Publication date: 15th December 2025
Lead halide perovskites are well known for their very large optical absorption coefficient in the visible spectrum, enabling thin film solar cells, few hundreds of nanometres in thickness, to fully absorb sunlight. Quantum mechanical unitarity of the absorption process ensures that the reciprocal process, optical emission, has an equally large matrix element and therefore is also very efficient, a feature that is known as reciprocity and that ensures that the best possible solar cell materials are also the best possible light emitting ones.
I will present evidence that light-matter is strong enough to give rise to polariton states, with a distinctive dispersion, even in unpatterned perovskite thin films, without external cavities or light confinement structures. Since correlated electron-hole pairs are the majority optical excitations in 3D hybrid perovskite thin films at room temperature, such polaritons are of a novel kind, not exciton-polaritons, but more appropriately band-edge polaritons. Not having to depend on the stability of bound excitons, band-edge polaritons survive at very large excitation densities.
Ultrafast optical spectroscopy experiments demonstrate that band edge polaritons even undergo Bose-Einstein condensation at the threshold for stimulated emission. Such findings open new avenues for perovskite optoelectronics thanks to the peculiar properties of polaritons: firstly, polariton lasers are potentially thresholdless because they do not require inversion nor optical gain; second, polaritons propagate at large speed in planar waveguides, and may be applied to integrated photonic circuits; third, and most important, polaritons inherit the strong nonlinearities from their matter component, so that efficient quantum gates may be realized.
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