3.2 Matter Excitations
79
data by the author’s team (based on the Fourier-space spectroscopy technique) with
trending parabolic dispersion relations for 2D excitons in hBN-encapsulated WSe 2
or MoSe 2 (the latter with weaker dispersion feature, in agreement with expectations
from theory for molybdenum-based TMDCs), a linear or nearly-linear behaviour
around k = 0 can neither be clearly verified nor ruled out.
In fact, given these examples, one has to abandon the common view that an
excitonic quasi-particle dispersion cannot be obtained optically. That view existed
due to the traditional knowledge for conventional semiconductors that the exciton
mass is too heavy (1–0.1 m 0 ) to feature angle-dependent energy shifts larger than a
few μeV within the light cone—typically impossible to measure reliably with exciton
linewidth of the order of meV.
Fine-Structure’s Pseudo-Spin and Angle-Resolved Valley Polarisation
Additional examinations of the fine-structure’s pseudo-spin texture for this specific
2D stack (i.e. the hBN-encapsulated WSe 2 monolayer) showed considerable differences for continuous-wave quasi-resonant excitation detunings and off-resonant
pumping cases [180]. A systematic decrease of the measurable dispersion feature in
Fourier-space resolved PL spectra was evidenced due to contributions of incoherent
excitons and emission from plasma when moving the excitation energy step-wise
above the electronic band gap. A step-wise increase of the electron–hole plasma
fraction and faster decoherence of the coherently mixed exciton states
12 was further
indicated by the step-wise reduction of the PL helicity (degree of circular polarisation) with its peculiar far-field pattern changes for the excitonic resonance. By having
revealed that there is a complex locking between valley-pseudo-spin and centre-ofmass momentum present [180], further investigations of angle-resolved polarisation
anisotropy in exciton-complexes-rich 2D semiconductors are encouraged, such as
time-resolved Fourier-space mapping in order to shed light on valley decoherence
mechanisms as a function of the centre-of-mass momentum.
While several theoretical predictions [183–187] previously claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch
(whereas the longitudinal one, which is the upper branch, exhibits an extraordinarily
strong dispersion in the meV range within the light cone), current understanding from
the studies by the author and co-workers let one recall that long-range exchange interactions (already proposed for quantum-well exciton–polaritons in III/V semiconductors [181, 182]) and strong far-field optical dipole coupling may indeed describe two
sides of the same coin.
12 Dipole resonance states that are coherently superposed due to long-range exchange interaction
between charge carriers within one valley and with those of opposite valleys, with the hybridised
states resembling the Rabi oscillations [183] between the Rabi-split states of strong optical dipole–
dipole coupling.
79
data by the author’s team (based on the Fourier-space spectroscopy technique) with
trending parabolic dispersion relations for 2D excitons in hBN-encapsulated WSe 2
or MoSe 2 (the latter with weaker dispersion feature, in agreement with expectations
from theory for molybdenum-based TMDCs), a linear or nearly-linear behaviour
around k = 0 can neither be clearly verified nor ruled out.
In fact, given these examples, one has to abandon the common view that an
excitonic quasi-particle dispersion cannot be obtained optically. That view existed
due to the traditional knowledge for conventional semiconductors that the exciton
mass is too heavy (1–0.1 m 0 ) to feature angle-dependent energy shifts larger than a
few μeV within the light cone—typically impossible to measure reliably with exciton
linewidth of the order of meV.
Fine-Structure’s Pseudo-Spin and Angle-Resolved Valley Polarisation
Additional examinations of the fine-structure’s pseudo-spin texture for this specific
2D stack (i.e. the hBN-encapsulated WSe 2 monolayer) showed considerable differences for continuous-wave quasi-resonant excitation detunings and off-resonant
pumping cases [180]. A systematic decrease of the measurable dispersion feature in
Fourier-space resolved PL spectra was evidenced due to contributions of incoherent
excitons and emission from plasma when moving the excitation energy step-wise
above the electronic band gap. A step-wise increase of the electron–hole plasma
fraction and faster decoherence of the coherently mixed exciton states
12 was further
indicated by the step-wise reduction of the PL helicity (degree of circular polarisation) with its peculiar far-field pattern changes for the excitonic resonance. By having
revealed that there is a complex locking between valley-pseudo-spin and centre-ofmass momentum present [180], further investigations of angle-resolved polarisation
anisotropy in exciton-complexes-rich 2D semiconductors are encouraged, such as
time-resolved Fourier-space mapping in order to shed light on valley decoherence
mechanisms as a function of the centre-of-mass momentum.
While several theoretical predictions [183–187] previously claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch
(whereas the longitudinal one, which is the upper branch, exhibits an extraordinarily
strong dispersion in the meV range within the light cone), current understanding from
the studies by the author and co-workers let one recall that long-range exchange interactions (already proposed for quantum-well exciton–polaritons in III/V semiconductors [181, 182]) and strong far-field optical dipole coupling may indeed describe two
sides of the same coin.
12 Dipole resonance states that are coherently superposed due to long-range exchange interaction
between charge carriers within one valley and with those of opposite valleys, with the hybridised
states resembling the Rabi oscillations [183] between the Rabi-split states of strong optical dipole–
dipole coupling.