Single-shot condensation of exciton polaritons and the hole burning effect
Spontaneous BEC of exciton polaritons is typically achieved with an optical pump and which is tuned far above the exciton resonance in the microcavity. The phonon-assisted and exciton-mediated relaxation of the injected free carriers then efficiently populates the available energy states of the lower polariton (LP) dispersion branch E(k), where k is the momentum in the plane of the QW. The reduced efficiency of the relaxation processes leads to accumulation of the polaritons in the bottleneck region at a high energy close to that of the exciton. When stimulated scattering from this incoherent, high-energy excitonic reservoir into the k = 0 takes place, transition to condensation in the ground state of the LP dispersion Emin (k = 0) is achieved.
In this work, authors perform single-shot real-space imaging of exciton polaritons created by a short laser pulse in a high-quality inorganic microcavity supporting long-lifetime polaritons. By utilising the highly non-stationary single-shot regime, they show the transition to ground-state condensation is driven primarily by reservoir depletion. This is in contrast to the quasistationary CW where this transition is driven by non-radiative energy relaxation processes that are more efficient for more excitonic polaritons. They, further, confirm that spatial fragmentation (filamentation) of the condensate density is an inherent property of a non-equilibrium, spontaneous bosonic condensation resulting from initial random population of high-energy and momenta states, and will persist even after relaxation to the lowest energy and momentum occurs. They unambiguously link this behaviour to the highly nonstationary nature of the condensate produced in a single-shot experiment, as well as to trapping of condensing polaritons in an effective random potential induced by spatially inhomogeneous depletion of the reservoir, i.e. the hole burning effect. They argue that the reservoir depletion and the resulting filamentation is the feature of the condensate growth rather than an indication of its dynamical instability. They also use a wide range of detuning between the cavity photon and QW excitons available in our experiments to vary the fraction of photon and exciton in a polariton quasiparticle, and demonstrate transition from a condensate of light, photonic polaritons with strong filamentation and large shot-to-shot density fluctuations to a more homogeneous state of heavy, excitonic polaritons with reduced density fluctuations, which is only weakly affected by the incoherent reservoir.
In conclusion, the formation of filaments can be interpreted as self-focusing of the polaritons due to effectively attractive nonlinear interactions produced by the hole-burning effect at the early stages of the condensate formation. Although the hole burning has been implied in most conventional theoretical models of the polariton condensation under non-resonant optical excitation conditions, here they present the direct observation of this effect and the associated self-focusing. Authors also succesfully show a remarkable agreement between theory and single-shot experimental results unambiguously.
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