ABSTRACT
Active phase-control metasurfaces show outstanding capability in the active manipulation of light propagation, while the previous active phase control methods have many constraints in the cost of simulation or the phase modulation range. In this paper, we design and demonstrate a phase controlled metastructure based on two circular split ring resonators (CSRRs) composed of silicon and Au with different widths, which can continuously achieve an arbitrary Pancharatnam-Berry (PB) phase between -π and π before or after active control. The PB phase of such a metasurface before active control is determined by the rotation angle of the Au-composed CSRR, while the PB phase after active control is determined by the rotation angle of the silicon-composed CSRR. And active control of the PB phase is realized by varying conductivity of silicon under an external optical pump. Based on this metastructure, active control of light deflection, metalens with arbitrary reconfigurable focal points and achromatic metalens under selective frequencies are designed and simulated. Moreover, the experimental results demonstrate that focal spots of metalens can be actively controlled by the optical pump, in accord with the simulated ones. Our metastructure implements a plethora of metasurfaces' active phase modulation and provides applications in active light manipulation.
ABSTRACT
A novel electromagnetically induced transparency (EIT) all-dielectric metamaterial is proposed, fabricated, and characterized. The unit cell of the proposed metamaterial comprises of two asymmetric split ring resonators (a-SRRs) positioned with a mirror symmetry. The asymmetric nature of a-SRRs results from the length difference of two arcs. Optical properties of the fabricated metamaterial are investigated numerically using finite difference method, as well as experimentally using a terahertz time-domain spectroscopy. The results confirm that the proposed metamaterial exhibits an EIT transparent window in the frequency range around 0.78THz with a Q-factor of ~75.7 and a time-delay up to ~28.9ps. Theoretical investigations show that EIT effects in our metamaterial are achieved by hybridizing two bright modes in the same unit cell, which are aroused by the excitation of magnetic moments. We also confirm that the proposed metamaterial has great potential for sensing applications with high sensitivity and high figure of merit (FOM), which guarantees potential applications in in situ chemical and biological sensing.
ABSTRACT
In this paper, a mid-infrared perfect absorber based on the dual gratings-coupled graphene-dielectric multilayer structures (DGC-GDM) is proposed, in which GDM is sandwiched between two Au gratings. The DGC-GDM absorber shows advantages of dual-band and tunable absorption, insensitive to polarization, ultrathin thickness and wide angle range absorption. Two kinds of SPPs in the GDM layer can be excited by the upper and lower Au gratings, respectively, which confine the incident light into the GDM and thus contribute to the dual-band absorption. The wavelength of the absorption peak can be effectively changed by varying the Fermi level of graphene. Most importantly, an analytic formulas describing the relationships between the parameters of the absorber and the absorption spectra is derived. And the accuracy of the theoretical formulas is verified by comparing the simulation results with the theoretically calculated ones. Therefore, the exact values of parameters of the structure for an absorption peak as required can be obtained. The proposed structure can be applied to absorbers that are working at other frequencies.
