5.6 Additional Methods
173
Other dark excitons ( 3 state or dipole-forbidden transitions) are still observable in
emission spectra when phonon-scattering processes are involved. For WSe 2 , the correspondingly long-living 3 dark state lies energetically below the bright states and
acts as a nonradiative decay channel for 6 excitons. This peculiarity distinguishes
WSe 2 from its MoSe 2 counterpart. For group-theory analysis and early experiments
on this matter see [150–152].
In fact, far-field beam characterisation is else used for laser beam divergence
studies, e.g., from VCSELs and edge emitters, and it can also reveal a phase transition in bosonic emitters from a Maxwell–Boltzmann distributed regime to a Bose–
Einstein-like condensate with narrow momentum distribution (commonly achieved
by Fourier-space imaging, cf. [4, 153, 154]).
5.6.2 Time-Domain Spectroscopy
Time-domain spectroscopy (TDS) is widely known from THz spectroscopy, which
often derives material information from amplitude and phase changes of transmitted
or backreflected THz pulses. In the literature, THz spectroscopy has been widely
used to characterise materials and photoconductivity with sub-ps resolution without
physical contact and non-destructively. Using pump–probe schemes, even excitonic
and free-charge-carrier behaviour can be distinguished through the analysis of the
frequency-dependent complex conductivity. More importantly, it has become a commercially available method after initial developments in this field took shape around
two decades ago [155]. It covers the far-infrared spectral range between approximately 3 to 300 wave-numbers, corresponding to frequencies between 0.1 to 10 THz.
In the following, a few examples are briefly provided.
For instance, THz spectroscopy allows one to study fundamental physical mechanisms such as the formation and decay of excitons in various semiconductor systems
[39, 42, 156]. In addition, it was recently demonstrated that the formation of crystals
out of a solution could be monitored with THz (attenuated-total-reflection) spectroscopy [157]. Note that the optics of THz waves are material-wise different than
those for visible light, which required all kinds of developments to obtain the simplest optics, such as lenses, wave-plates, and also beam splitters [158]. In the field
of non-destructive testing, THz experimentalists from the Semiconductor Photonics
Group in Marburg recently combined a THz system with a robotic arm to achieve a
novel THz tomography system for analysis of arbitrarily shaped samples [159].
Furthermore, THz spectroscopy can enable the exploration of carbon nanomaterials with regard to applications at THz frequencies [160, 161]. In addition,
recent optical ultrafast conductivity studies on graphene nanoribbons were carried
out using THz spectroscopy which showed how the conductivity varies with the
precise structure of graphene nanoribbons [162–164].
Often, THz-TDS systems are employed for material studies which are typically
composed of THz emitting antennae photoexcited by fs-pulsed laser pulses and THz
detecting antennae optically gated by delayed laser pulses from the same fs-laser
173
Other dark excitons ( 3 state or dipole-forbidden transitions) are still observable in
emission spectra when phonon-scattering processes are involved. For WSe 2 , the correspondingly long-living 3 dark state lies energetically below the bright states and
acts as a nonradiative decay channel for 6 excitons. This peculiarity distinguishes
WSe 2 from its MoSe 2 counterpart. For group-theory analysis and early experiments
on this matter see [150–152].
In fact, far-field beam characterisation is else used for laser beam divergence
studies, e.g., from VCSELs and edge emitters, and it can also reveal a phase transition in bosonic emitters from a Maxwell–Boltzmann distributed regime to a Bose–
Einstein-like condensate with narrow momentum distribution (commonly achieved
by Fourier-space imaging, cf. [4, 153, 154]).
5.6.2 Time-Domain Spectroscopy
Time-domain spectroscopy (TDS) is widely known from THz spectroscopy, which
often derives material information from amplitude and phase changes of transmitted
or backreflected THz pulses. In the literature, THz spectroscopy has been widely
used to characterise materials and photoconductivity with sub-ps resolution without
physical contact and non-destructively. Using pump–probe schemes, even excitonic
and free-charge-carrier behaviour can be distinguished through the analysis of the
frequency-dependent complex conductivity. More importantly, it has become a commercially available method after initial developments in this field took shape around
two decades ago [155]. It covers the far-infrared spectral range between approximately 3 to 300 wave-numbers, corresponding to frequencies between 0.1 to 10 THz.
In the following, a few examples are briefly provided.
For instance, THz spectroscopy allows one to study fundamental physical mechanisms such as the formation and decay of excitons in various semiconductor systems
[39, 42, 156]. In addition, it was recently demonstrated that the formation of crystals
out of a solution could be monitored with THz (attenuated-total-reflection) spectroscopy [157]. Note that the optics of THz waves are material-wise different than
those for visible light, which required all kinds of developments to obtain the simplest optics, such as lenses, wave-plates, and also beam splitters [158]. In the field
of non-destructive testing, THz experimentalists from the Semiconductor Photonics
Group in Marburg recently combined a THz system with a robotic arm to achieve a
novel THz tomography system for analysis of arbitrarily shaped samples [159].
Furthermore, THz spectroscopy can enable the exploration of carbon nanomaterials with regard to applications at THz frequencies [160, 161]. In addition,
recent optical ultrafast conductivity studies on graphene nanoribbons were carried
out using THz spectroscopy which showed how the conductivity varies with the
precise structure of graphene nanoribbons [162–164].
Often, THz-TDS systems are employed for material studies which are typically
composed of THz emitting antennae photoexcited by fs-pulsed laser pulses and THz
detecting antennae optically gated by delayed laser pulses from the same fs-laser