166
5 Optical Measurement Techniques
Fig. 5.15 Photo of a THz-generating dual-wavelength semiconductor disk laser device with nonlinear crystal, also referred to as THz-emitting external-cavity surface-emitting laser (TECSEL). Inset:
Comparison of the THz frequency tunability for an aperiodically-poled and a periodically-poled
lithium niobate (aPPLN/PPLN) crystal designed for 1-THz output. The laser operates on two laser
modes simultaneously at around 1 µm wavelength with the help of an intracavity etalon and with a
wavelength spacing around 3.4 nm, corresponding to about 1 THz. Intracavity difference-frequency
generation exploits high intensities inside the laser resonator for effective THz emission, which
is designed to take place perpendicular to the optical pump light (i.e. resonator mode beam). Due
to a chirp of the periods in the ferroelectric crystal, the aperiodic poling enables phase-matching
for a wider range of wavelengths than periodical poling, which allows phase-matching only for a
specific wavelength. Adapted under the terms of the CC-BY 4.0 Licence (http://creativecommons.
org/licenses/by/4.0/). [114] Copyright 2020 The Author(s), published by Wiley VCH
Bilayer areas reveal themselves in PL maps by their significantly reduced emission, whereas the twist angle of bilayers determines the strength of nonlinear frequency conversion and, thus, the recorded signal in SHG maps. In general, the SHG
response of both layers will cancel each other in an inversion symmetric case. In
contrast, the response will increase nearly 4-fold [133] in the scenario with broken
inversion symmetry.
Correspondingly, for AA’ and AB bilayers, destructive and constructive SHG
behaviours are observed [8]. The signal of the symmetry-dependent SHG process
cancels out for the symmetric AA’ bilayer (type abundant in nature as the energetically
favourable stacking configuration) and results in a dark bilayer area surrounded by
SHG from monolayer regions (for the studied CVD-grown stacks). In contrast, for the
AB-type bilayer (energetically less favoured configuration), the symmetry-break in
the stacking configuration provides increased signal in comparison to a single layer,
as the individual layers’ contributions constructively add up. Thus, for samples which
were grown to result in bilayers of different configuration, dark PL with bright SHG
indicates the AB-type bilayer, whereas bright SHG and bright PL is associated with
the monolayer parts. Indeed, a trilayer of naturally-stacked TMDC material should
5 Optical Measurement Techniques
Fig. 5.15 Photo of a THz-generating dual-wavelength semiconductor disk laser device with nonlinear crystal, also referred to as THz-emitting external-cavity surface-emitting laser (TECSEL). Inset:
Comparison of the THz frequency tunability for an aperiodically-poled and a periodically-poled
lithium niobate (aPPLN/PPLN) crystal designed for 1-THz output. The laser operates on two laser
modes simultaneously at around 1 µm wavelength with the help of an intracavity etalon and with a
wavelength spacing around 3.4 nm, corresponding to about 1 THz. Intracavity difference-frequency
generation exploits high intensities inside the laser resonator for effective THz emission, which
is designed to take place perpendicular to the optical pump light (i.e. resonator mode beam). Due
to a chirp of the periods in the ferroelectric crystal, the aperiodic poling enables phase-matching
for a wider range of wavelengths than periodical poling, which allows phase-matching only for a
specific wavelength. Adapted under the terms of the CC-BY 4.0 Licence (http://creativecommons.
org/licenses/by/4.0/). [114] Copyright 2020 The Author(s), published by Wiley VCH
Bilayer areas reveal themselves in PL maps by their significantly reduced emission, whereas the twist angle of bilayers determines the strength of nonlinear frequency conversion and, thus, the recorded signal in SHG maps. In general, the SHG
response of both layers will cancel each other in an inversion symmetric case. In
contrast, the response will increase nearly 4-fold [133] in the scenario with broken
inversion symmetry.
Correspondingly, for AA’ and AB bilayers, destructive and constructive SHG
behaviours are observed [8]. The signal of the symmetry-dependent SHG process
cancels out for the symmetric AA’ bilayer (type abundant in nature as the energetically
favourable stacking configuration) and results in a dark bilayer area surrounded by
SHG from monolayer regions (for the studied CVD-grown stacks). In contrast, for the
AB-type bilayer (energetically less favoured configuration), the symmetry-break in
the stacking configuration provides increased signal in comparison to a single layer,
as the individual layers’ contributions constructively add up. Thus, for samples which
were grown to result in bilayers of different configuration, dark PL with bright SHG
indicates the AB-type bilayer, whereas bright SHG and bright PL is associated with
the monolayer parts. Indeed, a trilayer of naturally-stacked TMDC material should