Size-selective two-photon spectroscopy of CuCl spherical quantum dots

We analyze luminescence spectra resulting from a resonant size-selective two-photon excitation of the low-energy confined ${\mathrm{Z}}_3$-exciton states in CuCl spherical nanocrystals. Excitation spectra of the two-photon-excited luminescence (TPL) bands and size dependences of band energies show that the TPL bands arise due to annihilation of the exciton in the lowest-energy state excited both directly and through the higher-energy confined states. It is shown that the oserved features of the TPL fine structure can be unambiguously interpreted in terms of longitudinal (L) and transverse (T) confined exciton states and resonant enhancement of phonon-assisted luminescence when the energy gap between 1S and 1P states of the T exciton is close to the LO-phonon energy.