All-optical control of exciton flow in a colloidal quantum well complex

Yu et al. Light: Science & Applications (2020)9:27 https://doi.org/10.1038/s41377-020-0262-7 ARTICLE Official journal of the CIOMP 2047-7538 www.nature.com/lsa Open Access All-optical control of exciton flow in a colloidal quantum well complex Junhong Yu1, Manoj Sharma 1,2 , Ashma Sharma1, Savas Delikanli1,2, Hilmi Volkan Demir1,2,3 and Cuong Dang 1,4 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Abstract Excitonics, an alternative to romising for processing information since semiconductor electronics is rapidly approaching the end of Moore’s law. Currently, the development of excitonic devices, where exciton flow is controlled, is mainly focused on electric-field modulation or exciton polaritons in high-Q cavities. Here, we show an alloptical strategy to manipulate the exciton flow in a binary colloidal quantum well complex through mediation of the Fö rster resonance energy transfer (FRET) by stimulated emission. In the spontaneous emission regime, FRET naturally occurs between a donor and an acceptor. In contrast, upon stronger excitation, the ultrafast consumption of excitons by stimulated emission effectively engineers the excitonic flow from the donors to the acceptors. Specifically, the acceptors’ stimulated emission significantly accelerates the exciton flow, while the donors’ stimulated emission almost stops this process. On this basis, a FRET-coupled rate equation model is derived to understand the controllable exciton flow using the density of the excited donors and the unexcited acceptors. The results will provide an effective alloptical route for realizing excitonic devices under room temperature operation. Introduction Exciton-based solid-state devices have the potential to be essential building blocks1,2 for modern information technology to surpass the performance of conventional electronic devices since excitonics combines an ultrafast operation speed3 with a highly compact footprint4. Because of the direct interaction with photons, excitonic devices effectively eliminate the interconnection delay between photon-based information communication and electronbased information processing. Unlike photonic devices with a diffraction limit of the footprint, the exciton thermal de Broglie wavelength is extremely small (~10 nm) at room temperature. Exploiting excitonic devices requires the Correspondence: Hilmi Volkan Demir () or Cuong Dang () 1 LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, The Photonics Institute (TPI), Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore 2 Department of Electrical and Electronics Engineering and Department of Physics, UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Bilkent, 06800 Ankara, Turkey Full list of author information is available at the end of the article ability to control the excitonic properties (e.g., exciton flow, exciton recombination rates, or exciton energy) in an optically active medium2,5. Until now, a series of excitonic schemes have been used to achieve controllability based on exciton–photon polaritons6,7, coupled indirect excitons4,5, DNA nanotechnology8,9, or electric-field modulation1,2. Unfortunately, these configurations have restricted application in integrated circuits because they either sacrifice the operation speed due to the application of a gate voltage or are inherently complex due to the requirement of high-Qfactor microcavities, engineered electron/hole wavefunction overlap, or nanometer-scale precision DNA scaffolds. Targeting the challenges described above, a scheme with an emphasis on all-optical control, bottom-up fabrication, and self-assembly offers a more attractive solution. Förster resonance energy transfer (FRET) is a particularly promising mechanism for excitonics, as dipolar coupling between a donor and an acceptor permits efficient and directed exciton flow in a simple solid mixture10–13. On the other hand, stimulated emission, in which the exciton recombination dynamics dramatically differs from that in spontaneous emission, is a possible © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Yu et al. Light: Science & Applications (2020)9:27 control mechanism for exciton flow considering that the FRET process significantly depends on the density of excited donors and unexcited acceptors14,15. Colloidal quantum wells (CQWs) with high quantum efficiency, a tuneable energy bandgap, and solution processability have recently attracted broad research interest for optoelectronic devices16–19. Particularly, CQWs have exhibited robust excitons at room temperature with a binding energy up to 150 meV16,18, extraordinary optical gain performance19,20, and near-unity FRET efficiency10,21, making them promising as an ideal platform to achieve all-optical control of exciton flow using FRET. Herein, we propose a strategy to control the exciton flow based on mediation of the FRET process by stimulated emission in a binary nanomaterial complex consisting of 4 monolayer (ML) core-only CdSe CQWs (donors) and 8 ML core-shell CdS/CdSe/CdS CQWs (acceptors). Specifically, at low pump fluence when the emission of both donors and acceptors is spontaneous (referred to as regime I), nearly 50% of the exciton population in the donors outflows into the acceptors via FRET. By increasing the pumping level to achieve stimulated emission in the acceptors (referred to as regime II), we can greatly enhance the exciton flow efficiency up to 90% since quick depletion of excitons in the acceptors significantly promotes the FRET process. Upon further increasing the fluence to initiate stimulated emission in the donors (referred to as regime III), the exciton flow towards the acceptors almost switches off because the stimulated emission rate in donors is much faster than the FRET rate. Furthermore, we develop a FRET-coupled kinetic model to identify the competing processes responsible for the variation in exciton flow. Our results, which demonstrate a prototype of an all-optical controllable exciton flow concept with multiple modulation stages, may inspire the design of all-optical (...truncated)