Perovskite nanocrystals exhibit exceptional optical properties, including ultrawide color tunability, high quantum efficiency and coherent single-photon emission. [1]
This renders them optimal candidates for optoelectronic devices, yet a comprehensive understanding of their optical characteristics is crucial.

Spectroscopy on single perovskite crystals helps to reveal their excitonic fine structure, which in thin films is concealed by inter-particle phenomena.
While photoluminescence (PL) studies have been conducted on single perovskite nanocubes, their wavelength-dependent absorption cross section has not been reported so far.
Since PL emission always originates from the energetically lowest state of the material, obtaining absorption data can contribute to a more complete picture of the energetic landscape.

The lack of these absorption measurements arises from the inherent difficulty in measuring the marginal absorption of such nanoscale samples.
To overcome this challenge we use an optical resonator, specifically a high-finesse microcavity [2], in which the light passes through the sample a thousand of times, thereby enhancing its absorption to a measurable amount.

This enables us to conduct ultra-sensitive and spectrally resolved measurements of the absorption cross section of perovskite nanocubes at room temperature.
The combination of this novel technique with PL and scanning electron microscopy (SEM) provides unique insight into the size- and morphology dependence of the energetic structure of perovskite nanocubes. Moreover, the degradation of the nanocrystals can be monitored by time-dependent changes in the spectral absorption signal.
An imminent replication of our measurements at cryogenic temperatures is expected to provide further understanding of the material’s optical properties.
The samples are provided by the groups of Prof. Urban and Prof. Kovalenko.

The findings subsume in a series of hyperspectral absorption measurements that were previously conducted on carbon nanotubes and defects in a two-dimensional material [3].
The progress towards routine measurements of samples on the nanoscale demonstrate that cavity-enhanced absorption spectroscopy has the potential to become a standard tool for the characterization of perovskite nanocrystals and other nanoscale samples, similar to the measurement of PL.