Plasmon-exciton coupling in semiconductor heterostructures represents a frontier in nanophotonics, where the interaction between localized surface plasmons in metallic nanostructures and excitons in quantum emitters like quantum dots (QDs) or transition metal dichalcogenides (TMDCs) leads to novel optical phenomena. This coupling can be tailored to enhance light-matter interactions, enabling applications in light-emitting diodes (LEDs), single-photon sources, and quantum information processing. The key mechanisms governing these interactions include strong coupling, weak coupling, and Purcell enhancement, each offering distinct advantages depending on the desired application.
When a metallic nanoparticle is placed in close proximity to a semiconductor quantum emitter, the plasmonic near-field can interact strongly with the excitonic transition. The strength of this interaction is determined by the overlap between the plasmon resonance and the exciton energy, as well as the local density of optical states (LDOS) provided by the plasmonic structure. In the weak coupling regime, the exciton-plasmon interaction results in Purcell enhancement, where the spontaneous emission rate of the emitter is increased due to the modified photonic environment. The Purcell factor, quantifying this enhancement, is given by the ratio of the emission rate in the presence of the plasmonic structure to that in free space. For quantum dots coupled to silver or gold nanoparticles, Purcell factors exceeding 100 have been experimentally demonstrated, leading to significant improvements in emitter brightness and radiative efficiency.
In contrast, the strong coupling regime occurs when the energy exchange rate between the plasmon and exciton exceeds the loss rates of both systems. This leads to the formation of hybrid quasi-particles known as plexcitons, characterized by a Rabi splitting in the energy dispersion. The Rabi splitting, observable in spectroscopic measurements, serves as a direct signature of strong coupling. For instance, monolayer TMDCs coupled to plasmonic nanocavities have shown Rabi splittings in the range of 50-200 meV, depending on the quality of the plasmonic resonator and the exciton oscillator strength. The strong coupling regime enables coherent energy transfer between plasmons and excitons, which is critical for applications requiring quantum coherence, such as single-photon sources and polariton lasing.
The choice of materials plays a crucial role in achieving efficient plasmon-exciton coupling. Noble metals like gold and silver are commonly used due to their strong plasmon resonances in the visible and near-infrared ranges. However, their high ohmic losses can limit the quality factor of the hybrid system. Alternative materials, such as aluminum or doped semiconductors, offer lower losses and tunable plasmon resonances, though at the cost of reduced field enhancement. On the semiconductor side, quantum dots provide high quantum yield and size-tunable emission, while TMDCs offer large exciton binding energies and valley-selective properties. Combining these materials requires precise nanofabrication techniques to control the separation distance, typically within a few nanometers, to maximize coupling while minimizing non-radiative quenching.
Applications of plasmon-exciton coupling in LEDs leverage Purcell enhancement to improve external quantum efficiency. By embedding quantum dots or TMDCs within plasmonic nanostructures, the radiative recombination rate can be enhanced while suppressing non-radiative pathways. This is particularly beneficial for colloidal quantum dot LEDs, where plasmonic arrays have been shown to increase device efficiency by over 50% compared to conventional structures. Additionally, plasmonic outcoupling structures can mitigate total internal reflection, further boosting light extraction.
Single-photon sources benefit from the enhanced emission rates and directionality provided by plasmon-exciton coupling. Strongly coupled systems can generate single photons with high purity and indistinguishability, essential for quantum key distribution and linear optical quantum computing. Plasmonic nanoantennas can also tailor the emission pattern, enabling efficient coupling to optical fibers or waveguides. Recent demonstrations have achieved single-photon emission rates exceeding 10 MHz with near-unity collection efficiency using hybrid quantum dot-plasmonic structures.
Challenges remain in scaling these systems for practical applications. Fabrication reproducibility, long-term stability, and integration with existing photonic platforms are critical areas of research. Advances in deterministic assembly techniques, such as DNA origami or pick-and-place methods, are improving the precision of hybrid nanostructure fabrication. Additionally, the development of low-loss plasmonic materials and alternative coupling schemes, such as dielectric resonators, may further enhance performance.
The study of plasmon-exciton coupling continues to reveal new physics and technological opportunities. From ultrafast optical switches to room-temperature quantum simulators, the interplay between plasmons and excitons in semiconductor heterostructures is driving innovations across photonics and quantum technologies. Future research will likely focus on extending these concepts to new material systems, exploring nonlinear interactions, and integrating hybrid devices into scalable architectures.