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Physical review. B, Condensed matter

Long phase-relaxation time in CuCl quantum dots: Four-wave-mixing signals analogous to dye molecules in polymers

Ryosuke Kuribayashi, Kuon Inoue, Kazuaki Sakoda, V. A. Tsekhomskiĭ, Alexander V. Baranov

Published Jun 15, 1998• 32 citations
Publisher

Abstract

Transient four-wave-mixing experiments reveal that the phase relaxation time ${T}_{2}$ 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 ${T}_{2},$ typically two orders of magnitude longer than the reported ones. The ${T}_{2}$ value is substantially governed by the energy relaxation time ${T}_{1}$ 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.

DOI
10.1103/physrevb.57.r15084

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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≈10\ensuremathμeV\ensuremath{\approx}10\ensuremath{\mu}\mathrm{eV}\ensuremath≈10\ensuremathμeV 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. 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.
We perform time-resolved degenerated four-wave mixing experiments on thin CuCl platelets (thickness of about 30 μm) and epitaxially grown layers (thickness of 5 to 10 μm) at low temperature (2K). The light source is a tunable picosecond dye-laser of 20 ps duration (FWHM) and of 0.3 meV spectral width (FWHM). It is synchronously pumped by the third harmonics of a mode-locked Nd:YAG laser. The dye-laser beam is split into two parts (pump and test) with an intensity ratio of about 8 to 1. The optical delay between both pulses is varied.
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.
Picosecond relaxation processes of excitons, including dephasing processes, are systematically studied in CuCl, CdSe and GaAs/AlAs MQW (multi-quantum well structures) by observing time-resolved induced absorption, luminescence and non-degenerate four-wave mixing. By studying the transient induced absorption from the exciton (polariton) state to the excitonic molecule state, time-resolved energy distribution of excitons in CuCl is determined (Fig.1).1) Another technique, i.e. time-resolved luminescence measurement, is applied to excitons in CdSe and GaAs/AlAs MQW (Figs.2 and 3).2,3) By analyzing the above-mentioned experiments the distribution of excitons in the two-dimensional energy-time coordinates is successfully determined. Thus, energy relaxation processes of excitons are clearly visualized. Prior to energy relaxation, coherently generated excitons are considered to be dephased. Phase relaxation of excitons in CuCl is directly measured for the first time in the picosecond time domain by means of time-resolved non-degenerate four-wave mixing (Fig.4).4) These new techniques will be demonstrated and the dephasing as well as energy relaxation mechanism of excitons and excitonic polaritons will be discussed. In the above experiments characteristics of a picosecond tunable optical parametric oscillator pumped by a mode-locked Nd3+:YAG laser as well as a picosecond tunable dye laser synchronously pumped by a mode-locked ion laser are complementarily utilized.
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.
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.