3.2 Matter Excitations
75
Caused by momentum conservation of the parallel part with respect to the matter–air
interface (see for instance [87]), only polaritons within the light cone can be emitted
to free space if phonon-assisted emission is neglected [167].
From Macroscopic Polarisation to Incoherent Excitons
For systems with a strong oscillator strength, propagating (oscillating) light fields in
a medium induce a coherent (oscillating) macroscopic polarisation, i.e. they result
in coherent excitons (oscillating polarisation coupled to the electromagnetic wave)
understandable as exciton–polaritons. In the literature, this coupling phenomenon for
light linked to strong refractive index changes around resonances is also heard of as
“slow-light”, “dark polaritons” and “stationary light pulses” (mentioned in Chap. 5 of
[87], see references therein). However, this macroscopic coherent polarisation wave
breaks down after a certain dephasing time into microscopic (local) polarisation, i.e.
incoherent excitons. In other cases with weak interaction with matter, light experience
retardation proportional to the refractive index n > 1 of the medium. In the extreme
case, when light states are not allowed in the medium, light simply cannot penetrate
into it and will be totally reflected, observed as Reststrahlenbande in the optical
spectrum. In fact, a negative permittivity around exciton resonances leads to the
same effect as total internal reflection or reflection from a one-dimensional photonic
crystal interface (Bragg mirror).
Light states can be easily tailored nowadays using optical resonators [11], and
therewith, the absorption, transmission, reflection and radiative-decay behaviour (i.e.
energetics, directionality, lifetime) can be drastically modified using weak [168] and
strong coupling phenomena [1, 22, 32, 33] (also see [4, 7, 169]).
While the formation of exciton–polaritons can be evidenced by Fourier-spaceresolved optical spectroscopy, in which their energy–momentum dispersion exhibits
the very light nature of the hybridised quasi-particles represented by the curvature
of the dispersion, the build-up of an (incoherent) exciton population on ps-to-ns
timescales can be probed effectively using ultrafast optical-pump–THz/IR-probe
schemes. With the probe pulses covering the intra-excitonic transitions (see [170,
171]), such pump–probe schemes can serve as a means of verification of formed
excitons (composite quasi-particles) and timescale investigations, even if the excitonic species does not couple to the light field. In fact, such method has been also
used for the measurement of a dark exciton fraction in light–matter-coupled systems
[172].
Ongoing Efforts to Characterise Excitonic Systems
Several achievements in Marburg in the last years opened the pathway to novel
studies including the characterisation of indirect excitons in semiconductors via
optical-pump–THz-probe spectroscopy [173], the characterisation of charge-transfer
excitons in heterostructures by four-wave-mixing and THz spectroscopy [174], the
ultrafast gain dynamics in quantum-well chip structures via transient reflectivity measurements [175], and the manipulation and control of quantum-well excitons [176].
The latter experiment combined a four-wave mixing experiment in a unique fashion
with a THz pulse that was used to perturb the coherent system in the two-opticalpulses experiment. The aim of such experiment was to monitor the time evolution
75
Caused by momentum conservation of the parallel part with respect to the matter–air
interface (see for instance [87]), only polaritons within the light cone can be emitted
to free space if phonon-assisted emission is neglected [167].
From Macroscopic Polarisation to Incoherent Excitons
For systems with a strong oscillator strength, propagating (oscillating) light fields in
a medium induce a coherent (oscillating) macroscopic polarisation, i.e. they result
in coherent excitons (oscillating polarisation coupled to the electromagnetic wave)
understandable as exciton–polaritons. In the literature, this coupling phenomenon for
light linked to strong refractive index changes around resonances is also heard of as
“slow-light”, “dark polaritons” and “stationary light pulses” (mentioned in Chap. 5 of
[87], see references therein). However, this macroscopic coherent polarisation wave
breaks down after a certain dephasing time into microscopic (local) polarisation, i.e.
incoherent excitons. In other cases with weak interaction with matter, light experience
retardation proportional to the refractive index n > 1 of the medium. In the extreme
case, when light states are not allowed in the medium, light simply cannot penetrate
into it and will be totally reflected, observed as Reststrahlenbande in the optical
spectrum. In fact, a negative permittivity around exciton resonances leads to the
same effect as total internal reflection or reflection from a one-dimensional photonic
crystal interface (Bragg mirror).
Light states can be easily tailored nowadays using optical resonators [11], and
therewith, the absorption, transmission, reflection and radiative-decay behaviour (i.e.
energetics, directionality, lifetime) can be drastically modified using weak [168] and
strong coupling phenomena [1, 22, 32, 33] (also see [4, 7, 169]).
While the formation of exciton–polaritons can be evidenced by Fourier-spaceresolved optical spectroscopy, in which their energy–momentum dispersion exhibits
the very light nature of the hybridised quasi-particles represented by the curvature
of the dispersion, the build-up of an (incoherent) exciton population on ps-to-ns
timescales can be probed effectively using ultrafast optical-pump–THz/IR-probe
schemes. With the probe pulses covering the intra-excitonic transitions (see [170,
171]), such pump–probe schemes can serve as a means of verification of formed
excitons (composite quasi-particles) and timescale investigations, even if the excitonic species does not couple to the light field. In fact, such method has been also
used for the measurement of a dark exciton fraction in light–matter-coupled systems
[172].
Ongoing Efforts to Characterise Excitonic Systems
Several achievements in Marburg in the last years opened the pathway to novel
studies including the characterisation of indirect excitons in semiconductors via
optical-pump–THz-probe spectroscopy [173], the characterisation of charge-transfer
excitons in heterostructures by four-wave-mixing and THz spectroscopy [174], the
ultrafast gain dynamics in quantum-well chip structures via transient reflectivity measurements [175], and the manipulation and control of quantum-well excitons [176].
The latter experiment combined a four-wave mixing experiment in a unique fashion
with a THz pulse that was used to perturb the coherent system in the two-opticalpulses experiment. The aim of such experiment was to monitor the time evolution