Interfacing differently oriented biaxial van der Waals crystals for negative refraction.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanophotonics

Background:

• Negative refraction is crucial for applications like imaging and sensing, often requiring complex hyperbolic metamaterials.
• Natural van der Waals (vdW) materials offer an accessible alternative to engineered metamaterials due to their layered structure.
• Most vdW materials exhibit uniaxial properties, limiting hyperbolicity to out-of-plane orientations, hindering planar integration.

Purpose of the Study:

• To investigate the potential of natural biaxial alpha-molybdenum trioxide (α-MoO3) for achieving in-plane hyperbolicity and negative refraction.
• To explore the integration of α-MoO3 with tunable graphene and a gold substrate for optical applications.
• To demonstrate novel optical phenomena including negative refraction, simultaneous positive and negative refractions, and diffractionless propagation.

Main Methods:

• Numerical simulations were employed to model the optical properties of heterostructures.
• The study focused on interfacing differently oriented α-MoO3 slabs coated with tunable graphene on a gold substrate.
• Analysis covered a broad range of frequencies and angles of incidence.

Main Results:

• Demonstrated negative refraction by interfacing oriented α-MoO3 slabs with graphene and gold.
• Observed simultaneous positive and negative refractions, offering versatile optical control.
• Achieved diffractionless propagation, a key phenomenon for advanced optical functionalities.

Conclusions:

• Biaxial α-MoO3 van der Waals materials enable in-plane hyperbolic properties and negative refraction.
• The proposed platform offers flexible manipulation of mid-infrared polaritons.
• Findings support the use of 2D vdW materials for nanoscale super-resolution imaging, molecular sensing, and photonic integrated circuits.