Mid-infrared deep subwavelength confinement in graphene plasmonic waveguides

Strong optical field intensity is greatly desired in mid-infrared (MIR) photonics to confine light on miniaturized compact-size chips. However, due to a fundamental trade-off between energy confinement and propagation loss, it has been a challenging task to obtain further deep subwavelength confinement. In this study, we present a simple graphene waveguide that generates image plasmons assisted by metallic cylindrical nanowires operating at a wavelength of 15 μm. Due to high-mobility graphene encapsulated by hexagonal boron nitride (hBN) and ultra-confined image plasmonic mode, the resulting waveguide has two orders of magnitude smaller mode area than the strongest confinement waveguides to date (up to an order of 10⁻⁸), in conjunction with comparable loss (a few micrometers of propagation distance). Furthermore, by the tunable graphene locations in the gap, we implement an efficient manipulation of the mode area across an order of magnitude. Such high-performance waveguides may be exploited for deep subwavelength field-enhanced devices, which promise to be unprecedented pathways for nanoscale transmission and manipulation of MIR light.