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Physical Review Letters

Investigation of femtosecond electronic dephasing in CdSe nanocrystals using quantum-beat-suppressed photon echoes

Robert W. Schoenlein, Daniel M. Mittleman, Joe J. Shiang, A. Paul Alivisatos, Charles V. Shank

Published Feb 15, 1993• 188 citations
Publisher

Abstract

We report the first direct measurements of femtosecond electronic dephasing in CdSe nanocrystals using three-pulse photon echoes and a novel mode-suppression technique. We are able to separate the dynamics of the coherently excited LO phonons from the underlying electron-hole dephasing by suppressing the quantum beats. The homogeneous linewidth of these materials at 15 K results from electronic dephasing in \ensuremath{\sim}85 fs, approximately half of which is due to acoustic phonon modes. Contributions from acoustic phonons dominate the homogeneous linewidth at room temperature.

DOI
10.1103/physrevlett.70.1014

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We report the first direct measurements of femtosecond electronic dephasing in CdSe nanocrystals using three-pulse photon echoes and a novel mode-suppression technique. We are able to separate the dynamics of the coherently excited LO phonons from the underlying electron-hole dephasing by suppressing the quantum beats. The homogeneous linewidth of these materials at 15 K results from electronic dephasing in \ensuremath{\sim}85 fs, approximately half of which is due to acoustic phonon modes. Contributions from acoustic phonons dominate the homogeneous linewidth at room temperature.
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Semiconductor nanocrystals are a novel class of materials intermediate between the molecular and solid state forms of matter. Their homogenous linewidths and electronic structure are of considerable interest for the understanding of the physics of quantum confinement. In this work we study the electronic and vibrational dephasing dynamics in InP nanocrystals using femtosecond three pulse photon echo (3PPE) experiments1 complemented by nanosecond transient hole-burning (HB) spectroscopy.2 The echo decay is explained by taking into account the multi-level electronic structure as observed in the hole-burning experiment.
The results of the femtosecond four-wave mixing measurements performed on the CdS crystal at room temperature are reported. The dephasing time T d = 180 fs was obtained. The anti-Stokes shift of the primary photon echo spectrum with respect to the excitation spectrum was observed. The some peculiarities of the coherent responses decay curves indicate that this optical dephasing is due to the exciton-exciton interaction. The stimulated photon echo decay curves appear to have a fast decay component (the time constant of approximately 4.5 ps) in case the energy of exciting pulses exceeds the threshold value of 0.4 μJ.
We demonstrate the hallmark concept of periodic collapse and revival of coherence in a room-temperature ensemble of quantum dots (QDs) in the form of a 1.5-mm-long optical amplifier. Femtosecond excitation pulses induce coherent interactions with a number of discrete homogeneous QD subgroups within an inhomogeneously broadened ensemble, which interfere constructively to induce coherent revival (CR). The amplitude decay of CR is dictated by the QD homogeneous linewidth, thus enabling its extraction in a double-pulse Ramsey-type experiment. The more common photon echo technique is also invoked and yields the same linewidth provided that both experiments use the same bias. A dephasing time T2{T}_{2}T2​ longer than 5 ps is extracted. This is a record long T2{T}_{2}T2​ for a room-temperature QD ensemble and testifies to the high quality of the InAs/InP QDs used in the experiments. Measured electrical bias and temperature dependencies of the transverse relaxation times (T2{T}_{2}T2​ and T2∗{T}_{2}^{*}T2∗​) enable the determination of the two main decoherence mechanisms: carrier-carrier and carrier-phonon scatterings.
Temperature dependence of the homogeneous linewidth of CuBr quantum dots in glass and NaBr matrices and CdSe quantum dots in glass matrix has been investigated using heterodyne-detected accumulated photon echo. We can reproduce the observed nonlinear temperature dependence considering two characteristic mechanisms: the small energy excitation in two-level system and the two-phonon Raman process of confined acoustic phonons. The interaction with the two-level system is dominant below 15 K and 10 K for CuBr and CdSe quantum dots, respectively. Above these temperatures the interaction with confined phonons is dominant. These results are similar to those of a previous study of CuCl quantum dots in glass, showing the universal influence of confined phonons and surrounding matrices on the low-temperature dephasing process of very small semiconductor quantum dots.
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 ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$=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{}${10}^{\mathrm{\ensuremath{-}}13}$ cm2{\mathrm{cm}}^{2}cm2. With increasing temperature, the dephasing of the bound excitons is determined by thermal release from the impurity bound state.