Confined acoustic vibration modes in CuBr quantum dots

Confined acoustic phonons in CuBr quantum dots (QDs) are studied for the first time using coherent phonon measurements and persistent spectral hole burning (PSHB) spectroscopy. A size-dependent damped oscillation with sub-THz frequency is observed in femtosecond pump-probe measurements of CuBr QDs in glass. Similarly, a size-dependent acoustic phonon hole with an energy shift of 1-2 meV is observed in PSHB spectra of CuBr QDs in glass and NaBr crystals. The energies of the confined acoustic phonons obtained by the two methods are in good agreement with each other. In addition, there is no difference in size-dependence between the glass matrix and NaBr matrix. The size dependence of acoustic phonons can be explained well as low frequency vibrational modes of CuBr QDs based on an elastic sphere model. The dominant mode is assigned to the l = 2 spheroidal mode, while the l = 0 mode is also observed.