Doubly resonant third-harmonic generation in near-zero-index heterogeneous nanostructures

The capability of near-zero-index media to bypass phase-matching requirements constitutes a promising platform for harmonic generation, which plays a crucial role for the development of compact ultraviolet radiation sources. Here, we investigate third-harmonic generation in near-zero-index heterogeneous nanostructures undergoing collision-driven nonlinear electron dynamics upon pulsed electromagnetic excitation. In particular, we model third-harmonic generation in sodium-aluminum bilayers by perturbatively solving Maxwell's equations in the undepleted pump approximation, finding that such a nonlinear process can become doubly resonant under the sodium and aluminum near-zero-index conditions for pump and third-harmonic waves, respectively. Furthermore, we find that third-harmonic generation is maximized at a peculiar pump incidence angle where attenuated total internal reflection enables surface plasmon polariton excitation at the sodium-aluminum interface.