Bidirectional Electromagnetically Induced Transparency Based on Coupling of Magnetic Dipole Modes in Amorphous Silicon Metasurface.

A bidirectional electromagnetically induced transparency (EIT) arising from coupling of magnetic dipole modes is demonstrated numerically and experimentally based on nanoscale a-Si cuboid-bar metasurface. Analyzed by the finite-difference time-domain (FDTD) Solutions, both the bright and dark magnetic dipole mode is excited in the cuboid, while only the dark magnetic dipole mode is excited in the bar. By breaking the symmetry of the cuboid-bar structure, the destructive interference between bright and dark magnetic dipole modes is induced, resulting in the bidirectional EIT phenomenon. The position and amplitude of simulated EIT peak is adjusted by the vertical spacing and horizontal spacing. The EIT metasurface was fabricated by Electron-Beam Lithography and deep silicon etching technique on the a-Si film deposited by Plasma-Enhanced Chemical Vapor Deposition. Measured by a convergent spectrometer, the fabricated sample achieved a bidirectional EIT peak with transmission up to 65% and 63% under forward and backward incidence, respectively. Due to the enhanced magnetic field induced by the magnetic dipole resonance, the fabricated bidirectional EIT metasurface provides a potential way for magnetic sensing and magnetic nonlinearity.

Broadened Angle-Insensitive Near-Perfect Absorber Based on Mie Resonances in Amorphous Silicon Metasurface.

A broadband near-perfect absorber is analyzed by an amorphous silicon (a-Si) hook shaped nanostructure metasurface. The transmission and reflection coefficients of the metasurface are investigated in the point electric and magnetic dipole approximation. By combining square and semicircle nanostructures, the effective polarizabilities of the a-Si metasurface calculated based on discrete dipole approximation (DDA) exhibit broadened peaks of electric dipole (ED) and magnetic dipole (MD) Mie resonances. The optical spectra of the metasurface are simulated with different periods, in which suppressed transmission are shifted spectrally to overlap with each other, leading to broadened enhanced absorption induced by interference of ED and MD Mie resonances. The angle insensitive absorption of the metasurface arrives 95% in simulation and 85% in experiment in spectral range from 564 nm to 584 nm, which provides potential applicability in nano-photonic fields of energy harvesting and energy collection.

Broadened band near-perfect absorber based on amorphous silicon metasurface.

A dielectric broadened band near-perfect absorber based on an amorphous silicon(a-Si) T-shaped nanostructure metasurface is investigated numerically and experimentally. The simultaneous suppressed transmission and reflection of the a-Si nanostructure metasurface are achieved by investigating the interference of the periodically adjustable electric dipole(ED) and magnetic dipole(MD) Mie resonances. The absorption of the a-Si nanostructure metasurface approaches the maximum of 95% in simulation and 80% in experiment with a top-hat shape in the spectral range from 580 nm to 620 nm by employing the T-shaped nanostructure. The proposed near-perfect absorber provides a new approach for expanding absorption bandwidth by integrating different nanostructures in metasurface, which is potentially applicable in nanophotonic fields of optical isolation, optical trapping and energy harvesting.

Tunable polarization-independent dual-band coherent perfect absorber based on metal-graphene nanoring structure.

A dual-band polarization-independent coherent perfect absorber(CPA) based on metal-graphene nanostructure is proposed, which is composed of golden nanorings with different sizes on graphene monolayer. Based on the finite-difference time-domain (FDTD) solutions, coherent perfect absorptions of the metal-graphene CPA are achieved at frequencies of 50.54 THz and 43.60 THz, which are resulted from the excited surface plasmon resonance induced by different size nanorings. Through varying the relative phase of two incident countering-propagating beams, the absorption peaks are all-optically tuned from 98.3 % and 98.4 % to nearly 0, respectively. By changing the gate-controlled Fermi energy of the graphene layer, the resonance frequencies of the CPA are tuned simultaneously without changing the geometrical parameters. And polarization independence of the metal-graphene CPA is revealed due to the center symmetry of nanoring structure. The electrical tunability of resonance frequency and polarization independence enable the proposed CPA to be widely applied in optoelectronic and engineering technology areas for tunable active multiple-band regulation and control.

Tunable polarization-independent plasmonically induced transparency based on metal-graphene metasurface.

