Steering and cloaking of hyperbolic polaritons at deep-subwavelength scales

The study addresses how to achieve practical manipulation of polaritons—hybrid light-matter excitations—over deep-subwavelength distances for on-chip photonic circuitry. While metamaterials and natural polaritonic media enable strong field confinement beyond the diffraction limit, integrating complex transmission characteristics across interfaces to steer polaritons at the nanoscale has remained challenging. Hyperbolic van der Waals materials, such as α-MoO3, support highly anisotropic polaritons with long lifetimes and extreme confinement, offering a promising route to directional control. The authors propose and demonstrate a materials-based strategy using stacked and assembled α-MoO3 building blocks with controlled twist and cut orientations to steer and cloak polaritons by exploiting mode hybridization and refraction at engineered interfaces.

The paper builds on extensive work in nanophotonics, metamaterials, and polaritonics showing deep-subwavelength field control and hyperbolic dispersion in natural vdW crystals. Prior studies established hyperbolic and shear polaritons with anomalous propagation, topological transitions, negative refraction, and lensing in anisotropic media and twisted heterostructures. Twisted α-MoO3 bilayers exhibit robust hybridization and topological transitions in isofrequency contours (IFCs), enabling phenomena like negative refraction and focusing. However, achieving practical in-plane steering across multiple interfaces with low loss and integrating such effects into functional devices (e.g., cloaks) has been elusive. This work leverages the low-loss, layered nature of vdW materials and image-polariton enhancement on gold substrates to realize high-transmittance steering and device-level cloaking.

• Materials and sample preparation: High-quality α-MoO3 and graphite crystals were synthesized by CVD and mechanically exfoliated onto SiO2(300 nm)/Si substrates. Suitable α-MoO3 films were identified via optical microscopy and thickness measured by AFM.
• Nanofabrication: Patterns with specified cut angles β were defined in ~1 µm PMMA950K using 100 kV EBL (Vistec 5000+ES). Structures were etched by RIE in SF6/Ar. Resist residues were removed by hot acetone (80 °C, 20 min) and IPA (3 min), followed by N2 drying.
• Deterministic assembly and twist control: A PDMS/PC stamp was used for dry transfer to stack α-MoO3 flakes onto Au(60 nm)/Si substrates. Individual α-MoO3 patterns (films and microribbons) were positioned and rotated in-plane using a plateau AFM tip to achieve designed twist angles θ and ribbon orientations β.
• Antenna fabrication and excitation: Gold antennas (~3.0 µm length, 50 nm thickness) were patterned in PMMA and deposited by e-beam evaporation (<5×10−6 Torr), then lifted off. Antennas served as resonant dipole launchers to efficiently excite polaritons in the bottom α-MoO3 film, spatially separating excitation from detection.
• Near-field imaging (s-SNOM): A commercial s-SNOM (Neaspec) with a ~25 nm radius Pt-coated AFM tip imaged polariton propagation. A QCL provided monochromatic mid-IR light (890–2000 cm−1), p-polarized, focused with a parabolic mirror at 55–65° incidence. The tip tapped at ~270 kHz with 30–50 nm amplitude; interferometric pseudoheterodyne detection demodulated the third harmonic (S3) to obtain background-free near-field amplitude images.
• Device geometries: (i) Twisted double films with an interface (cut edge perpendicular to antenna axis) to study refraction as a function of twist angle θ (0–90°). (ii) Steering devices using α-MoO3 microribbons with designed cut angles (e.g., β = 45° and 135°) placed atop a bottom α-MoO3 film to induce sequential refractions (zigzag guiding). (iii) A cloaking device consisting of four microribbons arranged symmetrically (β = 45° for ribbons 1 and 3; β = 135° for 2 and 4) to split, deflect, and recombine hyperbolic waves, hiding a central region; a graphite disk (50 nm thick, 1 µm diameter) served as a test defect.
• Analysis and simulations: Polariton refraction directions were derived by conserving the parallel wavevector at interfaces and analyzing IFCs of mixed (hyperbolic/hybrid) modes in twisted stacks. Numerical electromagnetic simulations reproduced near-field patterns, transmittance, and focusing behavior; simulated IFCs guided Poynting vector directions. An ad hoc overall quality factor Q was extracted from intensity decay along propagation paths, incorporating propagation and interface transmission. Transmission across interfaces vs twist angle was measured experimentally and compared to simulations, with error estimates (95% CI).

