In this letter, we theoretically investigate the coupling of far-infrared surface plasmon polaritons (SPPs) between spatially separated graphene sheets. By using the finite-difference frequency-domain method, we numerically illustrate the SPP propagation and modulation in the graphene sheets. The coupling of SPPs is employed to design zero insertion loss optical splitters, 1 × 2 digital optical spatial switches, and ultra-compact Mach-Zehnder interferometers with the arm length far below the diffraction limit. The study provides an effective way in designing graphene based high-speed and ultra-compact optoelectronic devices.

Metal heterostructures constructed surface plasmon polaritons (SPPs) Bragg reflectors and nanocavities on flat metallic surfaces are proposed and demonstrated numerically. A metal heterowaveguide structured by alternately stacking two kinds of metal gap waveguides (MGWs) shows periodically effective refraction index modulation to SPPs and produces SPP propagation on flat metallic surfaces a band gap in certain frequencies, known as plasmonic band gap, in which SPP propagation is forbidden. Changing the width of one MGW in the heterowaveguide, a SPP nanocavity with high quality factor can be created. Our results imply a broad possibility of constructed SPP-based Bragg reflectors, emitter, and filters, etc., on flat metallic surfaces for planar nanometeric photonic networks.

Here we investigate theoretically and numerically the coupling between surface plasmon polaritons (SPPs) in monolayer graphene sheet arrays that have a period much smaller than the wavelength. We show that when the collective SPP is excited with an out-of-phase illumination, the beam tends to propagate toward the opposite direction of the Bloch momentum, reflecting a negative coupling between the constituent SPPs. In contrast, for in-phase illumination, the incident beam is split into two collective SPPs that are highly collimated and display low propagation loss. Moreover, the coupling between the individual SPPs results in a reduction of the modal wavelength of the SPP in comparison with that of a single graphene sheet.

We present a wavelength tunable absorber composed of periodically patterned cross-shaped graphene arrays in the far-infrared and THz regions. The absorption of the single-layer array can essentially exceed the continuous graphene sheet by increasing the cross-arm width, even for small graphene filling ratio. As chemical potential and relaxation time increase, the absorption can be significantly enhanced. The complementary structure shows higher absorption compared to the original graphene array. Moreover, the wavelength of absorption maximum is angle-insensitive for both TE and TM polarizations. The absorption efficiency can be further improved with double layers of the cross-shaped graphene arrays, which are helpful to design dual-band and broadband absorbers.

In the past half-century, surface plasmon resonance in noble metallic nanoparticles has been an important research subject. Recent advances in the synthesis, assembly, characterization, and theories of traditional and non-traditional metal nanostructures open a new pathway to the kaleidoscopic applications of plasmonics. However, accurate and precise models of plasmon resonance are still challenging, as its characteristics can be affected by multiple factors. We herein summarize the recent advances of plasmonic nanoparticles and their applications, particularly regarding the fundamentals and applications of surface plasmon resonance (SPR) in Au nanoparticles, plasmon-enhanced upconversion luminescence, and plasmonic chiral metasurfaces.