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Colloidal semiconductor quantum dots (QDs) are excellent materials for combining photonics and nanoscience. They have been extensively studied to understand the photophysics of quantum confined excitons and to develop efficient optoelectronic devices such as QD solar cells and light-emitting diodes. The photophysical processes of QD devices are governed by a large number of QDs constructing the devices. Therefore, cooperative quantum dynamics of QDs, i.e., synchronous responses of QD ensembles, have a great potential for boosting optoelectronic device performances. Before the use of cooperative quantum dynamics in QD devices, the understanding of quantum dynamics of excitons, i.e., coherent properties of multiexcitons, is required to draw out quantum cooperativity from QD ensembles. In order to elucidate coherent dynamics of multiexcitons during resonant laser-pulse excitation, we performed a quantum interference spectroscopy of QD responses. We found that multiexcitons generate the high-frequency coherent oscillations with integer multiples of the exciton resonance frequency, which is called harmonic quantum coherence, and that the coherent responses govern the microscopic processes in the resonant multiphoton absorption 1-3. Furthermore, it was demonstrated that the coherent responses were detectable as photocurrent signals in thin films of QD ensembles 4. These developments build up an essential base of quantum cooperativities emerging in optoelectronics. Here, we report recent investigations of cooperative quantum dynamics in QDs. In optoelectronic devices, electronic couplings between QDs are expected to generate a new type of quantum cooperativities. To generate electronic coupling, we fabricated closely packed PbS QDs by using a ligand exchange method. We found that nonlinear photocurrent signals are strongly enhanced in the QD solids. The comparison analysis of different ligands clarified that the enhancement ratio increases monotonically with the decrease of the inter-QD distance 5. This means that the signal enhancement originates from the electronic coupling and cooperative behavior of QDs. These findings show that the cooperative quantum dynamics provide a new route to advanced optoelectronic technology such as nonlinear amplifiers. Part of this work was supported by JSPS KAKENHI (JP19H05465, JP22H01990, and JP23K17877), JST PRESTO (JPMJPR23H3), and JST CREST (JPMJCR21B4). References 1 Tahara, H.; Sakamoto, M.; Teranishi, T.; Kanemitsu, Y. Harmonic Quantum Coherence of Multiple Excitons in PbS/CdS Core-Shell Nanocrystals . Phys. Rev. Lett. 119, 247401 (2017). 2 Tahara, H.; Sakamoto, M.; Teranishi, T.; Kanemitsu, Y. Quantum coherence of multiple excitons governs absorption cross-sections of PbS/CdS core/shell nanocrystals . Nat. Commun. 9, 3179 (2018). 3 Tahara, H.; Kanemitsu, Y. Quantum Interference Measurements and Their Application to Analysis of Ultrafast Photocarrier Dynamics in Semiconductor Solar Cell Materials . Adv. Quantum Technol. 3, 1900098 (2020). 4 Tahara, H.; Sakamoto, M.; Teranishi, T.; Kanemitsu, Y. Collective enhancement of quantum coherence in coupled quantum dot films. Phys. Rev. B 104, L241405 (2021). 5 Tahara, H.; Sakamoto, M.; Teranishi, T.; Kanemitsu, Y. submitted .
Tahara et al. (Fri,) studied this question.