5.4 Nonlinearities
167
be dark in PL owing to its indirect gap, but not dark in SHG due to the symmetry
configuration, whereas a naturally-stacked (AA’) bilayer remains dark in both signal
maps (i.e. PL and SHG).
5.5 Fourier-Space Spectroscopy
Fourier-space imaging has become helpful with regard to single-shot acquisition
of angle-resolved emission. In fact, momentum-space-resolved spectroscopy can be
useful for many studies in which an energy–momentum dispersion can be probed,
such as for exciton–polaritons in optical microcavities. This usefulness can be also
evidenced from recent experiments on 2D semiconductor monolayers highlighted
below. The underlying Fourier-space projection method is explained in more detail
for instance in [4] in the context of polariton research.
5.5.1 Angle-Resolved Detection
Angle-resolved detection can be achieved by various means (for a more detailed
overview see [4]). The simplest approach employs a goniometer arm probing different
angles. Alternatively, one can spatially scan an aperture through the far field of an
objective-collected signal or directly map the Fourier-space plane on an imaging
camera.
Angle-variation experiments are in different ways also applicable to laser performance characterisation (as done for instance in [108, 132, 134]), but the main focus
remains here on the characterisation of angle-resolved PL and reflection contrast
(such as in [3, 46, 66]).
For angle-resolved PL (Fig. 5.16), measurements were done in [66] and followup work using a conventional 4- f μPL setup based on a self-built confocal optical
microscope (cf. Fig. 5.3) with Fourier-space imaging capabilities (similar to those
established by the author for polariton research in [14, 27, 59, 135, 136]). To reach
very low temperatures, the sample is typically mounted in a continuous-flow cryostat
at high vacuum.
14 Commonly, a microscope objective with long-enough working distance such as a 40x (NA 0.6) objective can be used to focus light onto the cooled sample. If discussed at the example of WSe 2 investigations (with its monolayer neutral
exciton located slightly below 720 nm at 10 K), a (pulsed) Ti:sapphire laser tuned to its
short-wavelength edge enables quasi-resonant excitation of monolayer excitons. For
100-fs-pulsed light, a short-pass filter for 700 nm was used to shape the pulse in front
of the sample [3]. To spectrally filter the excitation beam out of the collection path, a
14 To reduce the dependence on cryogens, closed-cycle cryostats are nowadays a popular option for
the replacement of flow cryostats. However, the noise specifications due to mechanical vibrations
have to be taken into account for high-precision measurements.
167
be dark in PL owing to its indirect gap, but not dark in SHG due to the symmetry
configuration, whereas a naturally-stacked (AA’) bilayer remains dark in both signal
maps (i.e. PL and SHG).
5.5 Fourier-Space Spectroscopy
Fourier-space imaging has become helpful with regard to single-shot acquisition
of angle-resolved emission. In fact, momentum-space-resolved spectroscopy can be
useful for many studies in which an energy–momentum dispersion can be probed,
such as for exciton–polaritons in optical microcavities. This usefulness can be also
evidenced from recent experiments on 2D semiconductor monolayers highlighted
below. The underlying Fourier-space projection method is explained in more detail
for instance in [4] in the context of polariton research.
5.5.1 Angle-Resolved Detection
Angle-resolved detection can be achieved by various means (for a more detailed
overview see [4]). The simplest approach employs a goniometer arm probing different
angles. Alternatively, one can spatially scan an aperture through the far field of an
objective-collected signal or directly map the Fourier-space plane on an imaging
camera.
Angle-variation experiments are in different ways also applicable to laser performance characterisation (as done for instance in [108, 132, 134]), but the main focus
remains here on the characterisation of angle-resolved PL and reflection contrast
(such as in [3, 46, 66]).
For angle-resolved PL (Fig. 5.16), measurements were done in [66] and followup work using a conventional 4- f μPL setup based on a self-built confocal optical
microscope (cf. Fig. 5.3) with Fourier-space imaging capabilities (similar to those
established by the author for polariton research in [14, 27, 59, 135, 136]). To reach
very low temperatures, the sample is typically mounted in a continuous-flow cryostat
at high vacuum.
14 Commonly, a microscope objective with long-enough working distance such as a 40x (NA 0.6) objective can be used to focus light onto the cooled sample. If discussed at the example of WSe 2 investigations (with its monolayer neutral
exciton located slightly below 720 nm at 10 K), a (pulsed) Ti:sapphire laser tuned to its
short-wavelength edge enables quasi-resonant excitation of monolayer excitons. For
100-fs-pulsed light, a short-pass filter for 700 nm was used to shape the pulse in front
of the sample [3]. To spectrally filter the excitation beam out of the collection path, a
14 To reduce the dependence on cryogens, closed-cycle cryostats are nowadays a popular option for
the replacement of flow cryostats. However, the noise specifications due to mechanical vibrations
have to be taken into account for high-precision measurements.