Nonlinear coupling in graphene-coated nanowires

Broadband continuous/discrete spectrum optical absorber using graphene-wrapped fractal oligomers

In this paper, a second-order fractal oligomer constructed by graphene-coated cylindrical nano-rods is proposed as the unit cell of a wideband optical absorber. Nano-rods have resided on a dielectric substrate with a thick metallic mirror. The fractional bandwidth of the designed structure is 88.67% for the absorption above 90%. Broadband absorption originates from the cooperative excitation of localized surface plasmon resonances (LSPRs) of the bottom, top, and lateral surfaces of the rods, engineered by the geometrical parameters through the fractal concept. Designed full absorber has an acceptable performance concerning the incident angles up to around 35° and it is polarization insensitive. Moreover, broadband absorption can be altered to multi-band performance in the same spectrum with the desired number of frequency bands. This feature is obtained by manipulating the substrate thickness to excite multiple orders of Fabry-Perot cavity resonances. Our proposed structure has potential applications in various optical devices such as filters, sensors, and modulators.

Graphene-Coated Nanowire Waveguides and Their Applications

In recent years, graphene-coated nanowires (GCNWs) have attracted considerable research interest due to the unprecedented optical properties of graphene in terahertz (THz) and mid-infrared bands. Graphene plasmons in GCNWs have become an attractive platform for nanoscale applications in subwavelength waveguides, polarizers, modulators, nonlinear devices, etc. Here, we provide a comprehensive overview of the surface conductivity of graphene, GCNW-based plasmon waveguides, and applications of GCNWs in optical devices, nonlinear optics, and other intriguing fields. In terms of nonlinear optical properties, the focus is on saturable absorption. We also discuss some limitations of the GCNWs. It is believed that the research of GCNWs in the field of nanophotonics will continue to deepen, thus laying a solid foundation for its practical application.

Multi-frequency near-field enhancement with graphene-coated nano-disk homo-dimers

In this paper, a 3D sub-wavelength graphene-coated nano-disk dimer (GDD) is proposed for multi-frequency giant near-field enhancement. We observed that the dual-band operation originates from the excitation of hybridized localized surface plasmons on top and bottom faces of the disks along with the mutual coupling from the adjacent particle. Due to the sub-wavelength nature of the disks, the excited localized surface plasmons on the sidewalls are weak but they still can affect the dual operating bands. On the other hand, the strength and resonance frequency of the enhanced fields can be simply modulated by tuning the relative distances of 2D graphene disks on top and bottom faces. Adjustable dual-band performance is hardly attainable using simplified 2D graphene disks, however, it naturally comes out through modal hybridization in the subwavelength 3D structure containing multiple resonant units. Our suggested configuration has better optical properties than its noble metal counterparts because of its higher field enhancement and lower ohmic losses. Moreover, the electromagnetic response is reconfigurable by varying the bias voltage. The influence of graphene quality, chemical potential, and dimer gap size on the electric field enhancement and the resonance frequency of the surface plasmons are investigated, as well. To further improve its performance, a double negative metamaterial core is considered. This mechanism of the performance improvement by the core material is feasible thanks to the 3D nature of the structure. Two possible applications of the presented design are in Surface-Enhanced Raman Spectroscopy (SERS) and optical absorbers.