe}_{2}$ and experimental methodology: (a) Schematic of crystal structure, associated Brillouin zone, and band structure for two adjacent layers in $2 H − {WSe}_{2}$ . Valley-dependent chiroptical selection rules within each layer are visualized by a black arrow, bright and dark excitons are represented as shaded areas, and the spin texture is shown with red and blue color code. (b) Experimental setup: a circularly polarized infrared pulse (IR— $800 nm / 1.55 eV$ ) photoexcites $2 H − {WSe}_{2}$ , while a linearly $p$ -polarized extreme ultraviolet probe pulse (XUV— $34.78 nm / 35.65 eV$ ) with a delay $Δ t$ ejects photoelectrons mainly from the first layer, which are collected by the hemispherical analyzer with spin resolution. $x$ and $y$ axes are each at 45° from the out-of-the-page direction. (c) Spin-resolved ( $z$ axis) EDCs of photoelectrons ejected from bright excitonic states in the majority $K^{'}$ valley ( $I_{up}$ in red and $I_{down}$ in blue) and the associated spin polarization (right subpanel) at $Δ t = 0 fs$ .
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Figure 2
Spin-integrated circular dichroism emerging from valley-resolved chiroptical selection rules: (a),(b) and (d),(e) Energy- and angle-resolved photoemission signal from bright excitons ( $K$ and $K^{'}$ valleys), for different pump pulse helicities as depicted on top of each panel. The magenta lines indicate the analyzer’s slit direction. (c),(f) Valley-resolved circular dichroism associated with the photoemission intensities shown in (a),(b) and (d),(e), respectively. Circular dichroism data points for photoemission intensity smaller than 5% of the peak intensity are not shown.
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Figure 3
Valley- and helicity-resolved hidden spin polarization of bright excitons: (a)–(d) Spin-resolved energy distribution curves ( $I_{up}$ in red and $I_{down}$ in blue) of photoelectrons ejected from bright excitonic states at both $K^{'}$ [(a),(b)] and $K$ [(c),(d)] valleys and associated spin polarization (right subpanel), for both pump pulse helicities, at the pump-probe overlap. The pump helicities and valley indexes are depicted on top of each panel.
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Figure 4
Ultrafast decay of the hidden spin polarization of bright excitonic states: Spin-resolved EDCs of photoelectrons ejected from bright excitonic states at $K^{'}$ valley ( $I_{up}$ in red and $I_{down}$ in blue) and the associated spin polarization (in black, right subpanel), (a) at pump-probe overlap, i.e., $Δ t = 0 fs$ and (b) at $Δ t = 33 fs$ .
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Figure 5
Hidden spin polarization dynamics of momentum-forbidden dark excitons. The black and grey dots represent measured spin polarizations of photoelectrons ejected from dark excitonic states at the $Σ^{'}$ valley, for three pump-probe delays (50 fs, 200 fs and 1000 fs) and for both pump pulse helicities ( $σ^{+}$ in black, and $σ^{-}$ , in grey). The dotted line represents the mean spin polarization value for both the polarization and all delays.
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Physical Review Letters

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H−WSe2

Mauro Fanciulli, David Bresteau, Jérôme Gaudin, Shuo Dong, Romain Géneaux, Thierry Ruchon, Olivier Tcherbakoff, Ján Minár, Olivier Heckmann, Maria Christine Richter, Karol Hricovini, and Samuel Beaulieu

Phys. Rev. Lett. 131, 066402 – Published 10 August 2023

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Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in $2 H − {WSe}_{2}$