A tunable polarization-independent dual-band plasmonically induced transparency (PIT) device based on metal-graphene nanostructures is proposed theoretically and numerically at mid-infrared frequencies, which is composed of two kinds of center-symmetric metallic nanostructure array with different sizes and element numbers placed on separate graphene interdigitated finger sets, respectively. The coupled Lorentz oscillator model is used to explain the physical mechanism of PIT effect at multiple frequency domains. The finite-difference time-domain (FDTD) solutions are employed to simulate the characteristics of the polarization-independent metal-graphene PIT device, which is consistent with the theoretical analysis. The PIT peaks, obtained at two frequency domains, are separately and dynamically modulated by varying the Fermi energy of corresponding graphene finger set without changing the geometrical parameter of the metallic nanostructure. By the carefully selected element numbers of nanostructure arrays, the resonance strength of the PIT peaks at two frequency domains are nearly close. And the PIT device has identical response to the various polarized incident field due to the center symmetry of the metallic nanostructure, which have advantages in practical applications with no polarization-dependent loss.

Independently tunable dual-band plasmonically induced transparency based on hybrid metal-graphene metamaterials at mid-infrared frequencies.

A tunable dual-band plasmonically induced transparency (PIT) device based on hybrid metal-graphene nanostructures is proposed theoretically and numerically at mid-infrared frequencies, which is composed of two kinds of gold dolmen-like structures with different sizes placed on separate graphene interdigitated finger sets respectively. The coupled Lorentz oscillator model is used to explain the physical mechanism of the PIT effect at multiple frequency domains. The finite-difference time-domain (FDTD) solutions are employed to simulate the characteristics of the hybrid metal-graphene dual-band PIT device. The simulated spectral locations of multiple transparency peaks are separately and dynamically modulated by varying the Fermi energy of corresponding graphene finger set, which is in good accordance with the theoretical analysis. Distinguished from the conventional metallic PIT devices, multiple PIT resonances in the hybrid metal-graphene PIT device are independently modulated by electrostatically changing bias voltages applied on corresponding graphene fingers, which can be widely applied in optical information processing as tunable sensors, switches, and filters.

Plasmonic wavelength splitter based on a metal-insulator-metal waveguide with a graded grating coupler.

A plasmonic wavelength splitter based on a sub-wavelength metal-insulator-metal (MIM) periodic rectangle wrinkle waveguide with a graded grating coupler is theoretically analyzed and experimentally demonstrated. The surface plasmon polaritons (SPPs), excited in the metal grating with wavelength selection, are deflected by the graded difference according to the aplanatic parametric principle. The wave vector of the deflected SPPs meets the phase-matching condition and couples into the periodic rectangle wrinkle waveguide with a plasmonic bandgap. The characteristic of the plasmonic wavelength splitter is simulated by finite difference time domain (FDTD) simulation, which agrees well with the theoretical analysis. By electron beam lithography and ion beam etching process, the plasmonic wavelength splitter was fabricated. The SPPs excited by incident 650 and 832 nm were successfully split and guided to opposite directions of the MIM waveguide with extinction ratios of 27.5 dB and 32.7 dB, respectively, which was observed under an optical microscope using a CCD camera. The proposed wavelength splitter is simple fabricated and has a large coupling aperture by utilizing the graded grating coupler.

Tunable multispectral plasmon induced transparency based on graphene metamaterials.

A dynamically wavelength tunable multispectral plasmon induced transparency (PIT) device based on graphene metamaterials, which is composed of periodically patterned graphene double layers separated by a dielectric layer, is proposed theoretically and numerically in the terahertz frequency range. Considering the near-field coupling of different graphene layers and the bright-dark mode coupling in the same graphene layer, the coupled Lorentz oscillator model is adapted to explain the physical mechanism of multispectral EIT-like responses. The simulated transmission based on the finite-difference time-domain (FDTD) solutions indicates that the shifting and depth of the EIT resonances in multiple PIT windows are controlled by different geometrical parameters and Fermi energies distributions. A design scheme with graphene integration is employed, which allows independent tuning of resonance frequencies by electrostatically changing the Fermi energies of graphene double layer. Active control of the multispectral EIT-like responses enables the proposed device to be widely applied in optical information processing as tunable sensors, switches, and filters.