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Abstract

Phonon polariton modes in layered anisotropic heterostructures are a key building block for modern nanophotonic technologies. The light-matter interaction for evanescent excitation of such a multilayer system can be theoretically described by a transfer-matrix formalism. This method allows us to compute the imaginary part of the p-polarized reflection coefficient Im(r_pp), whose resonant features are commonly used to evaluate the polariton dispersion of the multilayer structure. This reflection coefficient, however, does not reveal how the different layers contribute to these resonances. We present an approach to compute layer-resolved polariton resonance intensity in arbitrarily anisotropic layered heterostructures, based on calculating the Poynting vector extracted from the transfer-matrix formalism under evanescent light excitation. Our approach is independent of the experimental excitation conditions, and it fulfills a strictly proved conservation law for the energy flux. As a testing ground, we study two state-of-the-art nanophotonic multilayer systems, covering strong coupling and tunable hyperbolic surface phonon polaritons in twisted MoO₃ double layers. Providing a new level of insight into the polaritonic response, our method holds great potential for understanding, optimizing, and predicting new forms of polariton heterostructures in the future.