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Long-lived modulation of plasmonic absorption by ballistic thermal injection


Light–matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis1,2, energy harvesting3 and photodetection4, among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with this hot-electron injection remains unclear. Plasmon-assisted photocatalytic efficiencies can improve when intermediate insulation layers are used to inhibit the charge transfer5,6 or when off-resonance excitations are employed7, which suggests that additional energy transport and thermal effects could play an explicit role even if the charge transfer is inhibited8. This provides an additional interfacial mechanism for the catalytic and plasmonic enhancement at interfaces that moves beyond the traditionally assumed physical charge injection9,10,11,12. In this work, we report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, after the optical excitation of a metal/semiconductor heterostructure. We explicitly demonstrate that in cases of strong non-equilibrium, a novel energy transduction mechanism arises at the metal/semiconductor interface. We find that hot electrons in the metal contact transfer their energy to pre-existing free electrons in the semiconductor, without an equivalent spatiotemporal transfer of charge. Further, we demonstrate that this ballistic thermal injection mechanism can be utilized as a unique means to modulate plasmonic interactions. These experimental results are well-supported by both rigorous multilayer optical modelling and first-principle ab initio calculations.

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Fig. 1: Proposed mechanism of interfacial energy transfer and experimental schematic.
Fig. 2: Interpreting subsurface heat deposition with ab initio calculations.
Fig. 3: Ultrafast plasmonic modulation through BTI.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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We acknowledge funding from the US Department of Defense, Multidisciplinary University Research Initiative through the Army Research Office, Grant no. W911NF-16-1-0406. J.R.N. and J.D.C. appreciate support from the Office of Naval Research, Grant no. N00014-18-1-2107.

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Authors and Affiliations



P.E.H. and J.-P.M. conceived the idea and supervised the project; J.A.T. performed the TDTR and infrared pump–probe measurements and corresponding analysis; E.L.R., K.P.K., J.N. and A.C. grew and characterized the CdO films; Y.-S.W., W.C. and O.V.P. performed the ab initio calculations; J.A.T. and D.H.O. performed the two-temperature model (TTM) calculations; J.A.T., J.R.N. and J.D.C. performed the transfer matrix method (TMM) calculations and provided insight on the dispersion relations. J.A.T. wrote the manuscript with input from all the authors; all the authors discussed the results and commented on the manuscript.

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Correspondence to Patrick E. Hopkins.

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Supplementary Discussion and Figs. 1–9.

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Tomko, J.A., Runnerstrom, E.L., Wang, YS. et al. Long-lived modulation of plasmonic absorption by ballistic thermal injection. Nat. Nanotechnol. 16, 47–51 (2021).

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