Supersolidity—the simultaneous superfluid flow and crystalline order—has been realized in quantum atomic systems, but remains widely unexplored in photonic platforms operating at weak light-matter coupling. We predict a supersolid phase of light in a plasma-filled optical microcavity, where photons acquire effective mass and interact via nonlocal, plasma-mediated nonlinearities. By deriving a Gross-Pitaevskii equation with a tunable photon-photon interaction kernel, we show that the light field can spontaneously crystallize into a supersolid lattice via modulational instability. Crucially, this supersolid arises from a weak photon-electron coupling enabled by virtual electronic transitions and it does not require hybrid polariton formation. Using doped semiconductor microcavities, we identify accessible conditions (electron densities $\sim {10}^{10}–{10}^{11}{\mathrm{cm}}^{-2}$ and optical intensities $\sim {10}^3–{10}^5\mathrm{W}/{\mathrm{cm}}^2$) for experimental realization. This work establishes plasmonic cavities as a platform for correlated photonic matter with emergent quantum order.

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• Bose gases
• Light-matter interaction
• Nonlinear optics
• Optical coherence
• Photonics
• Semiconductor microcavities