172
5 Optical Measurement Techniques
In contrast to the optical wavelength region, there are only few of such measurements (e.g., [149]) in the THz frequency region. Recently, beam profile measurements
in the THz domain were for instance used to provide additional information about
the beam quality of a THz-generating VECSEL (TECSEL) [130], which featured
M
2 values of about 1.4 and 1.7 in x and y direction, respectively. Indeed, a simple
knife-edge method can already reveal the intensity profile of a THz source’s output
beam, as shown in [111] for the first TECSEL—something more trivial in the optical
domain when imaging cameras are available (intensity profiles acquired from a single
shot). With the rise of THz imaging cameras, such M
2 measurements are facilitated.
Nevertheless, care has to be taken when extracting values from raw data, because offset values and noise strongly influence the calculation of the second-moment width
D4σ derived from beam profile measurements. Thus, an additional processing of the
measured raw data was applied in [130], with the aim to correct for the background
offset signal, the camera non-uniformity, the background noise, where there was no
THz signal present, and smoothing of the signal. If no imaging device is available,
the knife-edge method in combination with a beam-waist scanning can also enable
the determination of beam quality factors. Therefor, a knife edge is translated in front
of a photodetector and sequential knife-edge scans at each position along the beam
are acquired.
Important information about a light source can also be obtained interferometrically. Characterisation of a semiconductor-disk-laser-driven terahertz source by
Michelson interferometry in the THz frequency domain, similar to Fourier-transform
infrared-spectroscopy (FTIR), can reveal the emission frequency and bandwidth
[112]. Such experiment by Wichmann et al. facilitated the estimation of the number
of difference-frequency-generated THz modes obtained from a TECSEL.
White-light interferometry using a comparison of balanced and unbalanced
Michelson interferometer arms can provide useful insights into the group-delay
dispersion of a transmissive sample. Therefor, the frequency-dependent phase
16 is
deduced via a fast Fourier transform of the interferogram from the time domain.
17 By
applying a second derivative (with respect to frequency) to this frequency-dependent
phase, the group-delay dispersion within a VECSEL introduced by the gain chip or
saturable-absorber mirror can be estimated for chirp compensation approaches. This
is currently pursued to improve self-mode-locked VECSELs with regard to pulse
stability and duration for ultrashort-pulse operation.
Recently, angle-resolved PL spectroscopy, which projects the far-field of an emission signal onto the spectrometer, was even used to identify excitonic species in an
encapsulated WSe 2 monolayer based on their spectrally-resolved angle-dependent
emission characteristics. Thereby, one type of dark exciton, which can only emit due
to symmetry-dependent selection rules in the plane but not out of the plane and which
is referred to as grey exciton ( 4 state with out-of-plane dipole orientation in WSe 2 ),
was clearly singled out from the pool of excitonic lines due to its radiation profile [3].
16 Part of the complex spectrum with amplitude and phase.
17 The interferogram results from the scanning of a length-change induced temporal delay in one
arm.
5 Optical Measurement Techniques
In contrast to the optical wavelength region, there are only few of such measurements (e.g., [149]) in the THz frequency region. Recently, beam profile measurements
in the THz domain were for instance used to provide additional information about
the beam quality of a THz-generating VECSEL (TECSEL) [130], which featured
M
2 values of about 1.4 and 1.7 in x and y direction, respectively. Indeed, a simple
knife-edge method can already reveal the intensity profile of a THz source’s output
beam, as shown in [111] for the first TECSEL—something more trivial in the optical
domain when imaging cameras are available (intensity profiles acquired from a single
shot). With the rise of THz imaging cameras, such M
2 measurements are facilitated.
Nevertheless, care has to be taken when extracting values from raw data, because offset values and noise strongly influence the calculation of the second-moment width
D4σ derived from beam profile measurements. Thus, an additional processing of the
measured raw data was applied in [130], with the aim to correct for the background
offset signal, the camera non-uniformity, the background noise, where there was no
THz signal present, and smoothing of the signal. If no imaging device is available,
the knife-edge method in combination with a beam-waist scanning can also enable
the determination of beam quality factors. Therefor, a knife edge is translated in front
of a photodetector and sequential knife-edge scans at each position along the beam
are acquired.
Important information about a light source can also be obtained interferometrically. Characterisation of a semiconductor-disk-laser-driven terahertz source by
Michelson interferometry in the THz frequency domain, similar to Fourier-transform
infrared-spectroscopy (FTIR), can reveal the emission frequency and bandwidth
[112]. Such experiment by Wichmann et al. facilitated the estimation of the number
of difference-frequency-generated THz modes obtained from a TECSEL.
White-light interferometry using a comparison of balanced and unbalanced
Michelson interferometer arms can provide useful insights into the group-delay
dispersion of a transmissive sample. Therefor, the frequency-dependent phase
16 is
deduced via a fast Fourier transform of the interferogram from the time domain.
17 By
applying a second derivative (with respect to frequency) to this frequency-dependent
phase, the group-delay dispersion within a VECSEL introduced by the gain chip or
saturable-absorber mirror can be estimated for chirp compensation approaches. This
is currently pursued to improve self-mode-locked VECSELs with regard to pulse
stability and duration for ultrashort-pulse operation.
Recently, angle-resolved PL spectroscopy, which projects the far-field of an emission signal onto the spectrometer, was even used to identify excitonic species in an
encapsulated WSe 2 monolayer based on their spectrally-resolved angle-dependent
emission characteristics. Thereby, one type of dark exciton, which can only emit due
to symmetry-dependent selection rules in the plane but not out of the plane and which
is referred to as grey exciton ( 4 state with out-of-plane dipole orientation in WSe 2 ),
was clearly singled out from the pool of excitonic lines due to its radiation profile [3].
16 Part of the complex spectrum with amplitude and phase.
17 The interferogram results from the scanning of a length-change induced temporal delay in one
arm.