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International Conference on Ultrafast Phenomena

Ultrafast dephasing dynamics and multi-level electronic structure of InP nanocrystals

Giulio Cerullo, Uri Banin, Andrew A. Guzelian, Christopher J. Bardeen, Robert W. Schoenlein, A. Paul Alivisatos, Charles V. Shank

Published Jan 1, 1996• 0 citations
Publisher

Abstract

Semiconductor nanocrystals are a novel class of materials intermediate between the molecular and solid state forms of matter. Their homogenous linewidths and electronic structure are of considerable interest for the understanding of the physics of quantum confinement. In this work we study the electronic and vibrational dephasing dynamics in InP nanocrystals using femtosecond three pulse photon echo (3PPE) experiments1 complemented by nanosecond transient hole-burning (HB) spectroscopy.2 The echo decay is explained by taking into account the multi-level electronic structure as observed in the hole-burning experiment.

DOI
10.1364/up.1996.thb.5

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Semiconductor nanocrystals are a novel class of materials intermediate between the molecular and solid state forms of matter. Their homogenous linewidths and electronic structure are of considerable interest for the understanding of the physics of quantum confinement. In this work we study the electronic and vibrational dephasing dynamics in InP nanocrystals using femtosecond three pulse photon echo (3PPE) experiments1 complemented by nanosecond transient hole-burning (HB) spectroscopy.2 The echo decay is explained by taking into account the multi-level electronic structure as observed in the hole-burning experiment.
We report the first direct measurements of femtosecond electronic dephasing in CdSe nanocrystals using three-pulse photon echoes and a novel mode-suppression technique. We are able to separate the dynamics of the coherently excited LO phonons from the underlying electron-hole dephasing by suppressing the quantum beats. The homogeneous linewidth of these materials at 15 K results from electronic dephasing in \ensuremath{\sim}85 fs, approximately half of which is due to acoustic phonon modes. Contributions from acoustic phonons dominate the homogeneous linewidth at room temperature.
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.
The three-dimensional confinement created by the ultrasmall semiconductor structures known as quantum dots greatly modifies the optical properties of the spatially localized carriers. These systems are presently of great interest to the research community, both for the improved understanding of the physics of confined structures, and for the potential applications of these systems to areas such as optoelectronics and quantum computing. Here, the techniques of transient coherent nonlinear optical spectroscopy are applied to the investigation of excitons confined within InAs/GaAs self-assembled quantum dots. These methods provide a powerful tool for the study of dynamic processes in these novel materials. The nonlinear measurements are based on the differential transmission experimental geometry, and utilize ultrafast pulsed lasers in order to temporally resolve the carrier dynamics after excitation. The experiments performed here are resonant with the ground state absorption of the quantum dots, as evidenced by comparison with the photoluminescence measurements which are used to characterize the sample. The incoherent relaxation dynamics of the population of photoexcited excitons are examined, allowing for a determination of both the lifetime of the excitons and the time scale for relaxation between the two polarization eigenstates of the system. The three-dimensional quantum confinement results in the suppression of these relaxation processes, as predicted. The use of coherent spectroscopic techniques allows for the first unambiguous demonstration of an optically-induced nonradiative Raman coherence in these quantum dots. Simultaneous excitation of both polarization states of the exciton results in a coherent superposition state, whose temporal evolution is observed through the homodyne-detection scheme. In these transient experiments, this is manifested as a beating in the nonlinear response, which allows us to directly measure the fine-structure splitting between the polarization states. Utilizing optical pulse shaping techniques, transient spectral hole burning is performed, which allows for the nonlinear response to be spectrally resolved. As expected for the strongly confined quantum dot excitons, the results show that diffusion among the localized states is not a significant effect. Temperature dependent measurements show that at elevated temperatures, extra dephasing processes, potentially related to acoustic phonon scattering, become important. Finally, this technique allows for the first direct observation of the induced absorption from the exciton to biexciton transition in these InAs quantum dots, and a measurement of the associated binding energy. These results lay the foundation for future studies in this material system, and for the possible development of quantum logic devices utilizing 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.