Single-photon superabsorption in CsPbBr₃ perovskite quantum dots

The absorption of light via interband optical transitions plays a key role in nature and applied technology, enabling efficient photosynthesis and photovoltaic cells, fast photodetectors or sensitive (quantum) light-matter interfaces. In many such photonic systems, enhancing the light absorption strength would be beneficial for yielding higher device efficiency and enhanced speed or sensitivity. So far, however, cavity-free light absorbers feature poorly engineerable absorption rates, consistent with the notion that the coupling strength between the initial and final states is an intrinsic material parameter. By contrast, greatly enhanced absorption rates had been theoretically predicted for superradiant systems, which feature giant oscillator strength through spatially extended coherent oscillations of the electron polarization. Unlike for emission processes, however, experimental realizations of superradiance in absorption-'superabsorption'-remain sparse and require complicated excited-state engineering approaches. Here we report superabsorption by the time reversal of single-photon superradiance in large CsPbBr₃ perovskite quantum dots. Optical spectroscopy reveals a bandgap absorption that strongly increases with the quantum dot volume, consistent with a giant exciton wavefunction. Configuration-interaction calculations, quantitatively agreeing with the experiment, attribute the observed single-photon superabsorption to strong electron-hole pair-state correlations. The approach brings new opportunities for the development of more efficient optoelectronic devices and quantum light-matter interfaces.