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SCIENCE ADVANCES | RESEARCH ARTICLE R ─RON(t ) −ROFF A (1) ─R (t)= ROFF 0.012 0.010 1.0-ps control film GSB A1 A2 A3 B1 B2 where RON (ROFF) is the probe reflectivity with (without) the pump 0.008 excitation. Note that control films (active layers without the micro- cavities) are measured under differential transmittivity T ∕ T. The 0.006 control films will allow us to identify the underlying photophysics 0.004 of the molecules. 0.002 In our experimental setup (shown schematically in Fig. 1E), 0.000 transient-absorption measurements were performed in a degenerate, - 0.002 SE almost collinear configuration. Pump and probe pulses were generated by a broadband noncollinear optical parametric amplifier (NOPA) (31) and spanned the wavelength range of 500 to 620 nm with a nearly transform-limited sub–20-fs duration (further details in Materials and Methods). An optical delay line was used to control the probe delay time, and a mechanical chopper was used to modulate the pump pulse, providing alternating probe-only and pump-probe pulses, allowing us to measure pump-induced absorption changes. Measurements at different molecular concentrations were performed, adjusting the pump fluence to maintain an approximately constant photon density (i.e., pump photons per LFO molecule) r = kN/N, where N is the total number of molecules in the excitation volume, N is the total number of pump laser photons, and k is the fraction of them that actually reaches the active layer of the microcavity. We estimate from the reflectivity data that only 6 to 8% of the initial pump excitation enters the cavity. We conducted our experiment in air at room temperature. Results We first show that ultrafast transient-absorption spectroscopy can monitor the population of excited molecules, even in a cavity, by comparing the control film and the microcavity spectra as shown in Fig. 2A. A representative control film T/T spectrum is shown for a probe delay time of 1.0 ps, and the R/R spectra of the microcavities are shown at a delay of 1.25 ps (further data are given in the Supple- mentary Materials). We found the control film spectra at all con- centrations to show two positive bands around 530 and 577 nm, which both reflect excited-state populations. By comparison with the spectra in Fig. 1B, we attribute the 530-nm band to ground state bleaching, i.e., suppression of absorption due to molecules already being in their excited state. The 577-nm band instead corresponds to stimulated emission by excited molecules. For each of the micro- cavity spectra, we have a single prominent peak, which corresponds to the transient signal filtered by the cavity mode. This implies that the time-dependent transient reflectivity signal is proportional to 510 525 540 555 570 585 600 Wavelength (nm) (eV) (eV) ∆T/T (∆R/R) the change in the number of excited molecules created by the pump Fig. 2. Experimental demonstration of superextensive charging. (A) Differential transmittivity (T/T) spectra for the control film (at 1% LFO concentration) at a probe delay time of 1.0 ps and the differential reflectivity (R/R) spectra for the microcavities at 1.25-ps probe delay. (B) Temporally resolved energy density of the microcavities shows that rise time decreases as stored energy density increases, indicating super- extensive charging. A1, A2, and A3 label results for microcavities containing LFO at concentrations of 10, 5, and 1%, as the ratio of pump photons to molecules is kept approximately constant at r ≃ 0.14. B1 and B2 label measurements for LFO at con- centrations of 1 and 0.5%, with r ≃ 2.4. The use of two different r values was necessary to achieve a sufficiently high signal-to-noise ratio. Points mark the experimental data, while continuous solid lines are the results of the theoretical model, with parameters given by a chi-squared minimization of the experimental data. Experimental un- certainties are estimated from the point-to-point variance of the data. GSB, ground state bleaching; SE, stimulated emission. undergoes a rapid rise followed by slow decay. The time scale of the rapid rise varies with concentration. We adjust the laser power to fix photon density r across comparable microcavities and compare be- havior with different LFO concentrations. Details of how r is estimated are provided in the Supplementary Materials. We found that to achieve a sufficiently high signal-to-noise ratio, it was not possible to compare all microcavities at the same r value; instead, a constant r value was maintained for matched structures. Specifically, measure- ments were made on microcavities with LFO concentrations of 10, 5, and 1% with approximately constant r ≃ 0.14 (respectively labeled as A1, A2, and A3), and 1 and 0.5% with r ≃ 2.4 (labeled as B1 and B2). Overlaying the experimental data are the corresponding theoret- ical predictions (see the “Theoretical model” section). To account (32), i.e., _R(t ) ∝ N↑(t). Since the energy stored in the molecules is R also proportional to the number of excited molecules E(t) ∝ N↑(t), we can thus monitor the stored energy. While the experiment directly provides the time dependence, estimating the absolute scale of energy density requires multiplying R/R by a time-independent constant. Estimating this constant from first principles is challenging, so we instead extract it through fitting to the theoretical model, which is discussed below. This fitting is discussed in section S3. We also note that two of the microcavity spectra show a negative R/R band, which results from the change in the refractive index induced by the pump pulse (33). Figure 2B shows the experimental values for the time-dependent stored energy density. In all microcavities studied, the energy density Quach et al., Sci. Adv. 8, eabk3160 (2022) 14 January 2022 3 of 7 B A1 A2 A3 B1 B2 conc. 10% 5% 1% 1% 0.5% 0.12 0.12 0.16 2.8 2.0 0.10 0.08 0.06 0.04 0.02 0.00 - 0.02 0.20 0.15 0.10 0.05 0.00 -0.5 0.5 1 1.5 Probe delay (ps) 2 2.5 Downloaded from https://www.science.org on June 26, 2022PDF Image | Superabsorption organic microcavity Toward a quantum battery
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