116
4 In the Field of Quantum Technologies
Manipulation and Control of Coherent States
In this context, also the ultrafast manipulation and control of the polariton state with
external fields gained importance, as recent works on optically modifying states on
the polariton’s Bloch sphere [198] or polarisation scrambling [199] show. External
fields were previously known as a successful tool to alter a polariton configuration
reversibly, such as by magnetic fields [118, 142, 143, 147, 200–202], even to control strong coupling in the single-quantum-emitter regime [203]. Similarly, electric
fields can be utilised, allowing one to spectrally tune individual quantum dots within
a quantum strong-coupling regime [204] or to switch between a polaritonic and
photonic regime of microcavity emission [137].
Advances in Polariton Research
Recently, the topic of 2D materials in optical microcavities gained strong momentum after pioneering works [205–208] as a logical consequence of first insights into
monolayer exciton properties (also see the summary in Chap. 2, and [209]). 2D semiconductors with their strong excitonic binding energy and correspondingly high oscillator strength in the ideal quantum-well-like 2D crystal were predestined for strong
light–matter coupling, which was previously already considered for graphene islands
([210]). The monolayers of TMDCs show a clear advantage with respect to high temperature operation and have become very promising testbeds for room-temperature
polaritonics and BEC studies. Similarly, few-layer materials have come into focus,
such as homobilayers (targeted with configurations such as in [211]), or heterobilayers with interlayer and moiré features (some of which indicating a pronounced
oscillator strength as reflection-contrast measurements in [212] show), leading to a
unique revival of polariton research. Recently, even exotic “topolaritons”—quasiparticles with topologically nontrivial states (polaritonic bands with so-called chiral
edge modes) and unidirectional polariton flow—were proposed in the literature [213]
which could possibly be achieved with monolayer TMDCs when the system is properly prepared.
The ability to use valley physics—already discussed with regard to “valleytronics” with 2D materials [214, 215]—in combination with polarisation-selective optical cavities, such as chiral microcavities based on polarisation-sensitive mirrors
and structures (addressed by the author’s team with preliminary design and fabrication attempts for chiral polaritons within a Master thesis project 2017/2018)
remains very attractive and requires advanced (nano-)fabrication capabilities. In fact,
helicity-favouring microcavities were previously investigated for monolithicallygrown quantum-well and quantum-dot microcavity systems [216–218]. Valleypolarised polaritons based on optical pumping schemes had been demonstrated
shorty after the first TMDC polaritons [219–222], whereas the natural next step
would involve the employment of chiral optical microstructure approaches which
could open up new possibilities related to polarisation- as well as spin-sensitive
electro-optical devices, i.e. optical valleytronics.
Tunable Light–Matter Interactions Around an Exceptional Point
One major advantage which is linked to the high oscillator strength of monolayer
semiconductors, other than the room-temperature operability, lies with the reduced
4 In the Field of Quantum Technologies
Manipulation and Control of Coherent States
In this context, also the ultrafast manipulation and control of the polariton state with
external fields gained importance, as recent works on optically modifying states on
the polariton’s Bloch sphere [198] or polarisation scrambling [199] show. External
fields were previously known as a successful tool to alter a polariton configuration
reversibly, such as by magnetic fields [118, 142, 143, 147, 200–202], even to control strong coupling in the single-quantum-emitter regime [203]. Similarly, electric
fields can be utilised, allowing one to spectrally tune individual quantum dots within
a quantum strong-coupling regime [204] or to switch between a polaritonic and
photonic regime of microcavity emission [137].
Advances in Polariton Research
Recently, the topic of 2D materials in optical microcavities gained strong momentum after pioneering works [205–208] as a logical consequence of first insights into
monolayer exciton properties (also see the summary in Chap. 2, and [209]). 2D semiconductors with their strong excitonic binding energy and correspondingly high oscillator strength in the ideal quantum-well-like 2D crystal were predestined for strong
light–matter coupling, which was previously already considered for graphene islands
([210]). The monolayers of TMDCs show a clear advantage with respect to high temperature operation and have become very promising testbeds for room-temperature
polaritonics and BEC studies. Similarly, few-layer materials have come into focus,
such as homobilayers (targeted with configurations such as in [211]), or heterobilayers with interlayer and moiré features (some of which indicating a pronounced
oscillator strength as reflection-contrast measurements in [212] show), leading to a
unique revival of polariton research. Recently, even exotic “topolaritons”—quasiparticles with topologically nontrivial states (polaritonic bands with so-called chiral
edge modes) and unidirectional polariton flow—were proposed in the literature [213]
which could possibly be achieved with monolayer TMDCs when the system is properly prepared.
The ability to use valley physics—already discussed with regard to “valleytronics” with 2D materials [214, 215]—in combination with polarisation-selective optical cavities, such as chiral microcavities based on polarisation-sensitive mirrors
and structures (addressed by the author’s team with preliminary design and fabrication attempts for chiral polaritons within a Master thesis project 2017/2018)
remains very attractive and requires advanced (nano-)fabrication capabilities. In fact,
helicity-favouring microcavities were previously investigated for monolithicallygrown quantum-well and quantum-dot microcavity systems [216–218]. Valleypolarised polaritons based on optical pumping schemes had been demonstrated
shorty after the first TMDC polaritons [219–222], whereas the natural next step
would involve the employment of chiral optical microstructure approaches which
could open up new possibilities related to polarisation- as well as spin-sensitive
electro-optical devices, i.e. optical valleytronics.
Tunable Light–Matter Interactions Around an Exceptional Point
One major advantage which is linked to the high oscillator strength of monolayer
semiconductors, other than the room-temperature operability, lies with the reduced