Observation of hyperbolic intersubband polaritons in native-dielectric-doped van der Waals semiconductor quantum wells.

Highly doped semiconductor quantum wells (QWs) exhibit strong intersubband transitions resulting from nanoscale electron confinement. Coupling photons to these collective dipoles in this anisotropic quantum structure enables intersubband polaritons with strong nonlinear optical response and hyperbolicity. Analogous to epitaxially grown multi-quantum wells, two-dimensional (2D) van der Waals (vdW) semiconductor heterostructures provide a compelling alternative platform, offering additional degrees of freedom and exceptional optoelectronic properties. Here we report intersubband polaritons in multilayer vdW WSe2 QWs with broadband tunability. By oxidizing the top WSe2 layer into a self-limiting native oxide, we activate charge transfer-induced efficient, high-density doping, enabling strong intersubband transitions and directly visualized polariton propagation. Lithographically defined nanostructures reveal their hyperbolic nature and sub-diffractional confinement, while electrostatic gating offers dynamic dispersion control. These results position vdW multilayers as a highly adaptable platform for tunable mid-infrared nanophotonics and integrated polaritonic circuits, detectors, and light sources.