Interplay between tightly focused excitation and ballistic propagation of polariton condensates in a ZnO microcavity

Abstract

13 pages, 3 tables and 9 figures. Réf Journal: Phys. Rev. B 92, 235308 (2015)International audienceThe formation and propagation of a polariton condensate under tightly focused excitation is investigated in a ZnO microcavity both experimentally and theoretically. 2D near-field and far-field images of the condensate are measured under quasi-continuous non-resonant excitation. The corresponding spatial profiles are compared to a model based on the Gross-Pitaevskii equation under cylindrical geometry. This work allows to connect the experiments performed with a small excitation laser spot and the previous kinetic models of condensation in a 2D infinite microcavity, and to determine the relevant parameters of both the interaction and the relaxation between the reservoir and the condensate. Two main parameters are identified: the exciton-photon detuning through the polariton effective mass and the temperature, which determines the efficiency of the relaxation from the reservoir to the condensate

Propagating Polaritons in III-Nitride Slab Waveguides

“…When comparing the present values to those obtained in GaAs-based waveguides, 16 the shorter decay lengths and lifetimes in III-nitride structures are likely due to the combination of an enhanced sensitivity to photonic disorder of short wavelength systems together with the increased QW absorption below the band gap and the high number of QWs. If we compare the obtained values for the decay length to the ballistic condensate propagation on the order of 10 µm reported by Hahe et al in a ZnO planar MC, 46 the much larger decay length in the present case can be well accounted for by TIR confinement and the larger polariton propagation velocity.…”

Polariton condensates at room temperature

“…The precise shape of the excitation spot (for example gaussian or flat-top) has a strong impact on the condensate wavefunction [63,87,88]. A second signature of the spatially out-of-equilibrium nature of the polariton condensate lies in the large contribution of k≠0 wave-vectors in the condensate wavefunction, as shown in the Fourier-space imaging of the condensate [38,41,89]. Finally, the photonic disorder [90] may strongly modify this first interpretative picture, and lead to a complex interplay between polariton localization, propagation and radiative losses [42], like in CdTe microcavities [91].…”

“…As theoretically investigated by M. Wouters et al [84], the polariton condensate nucleates under the excitation spot and it is propelled outwards by this repulsive potential, so that the condensation threshold is not defined anymore as the balance between the polariton loss rate and the stimulated relaxation rate from the reservoir to the condensate: the outwards flux of polaritons within the condensate also has to be compensated, and it dominates the polariton loss rate. In a recent study performed in a ZnO microcavity with a low photonic disorder, a typical 10-fold increase of the condensation threshold is extracted from the modeling of the condensate imaging and its ballistic propagation [41]. In most works on room-temperature polariton lasing, this threshold increase related to the tightly focused excitation regime therefore plays a very important role [36,38,[48][49][50]63,85,86].…”

Electrical Tuning of Nonlinearities in Exciton-Polariton Condensates

“…This yields g xp = 2.35 µeV µm 2 at F=0 kV/cm, g xp =3.98 µeVµm 2 at F=12.5 kV/cm and g xp = 4.7 µ eVµ m 2 at F=17 kV/cm. To obtain exciton-exciton interaction strengths from the corresponding exciton-polariton total, the latter has to be normalized by field-dependent exciton Hopfield coefficients [24]. For the electric field values above, after correcting for the exciton fraction we find g x =9.85 µeVµ m 2 (0 kV/cm), g xp = 12.98 µeVµm 2 (12.5 kV/cm) and g xp =13.6 µeVµm 2 (17 kV/cm).…”