Direct observation of optically forbidden energy transfer between CuCl quantum cubes via near-field optical spectroscopy.

We report, for the first time, evidence of near-field energy transfer among CuCl quantum cubes using an ultrahigh-resolution near-field optical microscopy and spectroscopy in the near UV region at 15 K. The sample was high-density CuCl quantum cubes embedded in a NaCl matrix. Measured spatial distributions of the luminescence intensities from 4.6-nm and 6.3-nm quantum cubes clearly established anticorrelation features. This is thought to be a manifestation of the energy transfer from the lowest state of exciton in 4.6-nm quantum cubes to the first dipole-forbidden excited state of exciton in 6.3-nm quantum cubes, which is attributed to the resonant optical near-field interaction.