Ultrastrong Plasmon-Phonon Coupling Enabled by Acoustic Graphene Plasmons

Area of Science:

• Condensed Matter Physics
• Nanophotonics
• Quantum Optics

Background:

• Vibrational ultrastrong coupling (USC) is crucial for controlling chemical reactivity and advancing mid-infrared nanophotonics.
• Conventional graphene plasmons (GPs) struggle with reduced dielectric thickness, weakening their coupling efficiency.

Purpose of the Study:

• Demonstrate USC in ultrathin polar dielectrics using acoustic graphene plasmons (AGPs).
• Explore the role of interface phonon polaritons in USC.
• Investigate the transition in anticrossing behavior by tuning AGP coupling.

Main Methods:

• Utilized acoustic graphene plasmons (AGPs) for enhanced field confinement in narrow dielectric gaps.
• Fabricated ultrathin polar dielectric films with thicknesses down to λ/8,000.
• Tuned AGP coupling to both transverse optical phonons and interface phonon polaritons.

Main Results:

• Achieved USC in polar films as thin as λ/2,000, overcoming limitations of conventional GPs.
• Identified interface phonon polaritons as a significant contributor to USC, alongside transverse optical phonons.
• Observed a transition from single to double anticrossing by tuning AGP coupling to different phonon types.

Conclusions:

• Acoustic graphene plasmons enable USC in extremely small volumes, advancing mid-infrared nanophotonics.
• The platform supports novel light-matter interactions, paving the way for quantum optics applications.
• The findings expand the understanding of USC mechanisms by including interface phonon polaritons.