78
3 Light–Matter Interactions for Photonic Applications
Fig. 3.8 Optical dispersion measurements acquired in a Fourier-space spectroscopy mode with a
microscope objective setup (see Fig. 5.17). Energy–k -resolved PL from hBN-encapsulated WSe 2
at 15 K (a), 100 K (b) and 150 K (c), obtained under quasi-resonant excitation. The dashed line
shows a parabolic fit to the data in a or a flat line in c, which exhibits no measurable dispersion.
This behaviour is attributed to the role of thermal effects (that can be faster dephasing times, i.e.
larger homogeneous broadening, correspondingly reduced oscillator strength, binding energy and
light–matter coupling, loss of macroscopic polarisation, increased fraction of ionised excitons, i.e.
e–h plasma) and polaron formation through exciton–optical-phonon interactions. d In comparison
to b, off-resonant excitation results in a loss of measurable dispersion and the data indicates a
k-independent constant emission energy, similar to c. This behaviour is attributed to an increased
fraction of e–h plasma and incoherent excitons (microscopic polarisation). e Extracted peak energies
in an energy–momentum chart labelled with corresponding effective masses from parabola fits. The
right side of e shows the respective linewidth values versus k . Here, the narrowest linewidth values
correspond to FWHM of 4–5 meV, given at 15 K. Reproduced with permission. [159] Copyright
2019 Optical Society of America
[181, 182]. In the theory literature, the quantum-mechanically considered long-range
exchange interactions of electrons and holes within one valley and across the valleys are understood to give rise to new excitonic eigen-states formed as coherent
superpositions with exciton normal mode splitting (see below). Some of these theories (predominantly based on 2D considerations) interestingly suggested a Dirac
cone or (nearly-)linear dispersion around zero momentum [183–186] with energy
shifts in the few meVs within the light cone, which would excitingly imply light-like
or relativistic behaviour, whereas 3D-based approaches [187] still obtain parabolic
branches with strongly different effective masses.
Very recently, by the vanishing of a dispersion feature as well as optical helicity with increasing laser–exciton detuning, it was even demonstrated that the formation of a macroscopic polarisation (coherent excitons) becomes highly unlikely
for excitation regimes well-detuned above the excitonic resonance, i.e. for offresonant compared to quasi-resonant pumping [180]. Thus, signatures of uncorrelated electron–hole plasma and microscopic polarisation (incoherent excitons) are
obtained for off-resonant pumping, in contrast to exciton–polaritons (coherent excitons) for resonances with nearly-homogeneous linewidth broadening [159] when
pumping between the A-exciton 1s and continuum state [180]. Yet, from the obtained
3 Light–Matter Interactions for Photonic Applications
Fig. 3.8 Optical dispersion measurements acquired in a Fourier-space spectroscopy mode with a
microscope objective setup (see Fig. 5.17). Energy–k -resolved PL from hBN-encapsulated WSe 2
at 15 K (a), 100 K (b) and 150 K (c), obtained under quasi-resonant excitation. The dashed line
shows a parabolic fit to the data in a or a flat line in c, which exhibits no measurable dispersion.
This behaviour is attributed to the role of thermal effects (that can be faster dephasing times, i.e.
larger homogeneous broadening, correspondingly reduced oscillator strength, binding energy and
light–matter coupling, loss of macroscopic polarisation, increased fraction of ionised excitons, i.e.
e–h plasma) and polaron formation through exciton–optical-phonon interactions. d In comparison
to b, off-resonant excitation results in a loss of measurable dispersion and the data indicates a
k-independent constant emission energy, similar to c. This behaviour is attributed to an increased
fraction of e–h plasma and incoherent excitons (microscopic polarisation). e Extracted peak energies
in an energy–momentum chart labelled with corresponding effective masses from parabola fits. The
right side of e shows the respective linewidth values versus k . Here, the narrowest linewidth values
correspond to FWHM of 4–5 meV, given at 15 K. Reproduced with permission. [159] Copyright
2019 Optical Society of America
[181, 182]. In the theory literature, the quantum-mechanically considered long-range
exchange interactions of electrons and holes within one valley and across the valleys are understood to give rise to new excitonic eigen-states formed as coherent
superpositions with exciton normal mode splitting (see below). Some of these theories (predominantly based on 2D considerations) interestingly suggested a Dirac
cone or (nearly-)linear dispersion around zero momentum [183–186] with energy
shifts in the few meVs within the light cone, which would excitingly imply light-like
or relativistic behaviour, whereas 3D-based approaches [187] still obtain parabolic
branches with strongly different effective masses.
Very recently, by the vanishing of a dispersion feature as well as optical helicity with increasing laser–exciton detuning, it was even demonstrated that the formation of a macroscopic polarisation (coherent excitons) becomes highly unlikely
for excitation regimes well-detuned above the excitonic resonance, i.e. for offresonant compared to quasi-resonant pumping [180]. Thus, signatures of uncorrelated electron–hole plasma and microscopic polarisation (incoherent excitons) are
obtained for off-resonant pumping, in contrast to exciton–polaritons (coherent excitons) for resonances with nearly-homogeneous linewidth broadening [159] when
pumping between the A-exciton 1s and continuum state [180]. Yet, from the obtained