3.2 Matter Excitations
77
Fig. 3.7 a Energy–momentum dispersion diagram for different monolayer WSe 2 exciton configurations and b sketch of the possible bright and dark exciton states for the respective monolayer.
Schematically, the non-degeneracy of excitonic resonances arising from the crystal symmetry is displayed in a with the labels according to group-theory analysis in the literature. Optical excitations
are indicated by nearly-vertical arrows. The Γ 6 branch represents the bright exciton X 0 , which is
energetically separated from the grey exciton X D,g branch Γ 4 and the dark exciton X D branch Γ 3.
The dipole orientation and emission pattern of the radiating exciton species (X) is sketched in b by
black double-arrows and red lobes, respectively. While the ordinary emitter (Γ 6) is oriented in the
plane and emits out of plane, the grey species (Γ 4), which is only dipole allowed for z-polarisation
is oriented out of the plane and emits into the TMDC plane (x–y plane). The dark species Γ 3
remains “dark” due to the lack of coupling to the electromagnetic field. Reproduced under the terms
of the CC-BY 4.0 Licence (http://creativecommons.org/licenses/by/4.0/). [160] Copyright 2020
The Author(s), published by Springer Nature
10
−5 m 0 , and typical III/V semiconductor excitons of the order of 10
−1 m 0 . Thus,
these 2D-excitonic dispersions imply very light quasi-particles,
11 that are the hybrid
exciton–polaritons (light-dressed excitons, macroscopic polarisation). Pump-densitydependent dispersion measurements in the linear excitation regime further indicated
a small change in the curvature due to an increased role of an electron–hole plasma
fraction in the emission signal [159].
Indeed, the origin and understanding of such a strong dispersion is still subject of ongoing investigations [180], revisiting the concept of renormalisation
of longitudinal-mode energies within the light cone and the degeneracy lifting
between longitudinal and transverse exciton modes, as historically discussed for
III/V quantum-well systems in the view of strong far-field optical dipole coupling
11 In the study of [159], the formation of polaritons as a result of any cavity effects by a possible
Fabry–Pérot structure of the 2D stack on the substrate material was ruled out, because instead of any
confined modes only a leaky mode could be found in a simulation for the relevant spectral range.
In addition, propagation effects modelled by theory colleagues of the author also did not yield a
curved dispersion. Moreover, attributing the effects to resonant coupling of monochromatic intense
laser light with the exciton resonance would not explain the dispersion in broadband white-light
reflection contrast measurements. In fact, reflection-contrast values of the effective masses as low as
few 10 −4 m 0 represent a zero-density excitation regime and are not affected by any density-related
effects.
77
Fig. 3.7 a Energy–momentum dispersion diagram for different monolayer WSe 2 exciton configurations and b sketch of the possible bright and dark exciton states for the respective monolayer.
Schematically, the non-degeneracy of excitonic resonances arising from the crystal symmetry is displayed in a with the labels according to group-theory analysis in the literature. Optical excitations
are indicated by nearly-vertical arrows. The Γ 6 branch represents the bright exciton X 0 , which is
energetically separated from the grey exciton X D,g branch Γ 4 and the dark exciton X D branch Γ 3.
The dipole orientation and emission pattern of the radiating exciton species (X) is sketched in b by
black double-arrows and red lobes, respectively. While the ordinary emitter (Γ 6) is oriented in the
plane and emits out of plane, the grey species (Γ 4), which is only dipole allowed for z-polarisation
is oriented out of the plane and emits into the TMDC plane (x–y plane). The dark species Γ 3
remains “dark” due to the lack of coupling to the electromagnetic field. Reproduced under the terms
of the CC-BY 4.0 Licence (http://creativecommons.org/licenses/by/4.0/). [160] Copyright 2020
The Author(s), published by Springer Nature
10
−5 m 0 , and typical III/V semiconductor excitons of the order of 10
−1 m 0 . Thus,
these 2D-excitonic dispersions imply very light quasi-particles,
11 that are the hybrid
exciton–polaritons (light-dressed excitons, macroscopic polarisation). Pump-densitydependent dispersion measurements in the linear excitation regime further indicated
a small change in the curvature due to an increased role of an electron–hole plasma
fraction in the emission signal [159].
Indeed, the origin and understanding of such a strong dispersion is still subject of ongoing investigations [180], revisiting the concept of renormalisation
of longitudinal-mode energies within the light cone and the degeneracy lifting
between longitudinal and transverse exciton modes, as historically discussed for
III/V quantum-well systems in the view of strong far-field optical dipole coupling
11 In the study of [159], the formation of polaritons as a result of any cavity effects by a possible
Fabry–Pérot structure of the 2D stack on the substrate material was ruled out, because instead of any
confined modes only a leaky mode could be found in a simulation for the relevant spectral range.
In addition, propagation effects modelled by theory colleagues of the author also did not yield a
curved dispersion. Moreover, attributing the effects to resonant coupling of monochromatic intense
laser light with the exciton resonance would not explain the dispersion in broadband white-light
reflection contrast measurements. In fact, reflection-contrast values of the effective masses as low as
few 10 −4 m 0 represent a zero-density excitation regime and are not affected by any density-related
effects.