Polaritons in Van der Waals Materials

Polaritons are hybrid quasiparticles that emerge when electromagnetic waves couple strongly to collective excitations in solids, such as lattice vibrations (phonons), electronic plasmons or bound electron–hole pairs (excitons). Layered van der Waals materials—crystals held together by weak interlayer forces—offer an ideal platform for sustaining and tailoring polaritonic modes at deeply subwavelength scales. By exploiting extreme optical anisotropy, natural hyperbolic dispersion can confine and guide light in two dimensions with propagation angles defined by the dielectric tensor. In addition, low‐symmetry and phase‐change heterostructures enable reconfigurable directional steering, wavefront shaping and enhanced light–matter interaction. These developments have opened routes towards on-chip mid-infrared imaging, ultra-compact sensors and programmable nanophotonic elements. Crucially, the atomic thinness allows direct integration with electronic, magnetic and mechanical degrees of freedom, suggesting pathways to active polaritonic devices and hybrid quantum systems.

Research from Nature Portfolio

Recent work on biaxial two-dimensional crystals has shown that planar hyperbolic polaritons arise naturally in many van der Waals materials, with tunability via layer orientation, temperature and external gating. This perspective highlights the prevalence of hyperbolic media in both uniaxial and biaxial slabs, and envisages polaritons coupled to ferroelectric, ferromagnetic or piezoelectric order parameters for multifunctional control. In monoclinic and triclinic crystals, the dielectric tensor cannot be simultaneously diagonalised, giving rise to hyperbolic shear polaritons. These modes display frequency-dependent wavefront tilts, asymmetric propagation and potential for non-Hermitian and topological photonic states. Foundational experiments in hexagonal boron nitride have demonstrated hyperbolic phonon-polaritons that enable sub-diffraction imaging, flat-lens focusing and ultra-low-loss waveguiding, establishing a blueprint for volume-confined polaritonic nanophotonics.

Polaritons in Van der Waals Materials publication trend

The graph below shows the total number of articles in polaritons in van der waals materials across all publications each year (not limited to Nature Index journals).

Technical terms

Polaritons: Hybrid quasiparticles formed by strong coupling between photons and material excitations such as phonons, plasmons or excitons.

Van der Waals materials: Layered crystals in which individual sheets adhere by weak dispersion forces, allowing exfoliation to atomic thickness.

Hyperbolic dispersion: A regime in which principal permittivities have opposite signs, producing open isofrequency contours and high-momentum modes.

Phonon-polaritons: Polaritons arising from coupling of light to optical phonon modes in polar crystals, often in the infrared.

Shear polaritons: Modes in low-symmetry crystals that originate from off-diagonal components of the dielectric tensor, exhibiting tilted wavefronts and asymmetric propagation.

Anisotropy: Direction-dependent variation of optical or electronic properties within a material.

References

1. Source-configured symmetry-broken hyperbolic polaritons. eLight (2023).
2. Exploring van der Waals materials with high anisotropy: geometrical and optical approaches. Light: Science & Applications (2024).
3. Planar hyperbolic polaritons in 2D van der Waals materials. Nature Communications (2024).
4. Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing. Nature Communications (2015).
5. Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material. Nature Communications (2015).
6. Reconfigurable infrared hyperbolic metasurfaces using phase change materials. Nature Communications (2018).
7. Hyperbolic shear polaritons in low-symmetry crystals. Nature (2022).
8. Phonon-polaritonics: enabling powerful capabilities for infrared photonics. Nanophotonics (2019).