Nonlocal-nonlinear phonon polaritons

Gonzalo Álvarez-Pérez; Line Jelver; Alexander Paarmann; Simone De Liberato

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arXiv · 2608.24344

Nonlocal-nonlinear phonon polaritons

Gonzalo Álvarez-Pérez·Line Jelver·Alexander Paarmann·Simone De Liberato

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Abstract

When light is confined to deeply subwavelength volumes, optical responses can become nonlocal. Yet how nonlocality reshapes nonlinear light-matter interactions remains largely unexplored, particularly in dielectrics. Here, we theoretically uncover a new class of nonlinear optical processes arising from the interplay between optical-phonon propagation and ionic-bond anharmonicity in polar crystals. We show that, unlike conventional local nonlinearities, this mechanism enables second-order responses even in centrosymmetric materials. Moreover, these responses can be tuned by the confinement and resonantly enhanced near optical-phonon resonances. Using density functional perturbation theory, we quantify the resulting nonlocal second-order susceptibilities in representative polar crystals. Our results establish nonlocal-nonlinear optics as a new regime of light-matter interaction in dielectrics.

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Gonzalo Álvarez-Pérez, Line Jelver, Alexander Paarmann, Simone De Liberato. 2026-08-25. Nonlocal-nonlinear phonon polaritons. https://arxiv.org/abs/2608.24344

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