We develop a systematic temporal coupled-mode theory tailored for a single Floquet-sheet resonator governed by time-modulated conductivities. By invoking photon-number conservation during frequency conversion, we derive characteristic radiative decay rates and coupling coefficients that explicitly account for the frequency ratio between different channels. We establish a direct analytical connection to the Floquet transfer-matrix method, providing closed-form analytical expressions that map scattering parameters to the Drude-type physics of the sheet. Our model captures the resonant coupling between the zeroth-order propagating channel and the negative first-order surface-mode channel. Validated by full-wave numerical simulations, the proposed theory remains accurate even with the inclusion of intrinsic material loss. This approach provides an intuitive pole-expansion representation for designing time-varying photonic interfaces and understanding the underlying physics in two-dimensional temporal systems.

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• Photonics