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Quantum Electronics and Laser Science Conference

Homogeneous linewidths of the resonant luminescence spectra of CuCl quantum dots

Keiichi Edamatsu, T. Itoh, Kyoko Matsuda, S. Saikan

Published Jan 1, 2000• 0 citations
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Abstract

Summary form only given. Excitons confined in nanocrystals or quantum dots attract much attention because they have a very long phase relaxation time that opens the way to the artificial coherent control of the excitons. CuCl quantum dot is a typical material in which the translational motion of the exciton is confined. In this paper, we report on the high resolution measurement of the resonant luminescence of CuCl quantum dots embedded in glass and NaCl matrices under weak excitation condition, and show that the zero-phonon line of the resonant luminescence has a narrow homogeneous linewidth reflecting the long phase relaxation time.

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physica status solidi (a)•Nov 1, 1997•4 citations
Summary form only given. Excitons confined in nanocrystals or quantum dots attract much attention because they have a very long phase relaxation time that opens the way to the artificial coherent control of the excitons. CuCl quantum dot is a typical material in which the translational motion of the exciton is confined. In this paper, we report on the high resolution measurement of the resonant luminescence of CuCl quantum dots embedded in glass and NaCl matrices under weak excitation condition, and show that the zero-phonon line of the resonant luminescence has a narrow homogeneous linewidth reflecting the long phase relaxation time.
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.
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.
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.
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.
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 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.