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Physical review. B, Condensed matter

Spontaneous photon echo from bound excitons in CdSe

Holger Schwab, Vadim G. Lyssenko, J. M. Hvam

Published Aug 15, 1991• 23 citations
Publisher

Abstract

We have observed spontaneous photon echoes from acceptor bound excitons in CdSe. The ${\mathit{I}}_{1}$ line is inhomogeneously broadened to about 1 meV. The homogeneous linewidth is, at low temperature and low excitation density, found to be extremely small and is determined by the recombination lifetime ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$=500 ps. The bound excitons have negligible interaction with phonons and interact only weakly with free excitons with a collision cross section \ensuremath{\sigma}=5.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$. With increasing temperature, the dephasing of the bound excitons is determined by thermal release from the impurity bound state.

DOI
10.1103/physrevb.44.3999

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We have observed spontaneous photon echoes from acceptor bound excitons in CdSe. The I1{\mathit{I}}_{1}I1​ line is inhomogeneously broadened to about 1 meV. The homogeneous linewidth is, at low temperature and low excitation density, found to be extremely small and is determined by the recombination lifetime \ensuremathτR{\mathrm{\ensuremath{\tau}}}_{\mathit{R}}\ensuremathτR​=500 ps. The bound excitons have negligible interaction with phonons and interact only weakly with free excitons with a collision cross section \ensuremath{\sigma}=5.8\ifmmode\times\else\texttimes\fi{}10\ensuremath−13{10}^{\mathrm{\ensuremath{-}}13}10\ensuremath−13 cm2{\mathrm{cm}}^{2}cm2. With increasing temperature, the dephasing of the bound excitons is determined by thermal release from the impurity bound state.
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.
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We report low-temperature results of accumulated and two-pulse picosecond photon-echo experiments on the optical transition of the nitrogen-bound exciton in GaP:N. The observed temperature dependence of the dephasing of the so-called A line at 534.95 nm can be explained in terms of (stimulated) phonon processes involving the lower B state and, at higher temperatures, the free-exciton band. The low-temperature-echo decay time of 25.5\ifmmode\pm\else\textpm\fi{}0.5 ps is shown to be determined by the lifetime of a A state. Analysis of the factors governing the intensity of the accumulated photon echo as a function of temperature leads to the conclusion that in the B state temperature-independent excitation transfer occurs. The rate constant of this phenomenon, which we ascribe to excitation tunneling, is 107{10}^{7}107 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.
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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.