In quantum-confined structures, the interaction between plasmons and excitons reaches the strong coupling regime, leading to hybrid quasiparticles known as plasmon-exciton polaritons. This phenomenon arises when the energy exchange between plasmons and excitons exceeds their individual decay rates, resulting in a coherent energy transfer. The strong coupling regime is characterized by an anti-crossing behavior in the dispersion relation, where two distinct polariton branches emerge with a Rabi splitting energy that quantifies the coupling strength. Typical Rabi splitting values in quantum dots or 2D materials range from 50 meV to 300 meV, depending on the material system and nanostructure geometry.

The Purcell effect plays a critical role in enhancing the spontaneous emission rate of excitons in plasmonic environments. When an exciton is placed near a metallic nanostructure, its radiative decay rate can be significantly increased due to the local density of optical states provided by surface plasmons. For instance, in a silver bowtie nanoantenna coupled to a quantum emitter, Purcell factors exceeding 1000 have been reported. This enhancement is highly dependent on the emitter's position and orientation relative to the plasmonic structure, with sub-10 nm gaps yielding the strongest effects. The Purcell factor is given by the ratio of the modified emission rate to the free-space rate, and it scales with the quality factor of the plasmonic mode and the overlap between the emitter's dipole moment and the local electric field.

Nanophotonic applications leverage plasmon-exciton strong coupling for advanced functionalities. One key area is the development of ultra-compact modulators and switches. By electrically or optically tuning the coupling strength, the polariton dispersion can be dynamically controlled, enabling modulation speeds in the terahertz range. Another application is in nonlinear optics, where the strong light-matter interaction enhances nonlinear processes such as four-wave mixing and harmonic generation. For example, second-harmonic generation efficiency can be improved by two orders of magnitude in strongly coupled systems compared to uncoupled plasmons or excitons. Additionally, these hybrid systems enable subwavelength light confinement, breaking the diffraction limit and allowing for nanoscale optical components.

Single-photon sources based on plasmon-exciton coupling offer advantages in quantum information technologies. The Purcell effect reduces the exciton lifetime, enabling high repetition rates for single-photon emission. Plasmonic nanostructures also improve photon collection efficiency by directing emission into specific optical modes. A common approach involves coupling quantum dots to plasmonic cavities, achieving single-photon purity with g(2)(0) values below 0.1. The emission wavelength can be tuned by adjusting the plasmon resonance or the quantum dot size, covering visible to near-infrared ranges. Furthermore, plasmonic structures can enhance the brightness of single-photon emitters by over 100 times, making them suitable for practical quantum communication systems.

The choice of materials is crucial for achieving strong plasmon-exciton coupling. Noble metals like gold and silver are commonly used for plasmonic components due to their high conductivity and low losses in the visible range. For excitonic materials, semiconductor quantum dots, transition metal dichalcogenides, and organic dyes are frequently employed. Monolayer MoS2 coupled to silver nanoprisms has demonstrated Rabi splittings of 180 meV, while CdSe quantum dots near gold dimers have shown splittings of 70 meV. The dielectric environment also affects the coupling strength, with higher permittivity materials generally reducing plasmon damping and increasing the interaction volume.

Fabrication techniques must achieve precise control over nanoscale dimensions to optimize coupling. Electron-beam lithography and focused ion beam milling are used to create plasmonic structures with feature sizes below 20 nm. Colloidal synthesis enables uniform quantum dots with size variations less than 5%. For 2D materials, mechanical exfoliation or chemical vapor deposition provides monolayers with minimal defects. Alignment between plasmonic and excitonic components is critical, with placement accuracy requirements often below 10 nm to ensure maximal overlap of their optical modes.

Challenges remain in reducing losses and improving the quality factors of plasmonic systems. Ohmic losses in metals limit the coherence times of plasmon-exciton polaritons, typically to a few femtoseconds. Alternative materials such as doped semiconductors or graphene can offer lower losses in certain spectral ranges. Another challenge is the temperature stability of the coupling, as thermal fluctuations can detune the plasmon and exciton resonances. Cryogenic operation at 4 K can mitigate this but complicates practical applications. Recent advances in hyperbolic metamaterials and epsilon-near-zero media provide new avenues for loss management and enhanced light-matter interaction.

Future directions include integrating plasmon-exciton systems with photonic circuits for on-chip quantum technologies. Hybrid platforms combining plasmonic waveguides with quantum emitters could enable deterministic single-photon routing and entanglement distribution. Another promising area is the use of strong coupling for chemical sensing, where molecular vibrations interact with plasmon-exciton polaritons to enable label-free detection at single-molecule sensitivity. Advances in nanofabrication and theoretical modeling will continue to push the limits of coupling strengths and device performance, opening new possibilities for quantum optics and nanophotonics.