Recent Progress of Strong Exciton–Photon Coupling in Lead Halide Perovskites

Abstract

photon and electronic resonance remain unchanged and the excitonic radiative recombination rate can be accelerated owing to Purcell effect. [1] Even more, when the excitons are strongly coupled to microcavity photo ns, i.e., γ γ > + g ( ) /2 exc 2 cav 2where g is the exciton-photon coupling strength, γ exc and γ cav are the half-widths of uncoupled exciton and microcavity resonances, the energy oscillates back and forth between photons and excitonic resonance state until scattering destroyed this coherent state. In this case, the excitonphoton coupling process becomes faster than their loss process, which makes the coherent energy exchange between them reversible. Thereby a hybrid state of exciton and photon is formed which is named excitonpolariton (Figure 1a). [2] The new energy states behave as two separated energy branches (upper polariton branch (UPB) and lower polariton branch (LPB), respectively) which are "anticrossing" in energy-momentum space, as schemed in Figure 1b. The minimum energy difference between the two branches is called Rabi-splitting energy (2ℏΩ), which is an index of exciton-photon coupling strength via the relation of Figure 1b). [3] The strong excitonphoton coupling has been demonstrated in III-V and II-VI semiconductors, such as GaAs quantum well (QW), [4] ZnO, and GaN nanowires (NWs). [5,6] In a typical dynamics process, polaritons experience creation by external source, occupying the LPB and relaxation to lower energy by phonon emission. If relaxation by phonon scattering becomes inefficient, polariton will decay at high momentum region, known as "bottleneck effect." [7] Meanwhile, at high polariton density, more efficient relaxation pathway such as polariton-polariton scattering becomes predominated, leading to the population buildup at zero momentum, i.e., polariton condensation.Exciton-polariton possesses interesting properties as a hybrid nature. They are boson and undergo a phase transition to Bose-Einstein condensation (BEC) below a critical temperature. [8] More importantly, they have exceedingly light effective mass (typically 10 −4 times the electron mass), [4] which leads to a large thermal de Broglie wavelength, thus a high critical temperature of BEC, even at room temperature. Unlike the atom condensation, polariton condensation must be replenished by stimulated polariton-polariton scattering over a threshold density of polariton since they immediately decay as light, known as polariton laser (Figure 1b). [9] Compared to photonic lasers, polariton lasers can be generated without population inversion, which makes them attractive for ultralow-threshold coherentThe semiconductor exciton-polariton, arising from the strong coupling between excitons and confined cavity photon modes, is not only of fundamental importance in macroscopic quantum effects but also has wide applica tion prospects in ultralow-threshold polariton lasers, slowing-light devices, and quantum light sources. Very recently, metallic halide perovskites have been considered as a great candidate for exciton-pola...

Light Generation in Lead Halide Perovskite Nanocrystals: LEDs, Color Converters, Lasers, and Other Applications

“…In theory, exciton–polariton lasing provides an alternative pathway to low threshold coherent emission without population inversion . In electrically pumped InGaAs exciton–polariton lasers, the threshold of exciton–polariton lasing is 1–3 orders of magnitude lower than that for population inversion lasing in the same diode .…”

“…In electrically pumped InGaAs exciton–polariton lasers, the threshold of exciton–polariton lasing is 1–3 orders of magnitude lower than that for population inversion lasing in the same diode . The sufficiently strong coupling between an exciton and a photon leads to a composite boson known as an exciton–polariton, a new quantum superposition state containing two branches that consist of both light and matter . Both the lower and upper branches are mixtures of the original photon and exciton states .…”

“…In theory, exciton-polariton lasing provides an alternative pathway to low threshold coherent emission without population inversion. [17,[172][173][174][175][176][177][178] In electrically pumped InGaAs excitonpolariton lasers, the threshold of exciton-polariton lasing is 1-3 orders of magnitude lower than that for population inversion lasing in the same diode. [175,176] The sufficiently strong coupling between an exciton and a photon leads to a composite boson known as an exciton-polariton, a new quantum superposition state containing two branches that consist of both light and matter.…”

“…[175,176] The sufficiently strong coupling between an exciton and a photon leads to a composite boson known as an exciton-polariton, a new quantum superposition state containing two branches that consist of both light and matter. [172][173][174][175][176][177][178] Both the lower and upper branches are mixtures of the original photon and exciton states. [172][173][174][175][176][177][178] Above a critical particle density, a macroscopic number of bosons undergoes a nonequilibrium cooling process, i.e., stimulated scattering caused by repulsive interactions, and then accumulates in the ground state, i.e., a single quantum state, of the lower excitonpolariton branch, which is regarded as Bose-Einstein condensation.…”

“…[172][173][174][175][176][177][178] Both the lower and upper branches are mixtures of the original photon and exciton states. [172][173][174][175][176][177][178] Above a critical particle density, a macroscopic number of bosons undergoes a nonequilibrium cooling process, i.e., stimulated scattering caused by repulsive interactions, and then accumulates in the ground state, i.e., a single quantum state, of the lower excitonpolariton branch, which is regarded as Bose-Einstein condensation. [172][173][174][175][176][177][178] Eventually, a large number of condensed exciton-polaritons decay from the single state by leaking coherent photons.…”