Direct observation of excitonic polaron in InAs/GaAs quantum dots

Abstract

Excitonic polaron is directly demonstrated for the first time in InAs/GaAs quantum dots with photoluminescence method. A new peak (s ′ ) below the ground state of exciton (s) comes out as the temperature varies from 4.2 K to 285 K, and a huge anticrossing energy of 31 meV between s ′ and s is observed at 225 K, which can only be explained by the formation of excitonic polaron.The results also provide a strong evidence for the invalidity of Huang-Rhys formulism in dealing with carrier-longitudinal optical phonon interaction in quantum dot. Instead, we propose a simple two-band model, and it fits the experimental data quite well. The reason for the finding of the anticrossing is also discussed.PACS numbers: 60.20. Kr, 78.67.Hc Electron-phonon interaction is a very important ingredient determining the physical properties of semiconductors, such as phonon-assisted hot carrier relaxation process [1,2], light absorption and emission process [3,4]. For electron-longitudinal optical (LO) interaction in the weak polar system (e.g., GaAs, α c = 0.067), the welldocumented Huang-Rhys model always gives good explanation in bulk material. However, in quantum dot (QD), this interaction is greatly enhanced owing to the discrete nature of energy levels with spacing comparable to the energy of LO phonon. Both theoretical and experimental findings show that it may have entered the strong coupling regime [3,5,6,7], which means that an accurate description of this interaction system should be the hybridation of electron state and the phonon state, thus the polaron as a new ground state will be formed [8]. Similar conclusion can also be drawn to hole and exciton interacting with LO phonons in QD.Although Huang-Rhys parameter S and Fröhlich coupling constant α c are both related to the average number of LO phonons, while the irreversible emission of LO phonons (0.1 ∼ 1 ps) provides an efficient channel for energy relaxation [9], the formation of polaron will suppress the LO phonon contribution to the carrier decoherence process and hence leads to long polaron lifetime [5,6,7] and everlasting oscillation of survival probability [5,10,11].In experiments, far-infrared (FIR) absorption results have evidenced the formation of electron-LO [10,11], hole-LO [12] and exciton-LO[13] induced magnetopolaron in QDs under ultra-high magnetic field (up to ∼ 28 T) at about 4 K, and anticrossing between the polaron levels differing by one LO phonon[6] is found. How- * email: cfli@ustc.edu.cn ever, hampered by the large full width at half maximum (FWHM), no direct observation of excitonic polaron has yet been reported.In this letter, as we vary the temperature from 4 K to 285 K, the FWHM of the s peak decreases sharply, thus we can directly observe the excitonic polaron without applying magnetic field. It is shown that at low temperature, the exciton-LO phonon interaction is weak and may be explained within the framework of Huang-Rhys model and enhanced S value is obtained. But at high temperature, due to the increasing of coupling strength, exciton...

Storage of multiple single-photon pulses emitted from a quantum dot in a solid-state quantum memory

“…When the power of the 910-nm heating laser reaches 24 mW, the emission line can be finely red-shifted to 879.7 nm (ref. 30 ), that is, the memory band. The autocorrelation of this emission line is measured to be ∼ g (2) (0)=0.14, which demonstrates that the photons were emitted from a good-quality single-photon source.…”

“…Figure 2(b) shows the local-heating spectrum for this QD (see Supplementary Note 3 for local heating, and the sketch of the local heating and some additional data are shown in Supplementary Figure 3). When the power of the 910-nm heating laser reaches 24 mW, the emission line can be finely red-shifted to 879.7 nm [30], i.e., the memory band. The autocorrelation of this emission line is measured to be approximately g (2) (0) = 0.14, which demonstrates that the photons were emitted from a good-quality single photon source.…”

“…Compared with global heating, local heating can induce less noise and less inconvenience due to heat expansion. The heating effect can red shift the emission line of the QD continuously by several 0.1 nm [30]. During this process, a solid etalon (see Figure 1), which is calibrated by the pump laser of the quantum memory (at 879.7 nm), is used to set the target wavelength of the QD emission.…”

“…The heating effect can red-shift the emission line of the QD continuously by several 0.1 nm (ref. 30 ). During this process, a solid etalon (see Fig.…”

Temperature dependent empirical pseudopotential theory for self-assembled quantum dots

“…[32][33][34][35] The temperature dependent optical spectra of (single and ensemble) QDs have been investigated intensively in experiments in the past decades. [10][11][12]14,15,36,[36][37][38] New physics, for example, the formation of excitonic polaron 37,39,40 which is due to strong coupling between exciton and optical phonons, may be found in QDs at high temperature. However, a theoretical understanding of the temperature effects in QDs is still missing.…”