Recycled Photons Traveling Several Millimeters in Waveguides Based on CsPbBr₃ Perovskite Nanocrystals

Abstract Reabsorption and reemission of photons, or photon recycling (PR) effect, represents an outstanding mechanism to enhance the carrier and photon densities in semiconductor thin films. This work demonstrates the propagation of recycled photons over several mm by integrating a thin film of CsPbBr₃ nanocrystals into a planar waveguide. An experimental set‐up based on a frequency modulation spectroscopy allows to characterize the PR effect and the determination of the effective decay time of outcoupled photons. A correlation between the observed photoluminescence redshift and the increase of the effective decay time is demonstrated, which grows from 3.5 to near 9 ns in the best device. A stochastic Monte Carlo model reproduces these experimental results and allows the extraction of the physical mechanisms involved. In the waveguide under study recycled photons follow a drift (directional enhancement) velocity ≈5.7 × 10⁵ m s⁻¹, dominating over the diffusive regime observed in a standard thin film (D ≈ 420 m² s⁻¹). This means that recycled photons propagate mm‐distances in shorter traveling times in the waveguide (≈5 ns) as compared to the film (>20 ns). These results are expected to pave the road for exploiting the PR effect in future optoelectronic and photonic devices.