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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Size-dependence of the excitonic dephasing time in CuCl spherical quantum dots

Kuon Inoue, Ryosuke Kuribayashi

Published Jan 20, 2003• 0 citations
Publisher

Abstract

We report on the dot-size dependence of the dephasing time T/sub 2/ value in CuCl spherical QDs, which contrasts with the existing data observed by other methods, and the respective temperature dependencies of the T/sub 2/ value in the different dot-sizes.
DOI
10.1109/cleopr.1999.814800

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Summary form only given. The dephasing time T/sub 2/ of an exciton in the weak confinement regime in semiconductor quantum dots (QDs) is expected to be quite long, or equivalently, the electronic structure should be composed of discrete ultranarrow or atomic-like levels. These characteristics are very attractive for potential application of QDs to devices. In view of no observation of such a long T/sub 2/ value, we directly measured T/sub 2/ of confined excitons in CuCl QDs by using both a degenerate fourwave-mixing (FWM) and a heterodyne photon echo (HPE) experiment, using very weak pumping power.
Transient four-wave-mixing experiments reveal that the phase relaxation time T2{T}_{2}T2​ of an exciton in CuCl spherical quantum dots embedded in glass is very long at low temperatures, e.g., 130 ps ($\ensuremath{\approx}10\ensuremath{\mu}\mathrm{eV}$ in homogeneous spectral width) at 2 K. The measurement at very weak pumping power enables us to extract the intrinsic value of T2,{T}_{2},T2​, typically two orders of magnitude longer than the reported ones. The T2{T}_{2}T2​ value is substantially governed by the energy relaxation time T1{T}_{1}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.
The temperature-dependent homogeneous width of confined excitons in CuCl, CuBr and CdSe quantum dots was investigated at very low temperatures down to 0.6 K by means of accumulated photon echo. The observed nonlinear temperature dependence of the homogeneous width at low temperatures is explained by the interaction with confined acoustic phonons for CdSe quantum dots or very small excitation in the two-level system for CuCl and CuBr quantum dots. At the lowest temperature, the homogeneous width of confined excitons in CuCl, CuBr and CdSe quantum dots becomes 1–2 μeV, 23–59 μeV and 0.16–0.25 meV, respectively. It depends on the surrounding matrix for CuCl quantum dots and the radius of dots for CuBr and CdSe quantum dots. These results suggest surroundings of quantum dots dominate the low temperature dephasing mechanism, i.e. homogeneous width of the optical spectra, of real quantum dots.
Ultranarrow homogeneous linewidth of a Z3{Z}_{3}Z3​ exciton in CuCl quantum dots was studied by using accumulated photon echo. CuCl quantum dots embedded in two kinds of matrices, namely, an alminoborosilicate glass and a NaCl crystal, were studied. The narrowest homogeneous linewidth at low temperature was 1 $\ensuremath{\mu}\mathrm{eV}.$ The temperature dependence of the homogeneous linewidth was similar, above 5 K for both kinds of samples, but in the lower-temperature region, a remarkable difference was observed between them. It indicates that the matrix definitely affects the dephasing process of a confined exciton in quantum dots at low temperatures. The observed temperature dependence was explained by the contribution of the confined phonon in quantum dots and the two-level system in the matrix.
A heterodyne-detected accumulated photon-echo spectroscopy has been applied for the first time to the study of the excitonic dephasing dynamics in CuCl spherical quantum dots embedded in a glass matrix; this spectroscopy has the advantage that a hole-burning spectrum can be extracted from the Fourier-cosine transformation of the signal profile. Owing to this advantage, we reveal that an anomalously slow rise of the time-integrated echo signal at low temperatures, which was also observed in the previous ordinary photon-echo spectroscopy but the origin was left unknown, is caused by a spectral antihole. From the decay (single exponential one) part of the signal which is found to be not influenced by this anomaly, we have obtained rich unambiguous information about the dephasing time T 2 such as the temperature dependence, which is found to be essentially the same as the previous one that one might cast some doubt on.
Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.