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5 Optical Measurement Techniques
The linear-absorption counterpart of the PL-up-conversion technique which does
not rely on nonlinear conversion on the other side is referred to as optical pump–probe
spectroscopy, also known as transient absorption spectroscopy (the probe signal can
be analysed in reflection or transmission
7 ). The transient reflection technique was
for instance used to examine the gain dynamics of type-I and type-II VECSEL chips
[37]. Derivatives of it are transient reflection contrast, popular in the domain of 2D
materials [9, 38], optical-pump–THz-probe [39, 40] and optical-pump–IR-probe
[41], to name but a few examples.
All pump–probe experiments have in common that a pulsed optical excitation
with ultrashort duration takes place, while a probe pulse with variable delay time
with respect to the pump pulse samples the situation in the studied material for
numerous temporal settings in order to acquire a transient signal (the time trace).
If the probe in such experiment is monochromatic and resonant, a simple photodetection scheme can be used to read out the signal modulation as a function of
delay time. If the probe is broadband such as a white-light supercontinuum, the whole
spectrum can be retrieved for every time step.
If the probe is done via a broadband THz pulse, intraexcitonic transitions can be
probed in common III/V semiconductors after optical excitation. This can for instance
be used to investigate the exciton formation times [39] or the fraction between dark
excitons in reservoir states and condensed ground-state polaritons in optical microcavities [40]. Recently, also indirect excitons in type-II heterostructures were probed
by such method [42]. Note that for large-binding energy materials such as TMDCs,
the probe pulse typically is in the IR range (see for instance [41]).
Opposite to transient absorption or reflection spectroscopy, which can be used for
instance to see ultrafast Rabi oscillations arising from the strong-coupling regime in
quantum-well microcavities [43], luminescence digital holography can be employed
to resolve Rabi oscillations in the emission behaviour on ultrashort time scales [44].
In this scheme, a laser beam is split into two paths, one excitation path and one
reference path, which are superposed to map the phase relation of the sample signal
with the help of the reference signal on a monochromator imaging CCD.
8 Involving
additional pulses or polarisation schemes, the polariton can be manipulated optically
and various states on the Bloch sphere of the cavity–polaritons set [44, 45]. Extending
these manipulation schemes to THz radiation could show the direct impact of strong
transient electric fields on the coherent states of the system, offering a new level of
control for light–matter coupled devices.
The coherent detection scheme provided by the digital holography method and
previously used in [44, 45] for cavity–polaritons in combination with polarisation
sensitivity could give interesting insights into coherent oscillations of hybridised
7 For absorbance A, reflectance R and transmittance T are needed, as A = 1 − R − T .
8 Amended information: In off-axis digital holography, the recorded real-space interferogram for
every delay step undergoes in the evaluation part a 2D Fourier-transform to the phase space, where
the diagonal phase components apart from zero momentum are isolated (truncating the information
in the phase map) and re-transformed into real space. Thereby, sample signal is deconvoluted from
the overall signal with strong reference laser contribution. For details, see [44] and its Supporting
Information.
5 Optical Measurement Techniques
The linear-absorption counterpart of the PL-up-conversion technique which does
not rely on nonlinear conversion on the other side is referred to as optical pump–probe
spectroscopy, also known as transient absorption spectroscopy (the probe signal can
be analysed in reflection or transmission
7 ). The transient reflection technique was
for instance used to examine the gain dynamics of type-I and type-II VECSEL chips
[37]. Derivatives of it are transient reflection contrast, popular in the domain of 2D
materials [9, 38], optical-pump–THz-probe [39, 40] and optical-pump–IR-probe
[41], to name but a few examples.
All pump–probe experiments have in common that a pulsed optical excitation
with ultrashort duration takes place, while a probe pulse with variable delay time
with respect to the pump pulse samples the situation in the studied material for
numerous temporal settings in order to acquire a transient signal (the time trace).
If the probe in such experiment is monochromatic and resonant, a simple photodetection scheme can be used to read out the signal modulation as a function of
delay time. If the probe is broadband such as a white-light supercontinuum, the whole
spectrum can be retrieved for every time step.
If the probe is done via a broadband THz pulse, intraexcitonic transitions can be
probed in common III/V semiconductors after optical excitation. This can for instance
be used to investigate the exciton formation times [39] or the fraction between dark
excitons in reservoir states and condensed ground-state polaritons in optical microcavities [40]. Recently, also indirect excitons in type-II heterostructures were probed
by such method [42]. Note that for large-binding energy materials such as TMDCs,
the probe pulse typically is in the IR range (see for instance [41]).
Opposite to transient absorption or reflection spectroscopy, which can be used for
instance to see ultrafast Rabi oscillations arising from the strong-coupling regime in
quantum-well microcavities [43], luminescence digital holography can be employed
to resolve Rabi oscillations in the emission behaviour on ultrashort time scales [44].
In this scheme, a laser beam is split into two paths, one excitation path and one
reference path, which are superposed to map the phase relation of the sample signal
with the help of the reference signal on a monochromator imaging CCD.
8 Involving
additional pulses or polarisation schemes, the polariton can be manipulated optically
and various states on the Bloch sphere of the cavity–polaritons set [44, 45]. Extending
these manipulation schemes to THz radiation could show the direct impact of strong
transient electric fields on the coherent states of the system, offering a new level of
control for light–matter coupled devices.
The coherent detection scheme provided by the digital holography method and
previously used in [44, 45] for cavity–polaritons in combination with polarisation
sensitivity could give interesting insights into coherent oscillations of hybridised
7 For absorbance A, reflectance R and transmittance T are needed, as A = 1 − R − T .
8 Amended information: In off-axis digital holography, the recorded real-space interferogram for
every delay step undergoes in the evaluation part a 2D Fourier-transform to the phase space, where
the diagonal phase components apart from zero momentum are isolated (truncating the information
in the phase map) and re-transformed into real space. Thereby, sample signal is deconvoluted from
the overall signal with strong reference laser contribution. For details, see [44] and its Supporting
Information.