• Directional refraction control by twist: Rotating the top α-MoO3 film induces a topological transition of IFCs in the twisted region, enabling a continuous change from normal to negative refraction of hyperbolic polaritons. Near-field images confirm refraction and propagation direction changes with θ = 0°, 22.5°, 45°, 67.5°, 90°.
• High interfacial transmittance and low loss: Measured transmittance across the single–double film interface remains ~85–95% over a wide range of twist angles; discrepancies with simulations arise from focusing that enhances apparent transmission under negative refraction. Overall quality factors are ~30 across θ due to low interface roughness in vdW stacks and image-phonon-polariton enhancement on gold substrates.
• In-plane steering with microribbons: Top α-MoO3 microribbons with specific cut angles (β) produce two refractions per ribbon, yielding controlled lateral deflections (zigzag waveguided trajectories). Two adjacent ribbons with mirror-symmetric edges (β = 45° and 135°) produce opposite deflections that cancel, restoring the initial propagation direction; identical edge angles yield cumulative deflection. Fringe intensities remain essentially constant, evidencing low added losses.
• Cloaking via refraction: A device with four microribbons splits and deflects the incident hyperbolic wave into two beams that bypass a central region and recombine, effectively hiding a defect (graphite disk). Near-field amplitude profiles show minimal changes in intensity and wavelength with the defect present in the hidden region compared to without the defect, whereas a defect without the cloaking structure causes substantial intensity reduction. Electromagnetic simulations (near-field Re{Ez}) agree with experiments and visualize complex propagation paths.
• Agreement with modeling: Experimental propagation directions, fringe wavelengths, and refraction behaviors align with numerical simulations and IFC analyses, confirming robust mode hybridization and strong modal-profile alignment in twisted α-MoO3 stacks.

The results directly address the challenge of practical nanoscale manipulation of deeply subwavelength polaritons by demonstrating that engineered vdW heterostructures of α-MoO3 can steer energy flow via anisotropic refraction with high interfacial transmittance. The twist-controlled topological transitions of IFCs provide a tunable mechanism to switch between normal and negative refraction, while microribbon assemblies implement sequential refraction for complex in-plane routing. The observed low losses and high quality factors stem from the layered vdW interfaces and image-polariton effects on gold, enabling device-level functionalities such as cloaking. These findings validate a materials-based path toward transformation polaritonics using natural anisotropic crystals, offering a versatile platform for on-chip nanophotonic circuits that require precise, low-loss control of polariton trajectories.

The study demonstrates in-plane steering and cloaking of hyperbolic polaritons at deep-subwavelength scales using customizable stacked and spliced α-MoO3 structures. By exploiting twist-induced IFC topology changes and anisotropic refraction at engineered interfaces, the authors realize high-transmittance steering elements and a cloaking device that hides defects without degrading polariton intensity or wavelength. This natural-materials approach provides high quality factors and low interface losses, paving the way for practical polaritonic circuitry and advancing transformation polaritonics. Future work could expand the cloaking region via optimized ribbon geometry, explore broader spectral operation, integrate alternative substrates and vdW materials, and develop reconfigurable or actively tunable devices.

• Experimental–simulation mismatch in transmittance trends versus twist angle, attributed to focusing under negative refraction that enhances measured transmission.
• Loss of high-wavevector components at interfaces due to asymmetric IFCs and finite fabrication tolerances, limiting perfect mode matching and coupling of all k-states.
• Cloaking region extent is constrained by ribbon cut angle, width, and thickness; larger hidden regions require more aggressive geometries.
• Demonstrations are within α-MoO3 Reststrahlen band II (e.g., ~893–900 cm−1) and on gold substrates; performance may vary with frequency and substrate choice, and generalization to other platforms requires validation.
• Near-field imaging measures normalized amplitudes; absolute loss quantification and scalability to complex circuit architectures will benefit from further studies.