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1. Harmonic-induced plasmonic resonant energy transfer between metal and semiconductor nanoparticles

   https://doi.org/10.1126/sciadv.adv1822  

   Yan, Yueming; Spear, Nathan J; Cummings, Adam J; Khusainova, Karina; Macdonald, Janet E; Haglund, Richard F (June 2025, Science Advances)

   Heterostructures combining two or more metal and/or semiconductor nanoparticles exhibit enhanced upconversion arising from localized nanoparticle resonances. However, plasmon-exciton coupling in semiconductor-metal nanostructures exhibits nanosecond relaxation times, and multi-plasmon metallic heterostructures are not broadly tunable. Here, we develop a biplasmonic heterostructure in which CuS and Au nanoparticle layers, separated by an alumina spacer of variable thickness, exhibit enhanced second- and third-harmonic generation due to dipole-dipole coupling between Au and CuS plasmons, as seen in the characteristic inverse sixth-power dependence of their separation in the measured harmonic enhancement and confirmed by numerical simulations of near-field CuS-Au nanoparticle coupling. Transient-absorption spectroscopy shows faster relaxation in Au/CuS (690 femtoseconds) compared to CuS heterostructures (929 femtoseconds). Moreover, nonlinear absorption measurements provide evidence for harmonic-induced plasmonic resonant energy transfer between the narrow Au and broad, tunable CuS plasmon resonances. This prototype for ultrafast upconversion showcases a strategy for high-efficiency, tunable plasmonic nonlinear devices with promising applications in photocatalysis, parametric down-conversion, and biomedical imaging. 

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2. Charge carrier concentration dependence of ultrafast plasmonic relaxation in conducting metal oxide nanocrystals

   https://doi.org/10.1039/C7TC00600D  

   Johns, Robert W.; Blemker, Michelle A.; Azzaro, Michael S.; Heo, Sungyeon; Runnerstrom, Evan L.; Milliron, Delia J.; Roberts, Sean T. (January 2017, Journal of Materials Chemistry C)

   Electronically doped metal oxide nanocrystals exhibit tunable infrared localized surface plasmon resonances (LSPRs). Despite the many benefits of IR resonant LSPRs in solution processable nanocrystals, the ways in which the electronic structure of the host semiconductor material impact metal oxide LSPRs are still being investigated. Semiconductors provide an alternative dielectric environment than metallically bonded solids, such as noble metals, which can impact how these materials undergo electronic relaxation following photoexcitation. Understanding these differences is key to developing applications that take advantage of the unique optical and electronic properties offered by plasmonic metal oxide NCs. Here, we use the two-temperature model in conjunction with femtosecond transient absorption experiments to describe how the internal temperature of two representative metal oxide nanocrystal systems, cubic WO 3−x and bixbyite Sn-doped In 2 O 3 , change following LSPR excitation. We find that the low free carrier concentrations of metal oxide NCs lead to less efficient heat generation as compared to metallic nanocrystals such as Ag. This suggests that metal oxide NCs may be ideal for applications wherein untoward heat generation may disrupt the application's overall performance, such as solar energy conversion and photonic gating. 

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3. Perspective on functional metal-oxide plasmonic metastructures

   https://doi.org/10.1063/5.0134141  

   Sadeghi, Seyed M.; Wing, Waylin J.; Gutha, Rithvik R. (February 2023, Journal of Applied Physics)

   Plasmonic nanostructures and metasurfaces are appealing hosts for investigation of novel optical devices and exploration of new frontiers in physical/optical processes and materials research. Recent studies have shown that these structures hold the promise of greater control over the optical and electronic properties of quantum emitters, offering a unique horizon for ultra-fast spin-controlled optical devices, quantum computation, laser systems, and sensitive photodetectors. In this Perspective, we discuss how heterostructures consisting of metal oxides, metallic nanoantennas, and dielectrics can offer a material platform wherein one can use the decay of plasmons and their near fields to passivate the defect sites of semiconductor quantum dots while enhancing their radiative decay rates. Such a platform, called functional metal-oxide plasmonic metasubstrates (FMOPs), relies on formation of two junctions at very close vicinity of each other. These include an Au/Si Schottky junction and an Si/Al oxide charge barrier. Such a double junction allows one to use hot electrons to generate a field-passivation effect, preventing migration of photo-excited electrons from quantum dots to the defect sites. Prospects of FMOP, including impact of enhancement exciton–plasmon coupling, collective transport of excitation energy, and suppression of quantum dot fluorescence blinking, are discussed. 

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4. Plasmon mode engineering with electrons on helium

   https://doi.org/10.1038/s41467-025-60305-3  

   Mikolas, Camille; Beysengulov, Niyaz; Schleusner, Austin; Rees, David; Undershute, Camryn; Pollanen, Johannes (May 2025, Nature Communications)

   An ensemble of electrons trapped above superfluid helium offers a paradigm system for investigating and controlling collective charge dynamics in low-dimensional electronic matter. Of particular interest is the ability to spatially control and engineer surface plasmons for integration with hybrid quantum systems and circuit quantum electrodynamic device architectures. Here we present experiments using an electron-on-helium microchannel device that hosts microwave-frequency plasmons, generated via local microwave excitation in an electrostatically defined central channel. By precisely varying the electron density, we demonstrate tunability of plasmon mode frequencies over several GHz. Additionally, we find that the power dependence of these modes can be used to investigate both homogeneous and inhomogeneous sources of spectral broadening. These results demonstrate the versatility of electrons on helium for probing collective excitations in low-dimensional Coulomb liquids and solids, and demonstrate a path for integrating engineered plasmons in electrons on helium with hybrid circuit quantum electrodynamic systems. 

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5. Comparative Analysis of the Near‐ and Far‐Field Optical Response of Thin Plasmonic Nanostructures

   https://doi.org/10.1002/adom.202102550  

   Zundel, Lauren; Gieri, Paul; Sanders, Stephen; Manjavacas, Alejandro (March 2022, Advanced Optical Materials)

   Nanostructures made of metallic materials support collective oscillations of their conduction electrons, commonly known as surface plasmons. These modes, whose characteristics are determined by the material and morphology of the nanostructure, couple strongly to light and confine it into subwave- length volumes. Of particular interest are metallic nanostructures for which the size along one dimension approaches the nanometer or even the subna- nometer scale, since such morphologies can lead to stronger light–matter interactions and higher degrees of confinement than regular three-dimen- sional nanostructures. Here, the plasmonic response of metallic nanodisks of varying thicknesses and aspect ratios is investigated under far- and near-field excitation conditions. It is found that, for far-field excitation, the strength of the plasmonic response of the nanodisk increases with its thickness, as expected from the increase in the number of conduction electrons in the system. However, for near-field excitation, the plasmonic response becomes stronger as the thickness of the nanodisk is reduced. This behavior is attrib- uted to the higher efficiency with which a near-field source couples to the plasmons supported by thinner nanodisks. The results of this work advance the understanding of the plasmonic response of thin metallic nanostructures, thus increasing their potential for the development of novel applications. 

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