5.3 Basic Material Response
153
pling phenomenon can be observed when a strong mono-mode resonant CW light
field is incident on an ideal two-level system: periodic absorption (excitation) and
stimulated emission (deexcitation) occur within the frame of so-called Rabi oscillations,
10 known for instance from atom–cavity experiments (cf. [52, 69, 70]).
In some structures, an additional pump can be used to probe low-signal gain with
white-light spectroscopy such as in the time-resolved optical pump–probe experiment
in reflection geometry on VECSEL chips [37]. A steady-state spectroscopy of gain
mirrors is similarly possible with pumped chips probed by white light.
Novel semiconductor materials such as 2D materials have been also investigated in
the literature with nonlinear spectroscopy using two-photon absorption, with which
optically dark states for one-photon transition processes (“dark” in terms of inactive,
due to optical selection rules) can be addressed [71].
5.3.2 Photoluminescence
Luminescence spectroscopy can be clearly separated into two different excitation modes that are (i) resonant or quasi-resonant excitation, (ii) non-resonant or
above-band excitation. A mix of both is obtained in a third variation that is (iii)
energy-tunable excitation (PL excitation spectroscopy, delivering relative excitation
efficiency as a function of excitation photon energy). The former two cases are
distinctions of static-excitation-energy spectroscopy (widely used to obtain emission spectra), whereas the latter gives a probing mechanism using the excitation
energy and the response of the system. Exciton Rydberg states have been for instance
observed for monolayer TMDCs with PL excitation spectroscopy in [72]. Thereby, it
allows one to scan over transitions usually hidden in time-averaged above-band excitation spectroscopy, the spectra of which are dominated by the most shiny (most probable) transitions (due to much higher oscillator strengths). This method and reflection
contrast in combination with the two-photon absorption principle can provide complementary information about higher-order excitonic states (cf. [64, 65, 71, 72]).
Still, fixed-wavelength excitation bears an important role in PL studies. An example spectrum for a TMDC heterobilayer is shown in Fig. 5.11 together with a reflection
contrast measurement of that heterostructure. Naturally, non-resonant excitation, be
it through single-photon or two-photon excitation, introduces excess energy into the
electronic system which is dissipated through multiple particle–particle or particle–
quasi-particle scattering processes, such as electron- or phonon-mediated relaxation.
This produces heat and hot non-thermalized charge carriers, which can reach the
lower states through different channels and irradiate light by recombination at different stages on the way to the system’s excitation ground-state. More importantly,
emission from the exciton mode is likely not anymore representing recombination
from Coulomb-bound electron–hole pairs, as the fraction of electron–hole plasma
10 The related Rabi flopping behaviour in the time domain and Rabi splitting phenomenon in the
frequency domain are named in reference to historic atom–field experiments [67, 68].
153
pling phenomenon can be observed when a strong mono-mode resonant CW light
field is incident on an ideal two-level system: periodic absorption (excitation) and
stimulated emission (deexcitation) occur within the frame of so-called Rabi oscillations,
10 known for instance from atom–cavity experiments (cf. [52, 69, 70]).
In some structures, an additional pump can be used to probe low-signal gain with
white-light spectroscopy such as in the time-resolved optical pump–probe experiment
in reflection geometry on VECSEL chips [37]. A steady-state spectroscopy of gain
mirrors is similarly possible with pumped chips probed by white light.
Novel semiconductor materials such as 2D materials have been also investigated in
the literature with nonlinear spectroscopy using two-photon absorption, with which
optically dark states for one-photon transition processes (“dark” in terms of inactive,
due to optical selection rules) can be addressed [71].
5.3.2 Photoluminescence
Luminescence spectroscopy can be clearly separated into two different excitation modes that are (i) resonant or quasi-resonant excitation, (ii) non-resonant or
above-band excitation. A mix of both is obtained in a third variation that is (iii)
energy-tunable excitation (PL excitation spectroscopy, delivering relative excitation
efficiency as a function of excitation photon energy). The former two cases are
distinctions of static-excitation-energy spectroscopy (widely used to obtain emission spectra), whereas the latter gives a probing mechanism using the excitation
energy and the response of the system. Exciton Rydberg states have been for instance
observed for monolayer TMDCs with PL excitation spectroscopy in [72]. Thereby, it
allows one to scan over transitions usually hidden in time-averaged above-band excitation spectroscopy, the spectra of which are dominated by the most shiny (most probable) transitions (due to much higher oscillator strengths). This method and reflection
contrast in combination with the two-photon absorption principle can provide complementary information about higher-order excitonic states (cf. [64, 65, 71, 72]).
Still, fixed-wavelength excitation bears an important role in PL studies. An example spectrum for a TMDC heterobilayer is shown in Fig. 5.11 together with a reflection
contrast measurement of that heterostructure. Naturally, non-resonant excitation, be
it through single-photon or two-photon excitation, introduces excess energy into the
electronic system which is dissipated through multiple particle–particle or particle–
quasi-particle scattering processes, such as electron- or phonon-mediated relaxation.
This produces heat and hot non-thermalized charge carriers, which can reach the
lower states through different channels and irradiate light by recombination at different stages on the way to the system’s excitation ground-state. More importantly,
emission from the exciton mode is likely not anymore representing recombination
from Coulomb-bound electron–hole pairs, as the fraction of electron–hole plasma
10 The related Rabi flopping behaviour in the time domain and Rabi splitting phenomenon in the
frequency domain are named in reference to historic atom–field experiments [67, 68].