Quantitative absorption spectroscopy of few perovskite nanocrystals using cavity-enhanced imaging

Single-particle optical spectroscopy has revealed the intrinsic photophysics of nanoscale materials, yet it remains largely confined to emission-based measurements. Direct and quantitative absorption spectroscopy at the level of individual nanosystems has been limited by insufficient sensitivity, leaving non-emissive and higher-lying electronic states experimentally inaccessible. Here, we introduce imaging cavity-enhanced absorption spectroscopy (iCEAS), a high-finesse microcavity platform that amplifies light–matter interaction by orders of magnitude and enables hyperspectral extinction imaging with parts-per-million sensitivity and sub-nanometer spectral precision. Using CsPbBr3 perovskite nanocubes as a model system, we directly measure spectrally resolved absorption cross-sections of clusters containing as few as three nanocubes and demonstrate linear scaling over nearly two orders of magnitude. Correlative atomic force microscopy enables extraction of an absolute single-nanocube absorption cross-section of (6.5 ± 0.6) × 10 ^-14 cm ² at 460 nm, in agreement with independent transient-absorption measurements. Beyond absolute quantification, iCEAS reveals size-dependent spectral shifts and degradation dynamics inaccessible to photoluminescence alone. By enabling correlative access to morphology, absorption, and emission within the same nanoscale object, iCEAS establishes a general framework for quantitative single-particle absorption spectroscopy across a broad range of nanomaterials.