3.1 Where Strong Interactions with Light Matters
63
the physics of superfluids were studied at cryogenic temperatures exploiting condensation phenomena in solids [59–62]. The availability of (novel) semiconducting
materials with large excitonic binding energies has even enabled the observation of
room-temperature polariton condensation and superfluidity [63–66].
Prospects for Strong Light–Matter Coupling in Applications
The amount of applications proposed for light–matter coupled systems is ever rising, with leading scientists suggesting microcavity-based devices for optical circuits
[67–70] (with increased mobility compared to their electronic counterparts), terahertz
generation [71–76] and bosonic cascade lasers [77]. Even qubits based on polariton
Rabi oscillators have been proposed [78]. Furthermore, also in the domain of quantum technologies, although much more fundamental in nature, a recent experiment
demonstrated entanglement of a polariton with a single photon [79]. Towards onchip photonic integration of devices with ultralow lasing thresholds, even plasmon–
exciton–polariton lasers have been reported at room temperature using plasmonic
structures with organic materials [80]. Such polariton lasers harness the active metamaterial planar technologies and lower thresholds than conventional lasers based on
stimulated-scattering effects.
Thus, the wealth of microcavity physics is evident [15, 46, 81] and can be conveniently studied using well-established optical-spectroscopy techniques [82].
Other Forms of Polariton Systems
In addition to conventional optical cavities, metamaterials nanocavities could also
be employed to achieve coupling in wavelength ranges beyond the infrared in the
form of terahertz polaritons with systems comprising metallic resonators on top of
semiconductor quantum-well structures, in which the intersubband transitions of
semiconductor heterostructures can couple to the enhanced light field in the metamaterial nanocavity, as used for instance in [83]. Polaritons, which are coupledoscillator systems, can be obtained with quantum-well or organic-molecule excitons
in microcavities, as exciton–polaritons in bulk crystals [84–87], in systems with
phonons [88–92], polarons [93, 94] as well as surface-plasmons [92, 95–97]. The
latter is particularly highlighted for the class of 2D materials in [98], and surely
there are many more examples not mentioned here (a wider overview is further given
in [81]). Strikingly, beyond weak and strong coupling, one can for instance with
strongly-confined light fields and organic molecules also obtain coupling strengths
comparable to the transition frequencies in the light–matter system, or even larger.
The corresponding regime is therefore referred to as the ultra-strong coupling or even
deep-strong coupling regime, respectively [99, 100].
Tunable Polariton Microcavities
In recent years, polariton studies at room temperature with organic and inorganic
high-binding-energy semiconducting materials gained from various tunable-cavity
concepts. The material-independent continuous adjustability of cavity resonances
could be for instance utilised for polariton-based chemistry, which can alter reaction
pathways and rates inside cavities by precisely adjusting polariton energy levels or
light–matter coupling situations (see for instance [102]). In addition, tunable cavities
63
the physics of superfluids were studied at cryogenic temperatures exploiting condensation phenomena in solids [59–62]. The availability of (novel) semiconducting
materials with large excitonic binding energies has even enabled the observation of
room-temperature polariton condensation and superfluidity [63–66].
Prospects for Strong Light–Matter Coupling in Applications
The amount of applications proposed for light–matter coupled systems is ever rising, with leading scientists suggesting microcavity-based devices for optical circuits
[67–70] (with increased mobility compared to their electronic counterparts), terahertz
generation [71–76] and bosonic cascade lasers [77]. Even qubits based on polariton
Rabi oscillators have been proposed [78]. Furthermore, also in the domain of quantum technologies, although much more fundamental in nature, a recent experiment
demonstrated entanglement of a polariton with a single photon [79]. Towards onchip photonic integration of devices with ultralow lasing thresholds, even plasmon–
exciton–polariton lasers have been reported at room temperature using plasmonic
structures with organic materials [80]. Such polariton lasers harness the active metamaterial planar technologies and lower thresholds than conventional lasers based on
stimulated-scattering effects.
Thus, the wealth of microcavity physics is evident [15, 46, 81] and can be conveniently studied using well-established optical-spectroscopy techniques [82].
Other Forms of Polariton Systems
In addition to conventional optical cavities, metamaterials nanocavities could also
be employed to achieve coupling in wavelength ranges beyond the infrared in the
form of terahertz polaritons with systems comprising metallic resonators on top of
semiconductor quantum-well structures, in which the intersubband transitions of
semiconductor heterostructures can couple to the enhanced light field in the metamaterial nanocavity, as used for instance in [83]. Polaritons, which are coupledoscillator systems, can be obtained with quantum-well or organic-molecule excitons
in microcavities, as exciton–polaritons in bulk crystals [84–87], in systems with
phonons [88–92], polarons [93, 94] as well as surface-plasmons [92, 95–97]. The
latter is particularly highlighted for the class of 2D materials in [98], and surely
there are many more examples not mentioned here (a wider overview is further given
in [81]). Strikingly, beyond weak and strong coupling, one can for instance with
strongly-confined light fields and organic molecules also obtain coupling strengths
comparable to the transition frequencies in the light–matter system, or even larger.
The corresponding regime is therefore referred to as the ultra-strong coupling or even
deep-strong coupling regime, respectively [99, 100].
Tunable Polariton Microcavities
In recent years, polariton studies at room temperature with organic and inorganic
high-binding-energy semiconducting materials gained from various tunable-cavity
concepts. The material-independent continuous adjustability of cavity resonances
could be for instance utilised for polariton-based chemistry, which can alter reaction
pathways and rates inside cavities by precisely adjusting polariton energy levels or
light–matter coupling situations (see for instance [102]). In addition, tunable cavities