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physica status solidi (b)

Homogeneous Width of Confined Excitons in Quantum Dots at Very Low Temperatures

Yasuaki Masumoto, Michio Ikezawa, Byung‐Ryool Hyun, K. Takemoto, Masahiro Furuya

Published Apr 1, 2001• 27 citations
Publisher

Abstract

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.

DOI
10.1002/(sici)1521-3951(200104)224:3<613::aid-pssb613>3.0.co;2-x

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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.
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.
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.
We have systematically investigated low-temperature excitonic dephasing in CuBr and CdSe quantum dots by means of heterodynedetected accumulated photon echo method. It has been clarified that two characteristic mechanisms determine the homogeneous broadening at low temperature: the small energy excitation in two-level system and the twophonon Raman process of confined acoustic phonons. The former brings about the matrix-dependent behaviour of the temperature dependence between CuBr quantum dots in glass and CuBr quantum dots in NaBr below 15 K. Confined acoustic-phonon contribution becomes dominant above 15 K and 10 K for CuBr and CdSe quantum dots, respectively. These results closely resemble the previous report of CuCl quantum dots in weak confinement regime, showing the universal role of surface-related effects and confined phonon scattering as the residual dephasing mechanism of semiconductor quantum dots in matrices at low temperature.
Summary form only given. Understanding dephasing in quantum dots, especially dephasing due to electron-phonon interactions, is essential for the use of quantum dots as artificial atoms in applications such as quantum information processing. In this paper, we report experimental studies on dephasing in CdSe/ZnS core/shell nanocrystals using high resolution spectral hole burning. These studies not only allow us to determine the homogeneous linewidth, but also reveal directly discrete acoustic phonon sidebands. The extremely small homogeneous linewidth (32 /spl mu/eV) obtained at low temperature along with the nonlinear temperature dependence of the homogeneous linewidth indicates that the quantization of acoustic phonon modes leads to strong suppression of dephasing associated with electron-phonon interactions in nanocrystals. The dependence of the spectral hole width on the modulation frequency also reveals exciton spectral migration occurring on a time scale of order of 100 /spl mu/s.
We have investigated nonlinear coherent transients of confined excitons in CuBr quantum dots with use of a three-pulse photon-echo technique. The echo signal shows quantum beats between LO-phonon coupled exciton levels, reflecting a strong electron–phonon interaction in the present quantum dots. The emission efficiency is found to be three orders of magnitude larger than those of ordinary semiconductors, implying that the significant accumulation effect is responsible for the nonlinear emission.
We study CuBr nanocrystals embedded in a borosilicate matrix. The excitonic absorption line is strongly inhomogeneously broadened by the size dispersion of the quantum dots. We explore two ways in order to study the dephasing processes of the transition. The homogeneous absorption linewidth is first determined by spectral hole-burning (SHB) measurements, using a nanosecond dye laser excitation. We then perform degenerate four-wave-mixing experiments (FWM), using 100 fs pulses from a frequency-doubled Ti:sapphire laser. Due to the inhomogeneous broadening, we observe a so-called photon-echo signal and we measure its dynamics. From SHB experiments, we obtain homogeneous linewidths larger than 0.3 meV. From the FWM experiment, we obtain values smaller than 0.2 meV. These discrepancies between SHB and FWM measurements can be explained if we take into account the high excitation intensity necessary to induce absorption saturation in SHB experiments. Moreover, during the 20 ns pulses used, spectral diffusion may occur due to interactions between the nanocrystals and their matrix. Spectrally resolved four-wave-mixing experiments (SR-FWM) are also performed. The oscillations with a 250 fs period we observe at short time delays could be interpreted in terms of quantum beats between two-exciton and one-exciton states.