The discovery of localized surface plasmon resonances (LSPRs) in doped semiconductor nanocrystals has opened a new regime in plasmonics. We address both the technological and fundamental advances made possible by the realization of LSPRs in semiconductor nanocrystals. LSPRs were originally thought to be specific only to metallic nanostructures, but since their manifestation in semiconductor nanostructures, LSPRs are being seen as ubiquitous optical signatures of charge carriers. As fingerprints of a charge carrier collection, LSPRs of semiconductors are emerging as optical probes of processes that involve carrier dynamics, including redox reactions, electrochemistry, phase transitions, and photocatalysis. Unlike their electrical counterparts, LSPRs allow remote contactless probing and minimal device design. Ultrasmall semiconductor quantum dots are now enabling access to plasmon resonances of a handful of charge carriers, allowing us to ask fundamental questions regarding the lower limit of charge carriers needed to sustain a plasmon resonance, the emergence of a collective mode from a single-electron transition, and the effect of quantum confinement on plasmon resonances. These fundamental issues are discussed here, along with the need for new physical models required to capture the unique aspects of semiconductor LSPRs.

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Localized surface plasmon resonances (LSPRs), known for their fascinating optical properties, have thus far been limited to nanostructures of materials with high steady-state charge carrier densities. Here, we show that even a nonequilibrium charge population can support a LSPR mode. Photocharged zinc oxide (ZnO) nanocrystals show an infrared LSPR, which can be dynamically turned off by discharging via redox activity. It is deduced that the photoinduced LSPR is a collective mode of as few as four conduction band electrons, the least observed thus far. The sustenance of a free-electron plasma in charged ZnO, supported by the LSPR observation, leads us to propose the existence of a many-body excitonic state and suggest a mechanism for previously unresolved charge trapping dynamics in ZnO. The LSPR, which serves as an optical signature of a charged state of the nanoparticle, is also demonstrated as a useful probe of surface redox reactions.

Strong coupling between light and quantum mechanical transitions historically observed in atomic optics is now being realized in the solid state using plasmon resonances. Recent experiments on hybrid plasmonic/excitonic systems have reported varied observations such as excitonic splitting, asymmetric line shapes, and dips in scattering spectra. Here, we unite these seemingly disparate empirical observations under a single theoretical framework, illustrating that the same generalized hybrid system allows access to diverse forms of coupling between plasmons and molecular transitions. Simply by modifying the damping rate of the plasmon resonance, it is possible to transition from one regime of coupling to another (e.g., from Rabi splitting to Fano interference). Common experimental handles such as size, shape, and nature of the metal can be varied to tune the regime of coupling, as shown by electrodynamic simulations. We also show that strong coupling can be achieved using simple nanostructure configurations such as a plasmonic core/excitonic shell geometry without the necessity of sophisticated design of near-field hotspots. The unified model developed here will allow rational predictive design of hybrid plasmonic systems for achieving unique control of light on the nanoscale.