Long phase-relaxation time in CuCl quantum dots: Four-wave-mixing signals analogous to dye molecules in polymers

Transient four-wave-mixing experiments reveal that the phase relaxation time T₂ of an exciton in CuCl spherical quantum dots embedded in glass is very long at low temperatures, e.g., 130 ps (≈10μeV in homogeneous spectral width) at 2 K. The measurement at very weak pumping power enables us to extract the intrinsic value of T₂, typically two orders of magnitude longer than the reported ones. The T₂ value is substantially governed by the energy relaxation time T₁ at 2 K and then becomes shorter with increasing temperature. The time-integrated signal behaves such that the temperature dependence is found to be analogous to that in a system of dye molecules in a polymer.