A mid-infrared biaxial hyperbolic van der Waals crystal (1809.03432v1)

Abstract: Hyperbolic media have attracted much attention in the photonics community, thanks to their ability to confine light to arbitrarily small volumes and to their use for super-resolution applications. The 2D counterpart of these media can be achieved with hyperbolic metasurfaces, which support in-plane hyperbolic guided modes thanks to nanopatterns which, however, pose significant fabrication challenges and limit the achievable confinement. We show that thin flakes of the van der Waals material {\alpha}-MoO3 can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without any patterning. This is possible because {\alpha}-MoO3 is a biaxial hyperbolic crystal, with three different Restrahlen bands, each for a different crystal axis. Our findings can pave the way towards new paradigm to manipulate and confine light in planar photonic devices.

• The paper demonstrates that α-MoO3 is a natural biaxial hyperbolic crystal with anisotropic permittivity across its three principal axes.
• It reveals in-plane hyperbolic phonon polaritons whose direction-dependent propagation is confirmed by advanced optical nano-imaging techniques.
• The study highlights α-MoO3’s potential for subwavelength electromagnetic confinement in photonic devices without the need for complex nanopatterning.

Mid-Infrared Biaxial Hyperbolic van der Waals Crystal: α-MoO3

The research paper titled "A mid-infrared biaxial hyperbolic van der Waals crystal" presents a comprehensive paper on the hyperbolic properties of alpha-phase molybdenum trioxide (α-MoO3), a van der Waals (vdW) material, in the mid-infrared regime. This work stands out for demonstrating the naturally occurring in-plane hyperbolic properties of α-MoO3 without the requirement for any sophisticated nanopatterning techniques typically used in hyperbolic metasurfaces.

Key Findings

1. Biaxial Hyperbolic Properties: The fundamental discovery delineated in the paper is that α-MoO3 is a natural biaxial hyperbolic crystal. Unlike prior vdW materials that were reported as uniaxial, α-MoO3 showcases biaxial characteristics due to its distinct permittivity tensor with anisotropic components along the three principal crystal axes. This anisotropy is attributed to the Mo‒O bonds varying along each crystalline direction, leading to three different Reststrahlen bands covering 545 to 1010 cm⁻¹.
2. In-Plane Phonon Polaritons: Detailed optical scanning probe nano-imaging techniques reveal that α-MoO3 supports in-plane hyperbolic phonon polaritons, characterized by unique isofrequency hyperbolic geometries and concave wavefronts. The paper illustrates that the phonon polariton propagation and wavefront architecture are direction-dependent, differing for various crystal axes and excitation frequencies.
3. High Confinement with Natural Materials: The research emphasizes the potential of α-MoO3 in achieving high electromagnetic confinement using its native periodicity, which is unrestricted by artificial structuring and avoids associated fabrication challenges and optical losses. Such natural hyperbolicity enables manipulating mid-infrared light at deeply subwavelength scales.

Numerical and Experimental Insights

The paper contains robust experimental validation combined with numerical simulations. At distinct excitation frequencies, the near-field optical imaging demonstrates anisotropic in-plane propagation of polaritons on thin α-MoO3 flakes. These experimental observations align well with theoretical predictions using the Fresnel equation adapted for biaxial crystals. Moreover, based on extracted dispersion relations, the team identifies orientation and frequency-dependent propagation that disallows circular symmetry, hence highlighting the unique directionally confined polaritonic behavior of α-MoO3.

Implications and Future Directions

The presence of these biaxial hyperbolic features in α-MoO3 paves an avenue toward the development of planar photonics sans complex nanopatterning. The strongly confined plasmonic waves foreseen from these findings could influence the design and functionality of photonic devices, potentially being used in sub-diffraction optical focusing and advanced optoelectronics. Moreover, given α-MoO3's intrinsic semiconducting properties, further exploration can integrate these optical characteristics for novel active photonic devices enhancing optical-to-electrical conversion efficiencies.

The insight into the natural biaxial hyperbolic characteristics forms a foundation upon which additional vdW materials could be examined to tailor optical responses across various wavelength regimes. This approach could lead to a new class of materials optimized for specific photonic applications by exploiting the inherent structural and compositional diversity in naturally layered crystals.

Conclusion

The essay distills the essence of the reported research by highlighting the meticulous demonstration of mid-infrared hyperbolicity in α-MoO3—a property advantageous for high-confinement photonics and practicality in avoiding nanopatterning challenges. The implications stretch across both conceptual understanding and technological realizations in photonics, heralding a refined direction in the exploitation and integration of naturally hyperbolic vdW materials.