82
3 Light–Matter Interactions for Photonic Applications
effect in magnetic fields) [211, 212], and density-dependent, detuning-dependent
or magnetic-field dependent intensity noise (second-order temporal coherence) [38,
209, 213–215].
Polariton Condensate Not a Real BEC
However, a BEC in the strict sense cannot occur in a 2D system such as in optical
microcavities (cf. explanations in the literature [5, 7, 42, 45]), but a similar phase
transition (Kosterlitz–Thouless transition) according to the Berezinski–Kosterlitz–
Thouless theory facilitated by vortex–anti-vortex pair formation, which was first
experimentally evidenced in 2011 [216]. The interacting nature of bosons in polariton
condensates was further demonstrated by the experimental evidence of a Bogoliubov
dispersion relation [201], with the linear excitation branches around the final state
named in reference to Bogoliubov’s theoretical work on superfluidity [217]. The
experimental optical characterisation of condensates and special features of polariton
systems—ranging from typical cavity–polariton condensates to those of the kind at
their extremes—are summarised for instance in [46].
Towards Room-Temperature Operation and Electrical Injection
The ability to achieve polariton condensation at room temperature [218] and in
organic materials [63, 64] opened a way to practical applications. It came into reach
due to technological achievements in the production of high-quality optical microcavities (cf. [4, 7, 169]). This motivated room-temperature polariton-lasing (condensation) studies using large-bandgap semiconductors such as GaN [219, 220] and
ZnO [221, 222], and the hunt for high-binding energy materials such as the currently
highly attractive TMDCs (2D semiconductors) [223–227] or polymers [66, 228].
In recent years, even electrically-driven polariton condensates, also referred to as
polariton lasers, have emerged [48–50].
Plethora of Polariton Systems and Features
Although polaritons are already well studied in many regards, still intriguing features
come up in late studies involving the dynamics of exciton–polaritons [34, 172,
215, 229] and the behaviour of condensed gases in fluids [59, 61, 62, 66, 230,
231], relaxation oscillations in the formation of polariton condensates [232] and
interactions at the quantum limit [79].
Within the last decade, various examples of potential landscapes (e.g. for quantum
simulators [8]) were explored with eyes towards zero-state, π -state, d-wave or Diracpoint condensates [233–235], gap solitons [236] and a flat band in lattices [237,
238], as well as condensates in engineered polariton band structures using patterned
structures [239–241], which can even give rise to topological edge states [241].
Many more exciting examples and important findings are not covered in this brief
overview, whereas even the scope of [9] could not provide a complete summary of
all activities in these directions. An overview of numerous examples of potential
landscapes, polariton devices and alternative material systems for polariton studies
is delivered in [242], of optical condensation studies in [46] and an overview of the
concept of polariton lasers is found in [58].
3 Light–Matter Interactions for Photonic Applications
effect in magnetic fields) [211, 212], and density-dependent, detuning-dependent
or magnetic-field dependent intensity noise (second-order temporal coherence) [38,
209, 213–215].
Polariton Condensate Not a Real BEC
However, a BEC in the strict sense cannot occur in a 2D system such as in optical
microcavities (cf. explanations in the literature [5, 7, 42, 45]), but a similar phase
transition (Kosterlitz–Thouless transition) according to the Berezinski–Kosterlitz–
Thouless theory facilitated by vortex–anti-vortex pair formation, which was first
experimentally evidenced in 2011 [216]. The interacting nature of bosons in polariton
condensates was further demonstrated by the experimental evidence of a Bogoliubov
dispersion relation [201], with the linear excitation branches around the final state
named in reference to Bogoliubov’s theoretical work on superfluidity [217]. The
experimental optical characterisation of condensates and special features of polariton
systems—ranging from typical cavity–polariton condensates to those of the kind at
their extremes—are summarised for instance in [46].
Towards Room-Temperature Operation and Electrical Injection
The ability to achieve polariton condensation at room temperature [218] and in
organic materials [63, 64] opened a way to practical applications. It came into reach
due to technological achievements in the production of high-quality optical microcavities (cf. [4, 7, 169]). This motivated room-temperature polariton-lasing (condensation) studies using large-bandgap semiconductors such as GaN [219, 220] and
ZnO [221, 222], and the hunt for high-binding energy materials such as the currently
highly attractive TMDCs (2D semiconductors) [223–227] or polymers [66, 228].
In recent years, even electrically-driven polariton condensates, also referred to as
polariton lasers, have emerged [48–50].
Plethora of Polariton Systems and Features
Although polaritons are already well studied in many regards, still intriguing features
come up in late studies involving the dynamics of exciton–polaritons [34, 172,
215, 229] and the behaviour of condensed gases in fluids [59, 61, 62, 66, 230,
231], relaxation oscillations in the formation of polariton condensates [232] and
interactions at the quantum limit [79].
Within the last decade, various examples of potential landscapes (e.g. for quantum
simulators [8]) were explored with eyes towards zero-state, π -state, d-wave or Diracpoint condensates [233–235], gap solitons [236] and a flat band in lattices [237,
238], as well as condensates in engineered polariton band structures using patterned
structures [239–241], which can even give rise to topological edge states [241].
Many more exciting examples and important findings are not covered in this brief
overview, whereas even the scope of [9] could not provide a complete summary of
all activities in these directions. An overview of numerous examples of potential
landscapes, polariton devices and alternative material systems for polariton studies
is delivered in [242], of optical condensation studies in [46] and an overview of the
concept of polariton lasers is found in [58].