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SCIENCE ADVANCES | RESEARCH ARTICLE PHYSICS Superabsorption in an organic microcavity: Toward a quantum battery James Q. Quach1*, Kirsty E. McGhee2, Lucia Ganzer3, Dominic M. Rouse4, Brendon W. Lovett4, Erik M. Gauger5, Jonathan Keeling4, Giulio Cerullo3, David G. Lidzey2, Tersilla Virgili3* The rate at which matter emits or absorbs light can be modified by its environment, as markedly exemplified by the widely studied phenomenon of superradiance. The reverse process, superabsorption, is harder to demonstrate because of the challenges of probing ultrafast processes and has only been seen for small numbers of atoms. Its central idea—superextensive scaling of absorption, meaning larger systems absorb faster—is also the key idea underpinning quantum batteries. Here, we implement experimentally a paradigmatic model of a quantum battery, constructed of a microcavity enclosing a molecular dye. Ultrafast optical spectroscopy allows us to observe charging dynamics at femtosecond resolution to demonstrate superextensive charging rates and storage capacity, in agreement with our theoretical modeling. We find that decoherence plays an important role in stabilizing energy storage. Our work opens future opportunities for harnessing collective effects in light-matter coupling for nanoscale energy capture, storage, and transport technologies. Copyright © 2022 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY). INTRODUCTION The properties of physical systems can typically be categorized as intensive (i.e., they are independent of the system size, such as density) or extensive (i.e., they grow in proportion to system size, such as mass). However, in some cases, cooperative effects can lead to superextensive scaling. A well-studied example of this is super- radiant emission (1). In its original form, this describes emission from an ensemble of N emitters into free space. Constructive inter- ference in the emission process means that the time for emission scales as 1/N, so that peak emission power is superextensive, scaling as N2. This behavior has been demonstrated on a number of plat- forms [low-pressure gases (2, 3), quantum wells (4, 5) and dots (6), J aggregates (7), Bose-Einstein condensates (8), trapped atoms (9), and nitrogen-vacancy centers (10)]. A less-studied example is super- absorption (11), describing the N-dependent enhancement of ab- sorption of radiation by an ensemble of N two-level systems (TLSs). Only very recently has this been demonstrated for a small number of atoms (12). In principle, superabsorption could have important implications for energy storage and capture technologies, particu- larly if realized in platforms compatible with energy harvesting, such as organic photovoltaic devices. However, there are challenges in engineering the precise environment in which such behavior can occur and in monitoring the ultrashort charging time scales. Here, we show how these can be overcome, by combining organic micro- cavity fabrication with ultrafast pump-probe spectroscopy. Superextensive scaling of energy absorption is also a key property of quantum batteries (QBs). These represent a new class of energy storage devices that operate on distinctly quantum mechanical RESULTS Device structure The structures fabricated consist of a thin (active) layer of a low-mass molecular semiconductor dispersed into a polymer matrix that is deposited by spin coating and positioned between two dielectric mirrors, forming a microcavity as illustrated schematically in Fig. 1A (see Materials and Methods for fabrication details). Organic semi- conductors are particularly promising for many applications as the high oscillator strength and binding energy of molecular excitons mean that light can be absorbed efficiently, and excitons can exist at principles. In particular, they are driven either by quantum entangle- ment, which reduces the number of traversed states in the Hilbert space compared to (classical) separable states alone (13–21), or by cooperative behavior that increases the effective quantum coupling between battery and source (22–24). These effects mean that QBs ex- hibit a charging time that is inversely related to the battery capacity. This leads to the intriguing idea that the charging power of QBs is superextensive, that is, it increases faster than the size of the battery. For a QB consisting of a collection of N identical quantum systems, a superextensive charging rate density (charging rate per subsystem) that scales as N or √_N in the thermodynamic limit (20) has been predicted. Here, we experimentally realize a paradigmatic model proposed as a Dicke QB (24), which displays superextensive scaling of energy absorption, using an organic semiconductor as an ensemble of TLSs coupled to a confined optical mode in a microcavity. We also demon- strate how dissipation plays a crucial role; in a closed system, the coherent effects that lead to fast charging can also lead to subsequent fast discharging. Hence, stabilization of stored energy remains an open question: Proposed stabilization methods include continuous measurements (25), dark states (21), and novel energy trapping mechanisms (26, 27). In our open noisy system, dephasing causes transitions between the optically active bright mode and inactive dark modes. This suppresses emission into the cavity mode, so that we have fast absorption of energy but slow decay, allowing retention of the stored energy until it can be used. 1Institute for Photonics and Advanced Sensing and School of Chemistry and Physics, The University of Adelaide, South Australia 5005, Australia. 2Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK. 3Istituto di Fotonica e Nanotecnologia–CNR, IFN–Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy. 4SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK. 5SUPA, Institute of Photonics and Quantum Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK. *Corresponding author. Email: quach.james@gmail.com (J.Q.Q.); tersilla.virgili@ polimi.it (T.V.) Quach et al., Sci. 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