2D crystals can, in effect, be thought of as huge J-aggregates, focusing electronic oscillator strength in a small set of dominant excitonic transitions. The radiative lifetime of excitons, created within the light cone of the dispersion relation, can be exceedingly short, of order 100 fs. With such short lifetimes it is possible to perform a range of unusual experiments which are hard to conceive with conventional materials. For example, excitation energy can be harvested from plasmon excitations of metallic nanostructures. Whereas metal nanoparticles are often used to quench the fluorescence of dye molecules by FRET, the opposite process is possible with 2D crystals. We excite the plasmon of a single nanoparticle electrically and funnel the spectrally broad excitation energy into the narrow excitonic band of the 2D crystal. In this way, EL can be tuned by the monolayer and is both down- and up-converted to the exciton resonance. Photon correlation spectroscopy demonstrates that emission occurs from nanoscale volumes and reveals signatures of single-photon generation on demand, at room temperature .
A second demonstration of the unusual excitonic properties of these materials is given by their pronounced non-linear optical characteristics. With careful tuning of the excitonic resonances, by temperature and dielectric environment, quantum interference phenomena between transitions of bright and dark excitonic states become apparent. This interference is manifested in resonantly enhanced second-harmonic generation, and is analogous to electromagnetically induced transparency, EIT, providing a pathway to ultrafast optical switching and inversion-less lasing.
 Puchert, Lupton et al., Nature Nano. 12, 637 (2017).