76
3 Light–Matter Interactions for Photonic Applications
of a disturbed excitonic polarisation in a multiple quantum-well system. This experiment revealed intra-excitonic Rabi-flopping between 1s and higher-order states in
the system in the time domain, whereas the transient four-wave-mixing signal could
be systematically reduced as a function of the THz-field strength. However, in the
case of exciton ionisation, the signal showed a lack of transfer back into the initial 1s
state. This allowed to create scenarios of reversible and irreversible intra-excitonic
transfer depending on the strength of the transient THz field [176].
In contrast to the experiment by Kaindl et al. which probes exciton population
formation, i.e. the generation of incoherent excitons in semiconductors after optical excitation (obtained at times > 100 ps) [170], coherent spectroscopy targets the
dynamics of the system in the regime of coherent polarisation, which is the essential situation in microcavities where the photon lifetime of a few ps to tens of ps
determines the frame of light–matter coupling phenomena. It is this regime which
deserves special attention using THz pulses for manipulation and control of exciton–
polaritons, which shall be addressed in one of the author’s current projects (DFG
RA2841/9-1) using methods such as digital holography [34, 35] and THz pulses (cf.
[36, 172]).
3.2.2 Rich Exciton Physics in 2D Semiconductors
Detailed group-theory analysis of the possible excitons’ branches of TMDCs [177,
178] such as WSe 2 indicated that four different exciton configurations coexist within
the light cone at the A-exciton peak as demonstrated in [179]: Among them are two
bright excitons (Γ 6) at K and K
, respectively, and two more possible states (Γ 4 and
Γ 3) arise which represent a coherent superposition of intervalley excitons composed
of the spin-forbidden transition across K and K
valley. While the Γ 4 state is dipoleallowed for z-polarisation corresponding to the out-of-plane dipole orientation and
is labeled “grey” exciton, the Γ 3 state cannot couple to the electromagnetic field and
remains completely ‘dark’ (see Fig. 3.7).
Encouraged by the rich landscape of exciton complexes in encapsulated tungsten
diselenide, direct measurements of the radiation pattern for different excitons at
cryogenic temperatures were very recently reported in an angle-resolved PL study
in [160], revealing the differences between the bright and grey species that were
obtained under strong quasi-resonant pulsed excitation of the high-quality 2D stack.
Demonstration of Dispersion Feature Within Light Cone
In addition, the polaritonic nature of the quasi-resonantly excited low-pump-density
resonance of high-quality encapsulated WSe 2 was investigated shortly before in
[159]. Thereby, a dispersion within the light cone as strong as about 2 meV with
nearly-homogeneous-linewidth-broadened PL and reflection contrast modes was
demonstrated, corresponding to effective masses between 10
−3 and 10
−4 m 0 (see
Fig. 3.8).
10 Note that cavity–polaritons exhibit effective masses of the order of
10 Free electron mass m 0 , natural constant.
3 Light–Matter Interactions for Photonic Applications
of a disturbed excitonic polarisation in a multiple quantum-well system. This experiment revealed intra-excitonic Rabi-flopping between 1s and higher-order states in
the system in the time domain, whereas the transient four-wave-mixing signal could
be systematically reduced as a function of the THz-field strength. However, in the
case of exciton ionisation, the signal showed a lack of transfer back into the initial 1s
state. This allowed to create scenarios of reversible and irreversible intra-excitonic
transfer depending on the strength of the transient THz field [176].
In contrast to the experiment by Kaindl et al. which probes exciton population
formation, i.e. the generation of incoherent excitons in semiconductors after optical excitation (obtained at times > 100 ps) [170], coherent spectroscopy targets the
dynamics of the system in the regime of coherent polarisation, which is the essential situation in microcavities where the photon lifetime of a few ps to tens of ps
determines the frame of light–matter coupling phenomena. It is this regime which
deserves special attention using THz pulses for manipulation and control of exciton–
polaritons, which shall be addressed in one of the author’s current projects (DFG
RA2841/9-1) using methods such as digital holography [34, 35] and THz pulses (cf.
[36, 172]).
3.2.2 Rich Exciton Physics in 2D Semiconductors
Detailed group-theory analysis of the possible excitons’ branches of TMDCs [177,
178] such as WSe 2 indicated that four different exciton configurations coexist within
the light cone at the A-exciton peak as demonstrated in [179]: Among them are two
bright excitons (Γ 6) at K and K
, respectively, and two more possible states (Γ 4 and
Γ 3) arise which represent a coherent superposition of intervalley excitons composed
of the spin-forbidden transition across K and K
valley. While the Γ 4 state is dipoleallowed for z-polarisation corresponding to the out-of-plane dipole orientation and
is labeled “grey” exciton, the Γ 3 state cannot couple to the electromagnetic field and
remains completely ‘dark’ (see Fig. 3.7).
Encouraged by the rich landscape of exciton complexes in encapsulated tungsten
diselenide, direct measurements of the radiation pattern for different excitons at
cryogenic temperatures were very recently reported in an angle-resolved PL study
in [160], revealing the differences between the bright and grey species that were
obtained under strong quasi-resonant pulsed excitation of the high-quality 2D stack.
Demonstration of Dispersion Feature Within Light Cone
In addition, the polaritonic nature of the quasi-resonantly excited low-pump-density
resonance of high-quality encapsulated WSe 2 was investigated shortly before in
[159]. Thereby, a dispersion within the light cone as strong as about 2 meV with
nearly-homogeneous-linewidth-broadened PL and reflection contrast modes was
demonstrated, corresponding to effective masses between 10
−3 and 10
−4 m 0 (see
Fig. 3.8).
10 Note that cavity–polaritons exhibit effective masses of the order of
10 Free electron mass m 0 , natural constant.