Quantum optics with II-VI quantum dots

People:
Régis André, Jean-Michel Gérard, Kuntheak Kheng, Henri Mariette, Jean-Philippe Poizat, Robert Romestain

Photon correlation experiments
We report correlation and cross-correlation measurements of photons emitted under continuous wave excitation by a single II–VI quantum dot (QD) grown by molecular-beam epitaxy. A standard technique of micro-photoluminescence combined with an ultrafast photon correlation setup allowed us to see an anti-bunching effect on photons emitted by excitons recombining in a single CdTe/ZnTe QD (see Fig. 1), as well as cross correlation within the biexciton-exciton radiative cascade from the same dot (see Fig.2).
  
The experimental results have been fitted with multi-exciton ladder model with a population of up to 4 exciton. After convolution with the time resolution of our setup, and taking into account the signal to background ratio, the bold line in Fig. 1 is the prediction of the ladder model for the autocorrelation function. There is a difference of a factor of 4 between the FWHM of the experimental curve and the model prediction. This mismatch has been observed on the three quantum dots of the same sample that we have studied. Previous groups have observed the same mismatch in the past for III–V compounds.
 
 
Our result in Fig. 2 displays very clearly the bunching part for positive times, and less clearly the anti-bunching part for negative times. The bold line is the fitting curve using the same ladder model with up to four excitons in the dot. Now, we observe that, contrary to the antibunching measurement in Fig. 1, we do have the correct decay time corresponding to the exciton lifetime for positive times. The discrepancy of a factor of 4 for the exciton autocorrelation is no longer present in crosscorrelation measurements. This indeed suggests an influence of dark excitons which would explain the discrepancy of Fig.1, since in cross-correlation experiments the recombination of a biexciton always leaves an exciton in a bright state.

Purcell effect on CdSe quantum dots
The Purcell effect is a direct consequence of the "Fermi golden rule" for an emitter located in an optical cavity. In the case of QDs embedded into micropillars the Purcell effect is the driving force for optimal monomode single-photon sources. Indeed, this effect allows the control of the spontaneous emission in three ways: (i) a good collection efficiency can be obtained, (ii) the monomode emission permits one to control the direction and the polarization of the emission, and (iii) the decay time shortening, below decoherence time, is crucial to obtain indistinguishable photons. Until now, research efforts in this domain have been focused on III-V materials. II-VI materials have been less investigated, mostly because of the lack of suitable microcavities. However, II-VI QDs present very interesting features in the context of single-photon emission. The large exciton-biexciton line separation (20 meV) allows one to consider operation close to room temperature and the photoluminescence (PL) decay time is much shorter (300 ps) than for InAs/GaAs (1 ns), which limits the impact of decoherence processes.
 
In contrast to GaAs/AlAs, no couple of II-VI binary compounds displays the same lattice parameter and a large refractive index contrast in order to grow Bragg mirrors lattice matched with ZnSe. To grow a fully epitaxial structure, there is no other choice than using ternary or even quaternary alloys. This approach is complex and we preferred using oxide deposition for the elaboration of the cavity mirrors after substrate removal. Then to obtain micropillars, aluminium masks from 0.9 to 6 µm in diameter are deposited and the pillars are processed by reactive ion etching for the oxide layers and chemical etching for the II-VI layer.
 
We measured (figure 3, bottom) the PL decay time for a series of 12 single QDs located in the same 1.1 µm diameter pillar excited by a pulsed frequency-doubled Ti:Sapphire laser at 400 nm. The light emitted by the sample is analyzed by a 30 cm focal length monochromator and detected with a synchronized streak camera. The effective time resolution of the setup is 3.5 ps, and the spectral resolution is 1 nm. Those measurements were done at 4K and under very low excitation (1 W/cm2). We observed the emission of single QDs in the spectral range from 2.45 eV to 2.62 eV. A large set of PL decay times from 75 ps to 307 ps were measured for those QDs. If we compare those decay times to the reference decay time of QDs in a non-processed part of the sample (250-300 ps), we see that for QDs inside the micropillar, a clear shortening of the PL decay time, by a factor of at least 3.3, is obtained. The dispersion of measured values is due to the random location of the QDs in the pillar and slight energy detuning. Consequently, different coupling of the emitters to the cavity modes are obtained.
 
We succeeded in obtaining a marked shortening of the PL decay time for CdSe/ZnSe QDs located in micropillars thanks to the Purcell effect. The effect is up to a factor of 3.3 in spite of the relatively low Q-factor of this hybrid microcavity. This structure provides good conditions for single photon collection but the Purcell effect should be strongly enhanced to hinder decoherence. We benefit from the high photon collection efficiency to perform anti-bunching measurements on QDs coupled to a micropillar cavity mode. The results are qualitatively similar to those presented in the previous part, without cavities, but with high signal to noise ration in spite of shorter integration time.

In collaboration with:
Christophe Couteau, Sebastian Moehl, Ivan-Christophe Robin, Franck Tinjod
Jan Gaj(1), Laurence Ferlazzo(2)
(1) Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland
(2) CNRS-Laboratoire de Photonique et Nanostructures, Route de Nozay, 91460 Marcoussis, France

References:

1. C. Couteau, S. Moehl, F. Tinjod, J. M. Gérard, K. Kheng, H. Mariette, J. A. Gaj, R. Romestain, and J. P. Poizat, Appl. Phys. Lett. 85, 6251 (2004)

2. I. C. Robin, R. André, A. Balocchi, S. Carayon, S. Moehl, J. M. Gérard, L. Ferlazzo, Appl. Phys. Lett. 87, 233114 (2005)

3. Vers une source de photons uniques indiscernables a l’aide de boites quantiques semiconductrices II-VI

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