Tunable manipulation of terahertz wavefront based on graphene metasurfaces

Abstract

We have systematically investigated the performances of a tunable graphene metasurface that can dynamically manipulate the terahertz wavefronts. The metasurface consists of a silver substrate, SiO2 interlayer and the top graphene ribbons that can exhibit plasmon resonances to realize a phase shift by changing the Fermi levels of graphene ribbons. The plasmon resonances in graphene ribbons and Fabry–Perot resonances in the SiO2 interlayer work together for making the designed metasurface cover 2π phase range nearly. In the simulations, we can realize anomalous reflection at any angle by using the continuous phase modulation. On this basis, a reflective focusing lens based on the graphene metasurface has also been designed, which is designed in the frequency of 5.0 THz with a reasonable operation bandwidth from 4.5 THz to 6.5 THz. The corresponding focal lengths are designed as 300 μm and 100 μm, and the depths of focus (full width at half maximum along the Z direction) are 114 μm and 104 μm, respectively. Especially, the diameters of focal points (full width at half maximum along the X direction) are 29.5 μm and 24.1 μm, which are smaller than a half-wavelength (30 μm) in the focusing plane. It indicates that our designed focusing lenses have superior performance and can provide an opportunity to develop a tunable wavefront-controlling device.

Actively Tunable Terahertz Switches Based on Subwavelength Graphene Waveguide

“…The influence of the Si substrate on the SP dispersion can be neglected in our proposed architectures due to the thick PMMA layer [ 25 ]. Therefore, the dispersion relation of graphene-based SPs can be derived as follows [ 26 , 27 , 28 , 29 ]: where is the free-space wave vector of light, is the incident wavelength in vacuum, is the effective refractive index of the SP mode, denotes the relative permittivity of air, and is the relative permittivity of PMMA. Here, we set and .…”

Recent Progress on Graphene-Functionalized Metasurfaces for Tunable Phase and Polarization Control

“…When the graphene ribbons are separately biased, more degrees of freedom are gained for controlling the functionality temporally and spatially [58,59]. Figure 5c shows that the smooth 360° phase variation is covered by tuning the Fermi level, and it is well maintained from 4 THz to 6 THz.…”

“…It leads to switching of the functionality by frequency, for example, from an anomalous reflector at 5 THz to a normal reflector at 3.5 THz. With the Fermi level of the graphene ribbon tuned spatially and temporally, a metalens of a large numerical aperture is numerically demonstrated with either a fixed or variable focal point over a wide bandwidth [58]. Completely different functionalities, such as cloaking, illusion, and focusing, can be implemented in one metasurface [60].…”

“…( c ) Smooth 360° phase variation is covered by tuning the Fermi level of graphene, and it is shifted with a well-maintained shape from 4 THz to 6 THz. Adapted from [58], Copyright IOP Publishing Ltd., 2017. ( d ) Far away from the optimum frequency, the phase variation is either too sharp or too weak to use.…”

Polarization Controlled Dual Functional Reflective Planar Metalens in Near Infrared Regime

“…Metasurfaces possess the advantages of small losses and are easy to manufacture [10,11], and have been attractive in recent years. Among various metasurfaces, metalens with its unit cell interacting with incident light and introducing abrupt phase shift along the interface of the metasurface have been widely presented to achieve the function of focusing [12,13] and anomalous reflection [14][15][16]. Lots of models like the nanoslit [17], nanohole [18,19], and graphene ribbons [20][21][22][23][24] have been introduced in metalens design, where 2π phase shift resulting from the designed antennas is needed in controlling the wavefront.…”