Abstract
The generation and recombination dynamics of multiple excitons in nanocrystals (NCs) have attracted much attention from the viewpoints of fundamental physics and device applications. However, the quantum coherence of multiple exciton states in NCs still remains unclear due to a lack of experimental support. Here, we report the first observation of harmonic dipole oscillations in PbS/CdS core-shell NCs using a phase-locked interference detection method for transient absorption. From the ultrafast coherent dynamics and excitation-photon-fluence dependence of the oscillations, we found that multiple excitons cause the harmonic dipole oscillations with ω, 2ω, and 3ω oscillations, even though the excitation pulse energy is set to the exciton resonance frequency, ω. This observation is closely related to the quantum coherence of multiple exciton states in NCs, providing important insights into multiple exciton generation mechanisms.
Highlights
The generation and recombination dynamics of multiple excitons in nanocrystals (NCs) have attracted much attention from the viewpoints of fundamental physics and device applications
Theoretical investigations of multiple exciton generation (MEG) have predicted that one high-energy exciton generates multiple low-energy excitons via a coherent superposition between the single and multiple exciton states [16,17]
The theoretically predicted coherent state is essential for discussions on MEG mechanisms and the dynamical nature of multiple excitons in NCs
Summary
The generation and recombination dynamics of multiple excitons in nanocrystals (NCs) have attracted much attention from the viewpoints of fundamental physics and device applications. We report the observation of coherent multiple excitons in PbS/CdS core-shell NCs with a spectroscopic technique using phase-locking of excitation pulses. We demonstrate that the phase-locked interference detection of the transient absorption (TA) clearly reflects the frequency-multiplied dipole oscillations in the NCs. From the coherent dynamics and the excitation-photon-fluence dependence of the oscillations, we found that the oscillations are caused by multiple excitons.
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