Polariton condensation

We report on the observation of polariton lasing in a negative detuned ZnO hybrid microcavity at room temperature (300 K) with threshold of 12.38 J/cm 2 and at high temperature (353 K) with a higher threshold of 53.05 J/cm 2 due to a stronger thermal escape process and a shallower polariton trap. The decoherence mechanism of the polariton laser due to the polariton self-interaction was investigated by power-dependent photoluminescence and Michelson interference measurements. The spatial coherence length r c 0:26 m of the polariton laser was obtained from Young's double slits.

Unlike conventional photon lasing, in which the threshold is limited by the population inversion of the electron-hole plasma, the exciton lasing generated by exciton-exciton scattering and the polariton lasing generated by dynamical condensates have received considerable attention in recent years because of the sub-Mott density and low-threshold operation. This paper presents a novel approach to generate both exciton and polariton lasing in a strongly coupled microcavity (MC) and determine the critical driving requirements for simultaneously triggering these two lasing operation in temperature <140 K and large negative polariton-exciton offset (<−133 meV) conditions. In addition, the corresponding lasing behaviors, such as threshold energy, linewidth, phase diagram, and angular dispersion are verified. The results afford a basis from which to understand the complicated lasing mechanisms in strongly coupled MCs and verify a new method with which to trigger dual laser emission based on exciton and polariton. A microcavity (MC), which confines photons in a wavelength scale resonator, has numerous applications, such as in vertical-cavity surface-emitting lasers, single quantum emitters, and other optoelectronic devices, depending on the active layer design 1–6. Because the active layer in the cavity exhibits excitonic behavior, the MC can enter a strong exciton-photon coupling regime and generate a hybrid quasiparticle called polariton 7–8. Within a strongly coupled MC, the polariton laser—a novel type ultralow threshold coherent light source emitted from a condensed polariton state that does not require the electron-hole plasma to be in a population inversion condition—has been reported in various types of MC 9–18. In addition to polaritons, the so-called P-band, generated through exciton-exciton inelastic scattering (X-X) mechanisms in the phase-space at an intermediate exciton density, can generate a high excitonic gain to compensate the loss to achieve the lasing action 19–23. These two exciton-related lasing actions greatly influence future ultralow threshold lasers and should exhibit some correlation. They can also be manipulated since they share the same exciton reservoir. However, these two lasing actions have not been simultaneously triggered or studied in strongly coupled MCs because of critical operation condition, which involves difficulty encountered when attempting to obtain adequate exciton numbers to generate excitonic gain in a strongly coupled MC. Considering this, ZnO, which exhibits strong excitonic behavior, including large exci-ton binding energy and oscillator strength, can possibly achieve excitonic P-band lasing in a strongly coupled MC. Although the polaritonic behavior and low-threshold polariton lasing in a ZnO MC have been reported, simultaneous excitonic P-band lasing and polariton lasing in a ZnO MC remain unreported 24–36. In this study, we investigated polariton lasing and P-band lasing in a strongly coupled ZnO MC. The polariton lasing can be maintained up to 353 K because of the robust exciton properties in ZnO. The conditions required to simultaneously trigger these two lasing actions were specified by the operating temperature and lower polariton branch (LPB)-exciton offset (Δ = E LPB − E x , where E LPB and E x represent the energy of LPB and exciton, respectively). The experimental investigations indicate that the P-band exciton lasing was effective when the negative LPB offset was large (|Δ |> 133 meV). However, achieving polariton lasing in this offset is difficult because of the relatively short lifetime of the polariton. The P-band exciton lasing can coexist with polariton lasing in a moderate pumping condition.

The polariton condensation phase diagram is compared in GaN and ZnO microcavities grown on mesa-patterned silicon substrate. Owing to a common platform, these microcavities share similar photonic properties with large quality factors and low photonic disorder, which makes it possible to determine the optimal spot diameter and to realize a thorough phase diagram study. Both systems have been investigated under the same experimental conditions. The experimental results and the subsequent analysis reveal clearly that longitudinal optical phonons have no influence in the thermodynamic region of the condensation phase diagram, while they allow a strong (slight) decrease of the polariton lasing threshold in the trade-off zone (kinetic region). Phase diagrams are compared with numerical simulations using Boltzmann equations, and are in satisfactory agreement. A lower polariton lasing threshold has been measured at low temperature in the ZnO microcavity, as is expected due to a larger Rabi splitting. This study highlights polariton relaxation mechanisms and their importance in polariton lasing.

We report on the room temperature polariton lasing and photon lasing in a ZnO-based hybrid microcavity under optical pumping. A series of experimental studies of the polariton lasing (exciton-photon detunings of δ = −119meV) in the strong-coupling regime are discussed and compared to a photon lasing (δ = −45meV) in the weak-coupling regime obtained in the same structure. The measured threshold power density (31.8kW/cm 2) of polariton lasing is one order of magnitude lower than that of the photon lasing (318.2kW/cm 2). In addition, the comparison between polariton lasing and photon lasing is done in terms of the linewidth broadening, blue-shift of the emission peak, and polarization.