Active control of micrometer plasmon propagation in suspended graphene

Doped graphene supports ultrahigh-confined, low-loss, and tunable surface plasmons that hold great potential for developing infrared optoelectronic nanodevices with unprecedented capabilities. However, due to the two-dimensional character of this material, graphene plasmons have been invariably studied in supported samples so far. The substrate provides stability for graphene but often causes undesired interactions (such as dielectric loss, phonon hybridization, and impurity scattering) that compromise the quality of graphene plasmons, and limits intrinsic flexibility of the material. Here, we demonstrate the visualization of plasmons in suspended graphene at room temperature, exhibiting high-quality factor Q~33 and long propagation length >3 μm. Suspended graphene also provides an exceptional undisturbed plasmonic environment to exploit the behavior of the intrinsic flexibility under extrinsic modification. We introduce the graphene suspension height as a novel plasmonic tuning knob that enables in situ change of the dielectric environment and substantially modulates the plasmon wavelength, propagation length, and group velocity. Such active control of micrometer plasmon propagation facilitates near-unity-order modulation of nanoscale energy flow that serves a new plasmonic switch with an on-off ratio above 14. The suspended graphene plasmons possess long propagation length, high tunability, and controllable energy transmission simultaneously, opening up new horizons for their integration in practical photonic devices.