5.3 Basic Material Response
155
Quasi-resonant excitation in contrast to off-resonant excitation can directly excite
a species, and it creates a coherent (macroscopic) polarisation of matter (coherent
excitons, also understood as exciton–polaritons [69]). The macroscopic polarisation
then dephases over time and forms an incoherent population of Coulomb-bound
electron–hole pairs (microscopic polarisation). Such incoherent population can be
for instance probed through intraexcitonic transitions (cf. [39, 41]). Furthermore,
for high intensities of the driving light field, resonant excitation result in coupledoscillator effects such as the aforementioned Rabi oscillations.
However, in time-averaged spectroscopy, spectral resonance of the excitation
source and the emitter drastically complicates detection of luminescence signal under
resonant pumping. Typically, (back-) scattered laser light collected by the detection
optics is the dominant signal contribution and even filtering using cross-polarisation
techniques may not always be sufficient to cancel out signal, although applicable in
some cases such as for cavity–polariton studies under optical side pumping [59]. In
contrast to CW acquisition, resonant ultrashort-pulsed excitation in time-resolved
spectroscopy can be filtered out temporally.
Recent investigations on the optical dispersion of 2D excitons in WSe 2 by means
of angle-resolved (Fourier-space) spectroscopy applied quasi-resonant excitation of
the 1s species with a photon energy well-enough below the 2s resonance and farenough away from the 1s resonance to be able to spectrally filter it out with the help
of a premium edge filter and by a monochromator [66]. Furthermore, it could be
recently experimentally demonstrated that the dispersion feature, which also occurs
in white-light reflection contrast spectra, is detuning dependent and its occurrence
weakens with increased pump energy (above the 2s resonance) until disappearance
for continuum pumping (well above the gap for the TMDC’s A exciton, and above
the B-exciton level) [46]. Simultaneously, these measurements revealed a reduction
in the valley-polarisation degree and change in the 1s-exciton’s pseudo-spin texture
within the light cone as a function of the excitation detuning.
One major advantage of ultrashort-pulsed excitation in time-averaged acquisition
is the reduced heat transfer to the lattice at simultaneously very-high peak intensities, for which under CW pumping the sample would not survive exposure due to
the thermal load incorporated into the investigated structure, allowing to address a
wider range of charge-carrier densities in optical experiments. Such strong irradiances with short temporal footprint allow one to reach higher-density regimes in
(2D) semiconductors on ultrashort time scales that give rise to the formation and
observation of various exciton complexes including biexcitons,
12 charged biexcitons
and other multi-particle states even formed across (TMDC) valleys (cf. [3, 7, 73–
75]), and the observation of out-of-plane “grey” excitons (see [3, 76]). Indeed, some
of these exciton complexes are also obtained under CW pumping, but their occurrence shows strong excitation energy dependencies [46] and the obtained densities
are much lower than peak densities in pulsed experiments.
12 Biexcitons are exciton molecules similar to the hydrogen or positronium molecule, which require
sufficiently high exciton populations to be established, whereas trions that are charged excitons, or
even charged biexcitons, need an (additional) excess in free carriers.
155
Quasi-resonant excitation in contrast to off-resonant excitation can directly excite
a species, and it creates a coherent (macroscopic) polarisation of matter (coherent
excitons, also understood as exciton–polaritons [69]). The macroscopic polarisation
then dephases over time and forms an incoherent population of Coulomb-bound
electron–hole pairs (microscopic polarisation). Such incoherent population can be
for instance probed through intraexcitonic transitions (cf. [39, 41]). Furthermore,
for high intensities of the driving light field, resonant excitation result in coupledoscillator effects such as the aforementioned Rabi oscillations.
However, in time-averaged spectroscopy, spectral resonance of the excitation
source and the emitter drastically complicates detection of luminescence signal under
resonant pumping. Typically, (back-) scattered laser light collected by the detection
optics is the dominant signal contribution and even filtering using cross-polarisation
techniques may not always be sufficient to cancel out signal, although applicable in
some cases such as for cavity–polariton studies under optical side pumping [59]. In
contrast to CW acquisition, resonant ultrashort-pulsed excitation in time-resolved
spectroscopy can be filtered out temporally.
Recent investigations on the optical dispersion of 2D excitons in WSe 2 by means
of angle-resolved (Fourier-space) spectroscopy applied quasi-resonant excitation of
the 1s species with a photon energy well-enough below the 2s resonance and farenough away from the 1s resonance to be able to spectrally filter it out with the help
of a premium edge filter and by a monochromator [66]. Furthermore, it could be
recently experimentally demonstrated that the dispersion feature, which also occurs
in white-light reflection contrast spectra, is detuning dependent and its occurrence
weakens with increased pump energy (above the 2s resonance) until disappearance
for continuum pumping (well above the gap for the TMDC’s A exciton, and above
the B-exciton level) [46]. Simultaneously, these measurements revealed a reduction
in the valley-polarisation degree and change in the 1s-exciton’s pseudo-spin texture
within the light cone as a function of the excitation detuning.
One major advantage of ultrashort-pulsed excitation in time-averaged acquisition
is the reduced heat transfer to the lattice at simultaneously very-high peak intensities, for which under CW pumping the sample would not survive exposure due to
the thermal load incorporated into the investigated structure, allowing to address a
wider range of charge-carrier densities in optical experiments. Such strong irradiances with short temporal footprint allow one to reach higher-density regimes in
(2D) semiconductors on ultrashort time scales that give rise to the formation and
observation of various exciton complexes including biexcitons,
12 charged biexcitons
and other multi-particle states even formed across (TMDC) valleys (cf. [3, 7, 73–
75]), and the observation of out-of-plane “grey” excitons (see [3, 76]). Indeed, some
of these exciton complexes are also obtained under CW pumping, but their occurrence shows strong excitation energy dependencies [46] and the obtained densities
are much lower than peak densities in pulsed experiments.
12 Biexcitons are exciton molecules similar to the hydrogen or positronium molecule, which require
sufficiently high exciton populations to be established, whereas trions that are charged excitons, or
even charged biexcitons, need an (additional) excess in free carriers.