Spatiotemporal visualization of long-range anisotropic plasmon polaritons in hyperbolic MoOCl₂

Area of Science:

• Nanophotonics
• Materials Science
• Condensed Matter Physics

Background:

• Manipulating light at the nanoscale with minimal loss is a key challenge for nanophotonics.
• Anisotropic materials support direction-dependent polariton modes, enabling control over light propagation.
• Hyperbolic materials are known for confined polaritons but also host challenging-to-observe long-range directional polaritons.

Purpose of the Study:

• To directly visualize and characterize the dynamics of long-range anisotropic plasmon polaritons (LRAPPs).
• To investigate the potential of molybdenum oxydichloride as a platform for low-loss nanophotonic applications.
• To overcome experimental challenges in achieving simultaneous nanometer and femtosecond resolution for polariton observation.

Main Methods:

• Utilized time-resolved photoemission electron microscopy (TR-PEEM) for nanoscale imaging.
• Studied LRAPP dynamics on a van der Waals hyperbolic material, molybdenum oxydichloride.
• Tracked spatiotemporal evolution of LRAPPs to determine their velocities and observe reflections.

Main Results:

• Directly imaged LRAPPs with propagation lengths exceeding 10 μm.
• Observed LRAPPs with approximately three times longer propagation length and lower optical loss compared to short-range polaritons.
• Determined nanoscale phase and group velocities of LRAPPs and observed their reflections at flake edges.

Conclusions:

• Molybdenum oxydichloride supports both low-loss directional transport and subwavelength field confinement.
• This material is a versatile platform for integrated nanophotonics in the visible spectral range.
• Demonstrated direct nanoscale visualization of long-range polariton dynamics.