Self-referenced, drift-tolerant dipole-resolved population retrieval using degeneracy-lifted dual quasinormal modes

Photoluminescence intensity is widely used to infer exciton populations, yet the detected signal inherently convolves occupancy with radiative-rate modification and collection efficiency, making quantitative retrieval vulnerable to pump and system drifts. Here, we realize a dual-channel self-referenced scheme enabled by two nearly degenerate quasinormal modes in a hybrid microcavity. Their shared optical path provides common-mode observables (i.e., overall spectral and intensity drift) that track global thermo-optic and pump fluctuations, while their differential-mode observables (i.e., spectral splitting and mode-contrasted emission) remain highly sensitive to local gap dielectric perturbations and dipole-dependent radiative weights. Using temperature as a control parameter in monolayer WSe₂, we exploit this common/differential-mode framework to robustly invert the relative populations of excitons with out-of-plane (⊥) and in-plane (||) dipole transitions without external absolute calibration. At the temperature of ∼50 K, we obtain N_⊥/N_|| ≈ 150 ± 30, coincident with the expected accumulation in the out-of-plane-emitting dark manifold. This internally referenced approach provides a practical route to drift-tolerant, dipole-resolved population metrology in nanogap photonic systems.