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European Quantum Electronics Conference (EQEC)

Low-temperature coherence-time measurements for CdSe colloidal nanocrystals emission

Laurent Coolen, Xavier Brokmann, E. Giacobino, Maxime Dahan, Jean‐Pierre Hermier

Published Jan 18, 2006• 0 citations
Publisher

Abstract

In this paper, precise measurement of the time coherence of CdSe nanocrystals fluorescence is reported. At low temperature, where emission linewidth is expected to be below 0.2 meV, direct linewidth measurements are not possible, due to insufficient spectrometer resolution and spectral diffusion of the emission wavelength. To obtain a higher resolution, Michelson interferometer is used. An original highly sensitive method, based on the time-correlation between the photons detected at the two interferometer outputs is developed.

DOI
10.1109/eqec.2005.1567438

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In this paper, precise measurement of the time coherence of CdSe nanocrystals fluorescence is reported. At low temperature, where emission linewidth is expected to be below 0.2 meV, direct linewidth measurements are not possible, due to insufficient spectrometer resolution and spectral diffusion of the emission wavelength. To obtain a higher resolution, Michelson interferometer is used. An original highly sensitive method, based on the time-correlation between the photons detected at the two interferometer outputs is developed.
We examine the possibility of measuring the emission linewidth $2\ensuremath{\hbar}{\ensuremath{\Gamma}}_{2}$ of a single-photon source by Michelson interferometry. Such an emitter is characterized by a limited fluorescence intensity and, when embedded in a solid matrix, a large spectral diffusion. We show that fast spectral diffusion renders standard Fourier spectroscopy irrelevant. We then calculate the correlations of the intensities detected at the two interferometer outputs, point out the existence of two-photon interferences (coalescence) even for a non-single-mode emission, and show that the correlations are not affected by spectral diffusion at correlation times shorter than spectral dynamics. This yields two ways to measure the decoherence rate ${\ensuremath{\Gamma}}_{2}$: the recently proposed photon-correlation Fourier spectroscopy, and the study of the coalescence dip. We examine the feasibility and spectral resolution of each method, depending on spectral-diffusion characteristics, and show that both methods could be applied to typical nanometer-sized emitters such as colloidal quantum dots.
The fluorescence of single colloidal CdSe quantum dots is investigated at room temperature by means of the autocorrelation function over a time scale of almost 12 orders of magnitude. At short time, the autocorrelation function shows a complete antibunching indicating single photon emission and atomic-like behavior. On longer time scales (up to tens of seconds), we measure a bunching effect due to fluorescence intermittency that can not be described by fluctuations between two states with constant rates. The autocorrelation function also exhibits a non-stationary behavior related to power-law distributions of "On" and "Off" times.
The fluorescence of single-colloidal CdSe quantum dots is investigated at room temperature by means of the autocorrelation function over a time scale of almost 12 orders of magnitude. Over a short time scale, the autocorrelation function shows complete antibunching, indicating single-photon emission and atomiclike behavior. Over longer time scales (up to tens of seconds), we measure a bunching effect that is due to fluorescence intermittency and that cannot be described by fluctuations between two states with constant rates. The autocorrelation function also exhibits nonstationary behavior related to power-law distributions of On and Off times.
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.
Using a beam-splitter and two photon detectors, we measured the cross-correlation function of a series of single nanocrystals, and found that the nanocrystals exhibit antibunching (zero correlation at zero time delay) as expected, indicating that the nanocrystals were indeed emitting only one photon at a time. Modeling the nanocrystals as a three-level energy system (ground, excited, and "dark"), we measured the characteristic time delay between photons emitted by a nanocrystal, which corresponds to the inverse of the transition rate from the excited state and the ground state. We performed this measurement using 532nm and 405nm wavelength light to excite the nanocrystals. Nanocrystals excited by 532nm wavelength light had a mean time constant of 22.7 ± 5.3ns, and those excited by 405nm wavelength light had a mean time constant of 22.4 ± 6.3ns.