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

Reconfigurable switching between reflecting/absorbing modes in VO2 assisted graphene-coated hemispherical dielectric hole arrays

In this paper, a graphene-coated dielectric hole array is used to design a reconfigurable switchable optical reflector/absorber device. The design benefits from the collective excitation of localized surface plasmon resonances of graphene-coated hole array, providing simpler fabrication fellow and more compact structure with respect to graphene-coated spherical nanoparticle array with similar plasmonic behavior. Geometrical parametric study of the reflecting mode shows that the device has lots of degrees of freedom for spectrum tuning and can highly tolerate fabrication imperfections. Moreover, the reflection rate is slightly affected by the dielectric substrate height, which can be tuned to achieve strong absorption by backing it with a metallic mirror. The designed absorber efficiently captures a wide range of obliquely incident transverse electric (TE) and transverse magnetic (TM) waves. Also, the operating frequency of both reflecting and absorbing modes can be tuned after fabrication, thanks to the two-dimensional nature of graphene material. Finally, using vanadium dioxide (VO₂) phase change material, the switchable reflector\absorber mode of the device is also exhibited.

Bi-functional tunable reflector/high-Q absorber design using VO2 assisted graphene-coated cylinder array

In this paper, a bi-functional tunable reflector/absorber device using an assembly of graphene-coated cylindrical wires, backed by a thermally controlled phase change material, is proposed. The reflection coefficient of the graphene-coated wire-grating manifests multiple resonances, originating from the hybridized excitation of localized surface plasmons in the graphene shells. The first plasmonic resonance (with the order of two), in the free-standing configuration, shows tunable near-perfect reflection while the second plasmonic resonance (with the order of three), in the reflector-backed array, exhibits near-perfect absorption. Because of the metal-insulator transition in the phase change material, it is feasible to switch between these two functionalities using a VO₂ back layer. Moreover, the high-quality factor of the absorption band (Q ∼ 128.86) is due to its Fano line shape, leading to a narrow bandwidth. Thus, the absorbing mode can be possibly used for refractive index sensing with the sensitivity of S ∼ 9000 nm/RIU (refractive index unit) and figure of merit of FOM ∼ 104 RIU^-1. In the proposed structure, different optical, material, and geometrical parameters affect the optical response of the operating bands, offering a flexible design.

Broadband absorption using all-graphene grating-coupled nanoparticles on a reflector

In this paper, the hybridized localized surface plasmon resonances (LSPRs) of a periodic assembly of graphene-wrapped nanoparticles are used to design a nanoparticle assisted optical absorber. Bandwidth enhancement of this structure via providing multiple types of plasmonic resonances in the associated unit cell using two densely packed crossly stacked graphene strips is proposed. The designed graphene strips support fundamental propagating surface plasmons on the ribbons, and gap plasmons in the cavity constructed by the adjacent sections. Graphene strips exhibit a hyperbolic dispersion region in the operating spectrum and assist in the bandwidth enhancement. Moreover, since the nanoparticles are deposited on the top strips, real-time biasing of them can be easily conducted by exciting the surface plasmons of the strip without the necessity to electrically connect the adjacent nanoparticles. The overall dynamic bandwidth of the structure, using a two-state biasing scheme, covers the frequencies of 18.16–40.47 THz with 90% efficiency. Due to the symmetry of the structure, the device performs similarly for both transverse electric (TE) and transverse magnetic (TM) waves and it has a high broadband absorption rate regarding different incident angles up to 40°. Due to the presence of 2D graphene material and also using hollow spherical particles, our proposed absorber is also lightweight and it is suitable for novel compact optoelectronic devices due to its sub-wavelength dimensions.

Broadband/multiband absorption through surface plasmon engineering in graphene-wrapped nanospheres

In this paper, a thin film constructed by a periodic assembly of graphene-wrapped particles with spherical geometry has been proposed as a polarization-insensitive reconfigurable perfect absorber. The performance of the proposed structure is based on the cooperative excitation of the quadrupole localized surface plasmons on graphene shells. By sweeping the quality of graphene shells, it is recognized that the low-quality graphene material is the best choice for the absorber design. Moreover, the effect of graphene chemical potential and periodicity of the particles on the absorptivity of the structure is investigated. The physical mechanism of performance is clarified by investigating the excited localized surface plasmon resonances. In addition, the angle-independent behavior up to around 60 degrees for both transverse electric (TE) and transverse magnetic (TM) waves is proved. Interestingly, by engineering the substrate height, our proposed absorber exhibits dynamic broadband performance due to the impedance matching and multiband absorption by enhancing the Fabry-Perot resonances of a micrometer-sized substrate. The possibility of attaining a similar static broadband response by stacking multiple layers is also proved. Our proposed sub-wavelength absorber can be suitable for novel optoelectronic devices due to its simple geometry.

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.