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Advances in the design and manufacturing processes of photonic materials has realized new models and multifunctional devices which possess an unprecedented control of light.
In a new approach, a team of International Researchers have coated nanowires and nanowire dimers with graphene to create polarization-dependent materials which possess a tuneable invisibility cloaking mechanism.
Recent developments in photonic materials have seen the creation of multifunctional devices with never before seen light control. In particular, advancements surrounding the manipulation of transformation optics has allowed regions of space to be electromagnetically isolated at certain frequencies (or in a range of frequencies). Such processes have caused objects within such regions to stop interacting with illuminating light and to become invisible (technically optically cloaked) to an observer.
Despite their interest and ‘science-fiction coolness’, the materials to develop practical cloaking devices require extreme permittivity and permeability values and so have stagnated in some respects. However, advances in coating technologies has opened new doors into cloaking devices, especially where the coating possesses a metal-dielectric multilayer or metasurface which lowers the scattered signal from sub-wavelength particles.
Researchers from various locations around the world have now proffered graphene coated nanowires and nanowire dimers as cloaking devices. The Researchers used a combination of numerical and theoretical calculations to simulate the illumination and cloaking abilities of the coated materials, using a range of frequencies in the far-infrared. A p-polarized incident plane wave was utilized to illuminate the graphene coated dielectric material and produce localized surface plasmon polaritons.
The Researchers used a full-wave Lorenz-Mie scattering method to estimate the scattering efficiency of the coated nanowire(s) and employed a combination of TEz and TMz polarized propagating waves along the x-axis of the nanowires. The Researchers also employed numerical simulations to combat common problems associated with multiple scattering from non-overlapping parallel nanowires. The Researchers used COMSOL Multiphysics software to perform the numerical analyzes and was implemented to solve the Maxwell’s equations within the calculations based on a finite element analysis (FEA) method.
The nanowires adopted a cylindrical shape during the calculations, and the Researchers derived an analytical formula which enabled a fast evaluation of the spectral window alongside a significant reduction of the scattering efficiency for thin cylinders.
The team also optimized the scattering cancellation within the nanocavities of the nanowires and subsequently developed a spectral distribution model for the scattering efficiency. This model showed that the index ratio between the core and the surrounding medium produced a characteristic lineshape comparable to that of a Fano resonance lineshape.
The Researchers found that the very thin graphene coated nanowires exhibited a scattering behavior colloquial to that of solid nanowires with bulk conductivity. The scattering efficiency was also found to arise from the ratio of graphene surface conductivity and the radius of the nanowire.
The analytical expressions derived by the Researchers also enabled the spectral window to be characterized for invisibility cloaking. The invisibility window was found to be tuneable by modifying the graphene chemical potential and by monitoring the applied gate voltage. This led to a polarization-dependent effect which lead to a tuneable resonant invisibility.
Not content with single channel invisibility cloaking, the Researchers also investigated the prospect of a multi-frequency cloaking mechanism based on coated nanowire dimers. The investigated multi-frequency mechanism was based on a hybrid picture which employed clusters of several invisible graphene-coated nano-cylinders.
It was also found that the mechanism and multi-frequency scheme was tuneable enough to be used in ultra-thin reconfigurable cloaking devices. The multi-frequency system operated within the far-infrared range and the cylindrical nanowires were found to be undetectable when applied to subwavelength scatters.
The cloaking devices, if realized in real-life, could be used for sensing within a dynamic operation bandwidth, as the receiving field can efficiently reach the interior and stimulate its electromagnetic response. Such materials could also be realized in sensitive spectroscopy and novel nonlinear miniaturized systems, where the spectral minima from the excitation within the coupled modes (in the dimers) could allow for an increased gap field, without radiation to the far field.
Image Credit:
Tatiana Shepeleva/ Shutterstock.com
Source:
“Tunable invisibility cloaking by using isolated graphene-coated nanowires and dimers”- Naserpour M., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-12413-4
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