Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities

Abstract

We report on the direct observation of biexcitons in a III-nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.

Biexcitonic molecules survive excitons at the Mott transition

“…This interpretation is fully supported by the PL transients displayed in Fig. 2d: for time delays longer than 600 ps, the biexciton PL intensity follows the squared slope of the exciton intensity, which indicates thermal equilibrium between exciton and biexciton populations 21 .…”

“…Since the existence of biexcitons was neglected in most of theoretical lines of work accounting for a varying carrier density, likely because of their reduced stability in smaller bandgap QW heterostructures such as GaAs, our results call for more theoretical investigations considering the important role that can be played by those quasi-particles at the MT. However, our results can also be understood by applying Saha's law to excitons and biexcitons 21 . In such a case, the biexciton is expected to be the predominant species at high injection.…”

Temperature-Dependence of Exciton Radiative Recombination in (Al,Ga)N/GaN Quantum Wells Grown on a-Plane GaN Substrates

“…22, the E a obtained from temperature-dependent PL experiments cannot be directly compared to the electron and hole confinement energies. When aiming at describing the thermal escape of carriers from a QW, one needs to account for (i) the ionization of excitonic complexes into electron-hole pairs, 10,23) (ii) the difference in density of states between the (Al,Ga)N barriers and the GaN QW (three and two dimensional, respectively), 10) (iii) B and C excitons, as well as the excited states of the three exciton branches 22) and (iv) the disorder in both the QW plane and the (Al,Ga)N barriers. 10,23) As expected, we manage to fit the temperature dependence of PL for the different excitation spots keeping a constant E a , which we find equal to 43 meV (Fig.…”

Polariton condensation phase diagram in wide-band-gap planar microcavities: GaN versus ZnO

“…Both systems are compared and confronted to numerical simulations based on Boltzmann equations 39 in order to gain further insight into the physics and efficiency of the relaxation processes. Finally, the large impact of the excitonlongitudinal optical (LO) phonon interaction on the polariton relaxation, which has been previously demonstrated in wide bandgap semiconductor microcavities 40,41 , is addressed and its effect on lasing threshold reduction is analyzed showing that its efficiency is strongly dependent on the condensation/lasing regime in which the microcavity operates.…”