64
3 Light–Matter Interactions for Photonic Applications
Fig. 3.2 Modelling data for tunable exciton–photon coupling in an open Fabry–Pérot-type optical
microcavity with movable 2D semiconductor sheet. The structure is sketched in [101]. Spatial
translation of the monolayer WS 2 in this theoretical work along the standing wave electric-field
profile in z direction results in different coupling strengths which is evidenced by an extractable
mode splitting. a Energies of the system (left axis): Blue squares and red triangles indicate the
eigen-energies of the strongly-coupled system (upper and lower polariton states, respectively) for
each sheet distance z to the left mirror, green circles correspondingly indicate the central peak
energy attributed to the weakly-coupled system, according to calculated model reflectivity spectra.
The black dashed line marks the 2D-exciton’s energy and the solid black line the modelled cavity’s
resonance corresponding to an uncoupled system, i.e. E res = ω cav , for which the imaginary part
of the refractive index of WS 2 was neglected. b Absolute relative electric field strength (cavity
photon probability amplitude) calculated for the empty cavity (black line), and the coupled-mode
splitting E = Rabi (dots) obtained from calculated spectra and normalised to its maximum
(left axis). The refractive index modulation of the empty DBR–DBR cavity with air spacer is
displayed for clarity (grey dotted line, right axis). Reproduced under the terms of the CC-BY
Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/
4.0/). [101] Copyright 2020 The Author(s), published by Springer Nature
such as the one in [103] (cf. Fig.7.7) are not only interesting for single-photon
sources and nanolasers, but promising for the investigation of the transition between
weak and strong coupling (see Fig. 3.2), which may become possible by smoothly
tuning the coupling across the ‘exceptional point’ [101], which links the two regimes.
Interesting phenomena are expected at that specific point where only one complex
solution exists for the coupled-oscillator system [104–106].
Quanta of Polarisation in Action
Indeed, polariton physics is not limited to cavity–polaritons. The mere presence of
bulk excitons freely propagating through the host lattice of a solid as a polarisation
wave manifests the existence of exciton–polaritons [87], the quanta of polarisation
in the quasi-particle picture. These examples render the subject of matter excitations
highly interesting and rich of physics and phenomena. More about polariton physics
with an emphasis on Bose–Einstein condensation and polariton lasing can be found
in [9].
3 Light–Matter Interactions for Photonic Applications
Fig. 3.2 Modelling data for tunable exciton–photon coupling in an open Fabry–Pérot-type optical
microcavity with movable 2D semiconductor sheet. The structure is sketched in [101]. Spatial
translation of the monolayer WS 2 in this theoretical work along the standing wave electric-field
profile in z direction results in different coupling strengths which is evidenced by an extractable
mode splitting. a Energies of the system (left axis): Blue squares and red triangles indicate the
eigen-energies of the strongly-coupled system (upper and lower polariton states, respectively) for
each sheet distance z to the left mirror, green circles correspondingly indicate the central peak
energy attributed to the weakly-coupled system, according to calculated model reflectivity spectra.
The black dashed line marks the 2D-exciton’s energy and the solid black line the modelled cavity’s
resonance corresponding to an uncoupled system, i.e. E res = ω cav , for which the imaginary part
of the refractive index of WS 2 was neglected. b Absolute relative electric field strength (cavity
photon probability amplitude) calculated for the empty cavity (black line), and the coupled-mode
splitting E = Rabi (dots) obtained from calculated spectra and normalised to its maximum
(left axis). The refractive index modulation of the empty DBR–DBR cavity with air spacer is
displayed for clarity (grey dotted line, right axis). Reproduced under the terms of the CC-BY
Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/
4.0/). [101] Copyright 2020 The Author(s), published by Springer Nature
such as the one in [103] (cf. Fig.7.7) are not only interesting for single-photon
sources and nanolasers, but promising for the investigation of the transition between
weak and strong coupling (see Fig. 3.2), which may become possible by smoothly
tuning the coupling across the ‘exceptional point’ [101], which links the two regimes.
Interesting phenomena are expected at that specific point where only one complex
solution exists for the coupled-oscillator system [104–106].
Quanta of Polarisation in Action
Indeed, polariton physics is not limited to cavity–polaritons. The mere presence of
bulk excitons freely propagating through the host lattice of a solid as a polarisation
wave manifests the existence of exciton–polaritons [87], the quanta of polarisation
in the quasi-particle picture. These examples render the subject of matter excitations
highly interesting and rich of physics and phenomena. More about polariton physics
with an emphasis on Bose–Einstein condensation and polariton lasing can be found
in [9].