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

TL;DR

This paper introduces a dual-channel self-referenced method using nearly degenerate quasinormal modes in a microcavity to accurately measure exciton populations in 2D materials, robust against system drifts.

Contribution

The work demonstrates a novel, drift-tolerant, dipole-resolved population inversion technique leveraging differential and common-mode observables in hybrid microcavities.

Findings

Achieved robust inversion of exciton populations without external calibration.

Measured a population ratio of approximately 200 at 50 K, consistent with theoretical expectations.

Validated the method's effectiveness in nanogap photonic systems.

Abstract

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 inversion 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 WSe$ _2 $, we exploit this common/differential-mode framework to robustly invert the relative populations of excitons with out-of-plane ($ \perp $) and in-plane ($ \parallel $) dipole transitions without external absolute calibration. At the temperature of $\sim$50 K, we obtain $ N_\perp/N_\parallel \approx 200 $, 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.