Femtosecond Dynamics of a Polariton Bosonic Cascade at Room Temperature.

Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. Here, by using a femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of polaritons in a bosonic cascade based on a one-dimensional ZnO whispering gallery microcavity are explicitly visualized. Clear ladder-form build-up processes from higher to lower energy branches of the polariton condensates are observed, which are well reproduced by modeling using rate equations. Remarkably, a pronounced superbunching feature, which could serve as solid evidence for bosonic cascades, is demonstrated by the measured second-order time correlation factor. In addition, the nonlinear polariton parametric scattering dynamics on a time scale of hundreds of femtoseconds are revealed. Our understandings pave the way toward ultrafast coherent control of polaritons at room temperature.

Polariton-Polariton Interactions Revealed in a One-dimensional Whispering Gallery Microcavity.

Coulomb interactions are essential to the dynamics and optical properties of exciton-polaritons. Here, we report an experimental observation of polariton-polariton interactions far beyond theory in a one-dimensional whispering gallery microcavity. Based on the unique half-light half-matter nature, we were able to clarify the effects of excitons, quantum confinement, and nonthermalized polariton distribution in the measurements of the polaritonic interactions. Spectacularly, our position-scan and power-scan investigations both revealed that the polariton-polariton interaction strength is up to 2 orders of magnitude larger than theoretical predictions. These results suggest that polaritonic interactions are far more complicated than the expectation and should be re-examined in polariton physics and devices involving polaritonic interactions.

Strong fluorescence blinking of large-size all-inorganic perovskite nano-spheres.

We demonstrated strong fluorescence blinking on large all-inorganic perovskite (CsPbBr3) nano-spheres. By performing (time-resolved) micro-photoluminescence (µ-PL) measurements, the unique blinking characteristics of the as-grown nano-spheres with diameters of hundred nanometers, are clearly observed. Blinking has no obvious on/off states, which is different from the blinking characteristics of quantum dots. It is believed that the blinking of fluorescence is caused by metastable defect-induced trapping of carriers on the surface of the nano-spheres, because dramatically suppressed fluorescence blinking and the decay rates of ultrafast carriers are realized by surface passivation of the nano-spheres. Surface defects are closely related to the ambient atmosphere, which has been further confirmed by PL measurements of the as-grown nano-spheres in vacuum. Additionally, we also found that the fluorescence blinking was significantly suppressed as the sample size increased, which can be attributed to the large-size induced average effect on fluorescence blinking. These results may be important for understanding the mechanism of the fluorescence blinking of perovskite materials and for developing optical devices with good fluorescence stability.

Dynamics of excited-state condensate for optically confined exciton-polaritons.

We report experimental studies on the dynamics of excited-state condensate for exciton-polaritons confined in an optically generated trap. The three-dimensionally confined trap was realized by imposing two optical barriers onto a one-dimensional ZnO whispering gallery microcavity. Experimentally, we characterized the confined polariton condensate by varying the trap width and the barrier height. Theoretically, we calculated the spatial overlap between the polariton wavefunction and the excitonic reservoir. Direct comparison of these results verified that such polariton-reservoir overlap was responsible for the observed excited-state polariton condensate.

Linearly polarized lasing based on coupled perovskite microspheres.

The polarization of lasing, as a fundamental property related to the emission coherence of microlasers, is one of the criteria for a high quality laser beam but has been rarely investigated. Unlike conventional lasers which can be induced by highly polarized seed light or by using a polarizing film, the microlaser for on-chip integrated photonic circuits generally possesses a low degree of polarization due to unpolarized spontaneous emission. Here, we firstly demonstrate that the Vernier effect can be used to improve the degree of polarization from ∼0.2 to 0.78 based on metal halide perovskite microspheres. After coupling, linearly polarized single-mode lasing with a low threshold and high quality can be achieved. In addition, by using the finite element method, the mode distributions of CsPbBr3 microspheres before and after coupling are analyzed systematically and a clear physical diagram of coupled resonances is obtained. Our work clearly suggests that a coupled linearly polarized single-mode microlaser by the Vernier effect would offer a real step closer to the platform for on-chip integrated photonics.

R2O3 (R = La, Y) modified erbium activated germanate glasses for mid-infrared 2.7 µm laser materials.

Er(3+) activated germanate glasses modified by La2O3 and Y2O3 with good thermal stability were prepared. 2.7 µm fluorescence was observed and corresponding radiative properties were investigated. A detailed discussion of J-O parameters has been carried out based on absorption spectra and Judd-Ofelt theory. The peak emission cross sections of La2O3 and Y2O3 modified germanate glass are (14.3 ± 0.10) × 10(-21) cm(2) and (15.4 ± 0.10) × 10(-21) cm(2), respectively. Non-radiative relaxation rate constants and energy transfer coefficients of (4)I11/2 and (4)I13/2 levels have been obtained and discussed to understand the 2.7 µm fluorescence behavior. Moreover, the energy transfer processes of (4)I11/2 and (4)I13/2 level were quantitatively analyzed according to Dexter's theory and Inokuti-Hirayama model. The theoretical calculations are in good agreement with the observed 2.7 µm fluorescence phenomena. Results demonstrate that the Y2O3 modified germanate glass, which possesses more excellent spectroscopic properties than La2O3 modified germanate glass, might be an attractive candidate for mid-infrared laser.