Periodical Subwavelength Metal-graphene Optoelectronic Devices And High-contrast Gratings Optical Differentiator

Recently,the subwavelength periodical photonic device based on metal,graphene and dielectric have attracted much attention due to the application potential in integrated optics field.Many periodical structures have been proposed to control and manipulate the amplitude,phase and scattering pattern of optical field at subwavelength scale.In this paper,the optical properties of metal-graphene and high-contrast grating have been studied,and the polarizationindependent dual-band plasmonically induced transparency(PIT)metasurface,tunable plasmonic antenna based on double nanorods and the spatial differentiator based on high-contrast grating have been proposed,which are as follows,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 finite-difference time-domain(FDTD)solutions are employed to simulate the characteristics of the polarization-independent metal-graphene PIT device.The PIT peaks,caused by the coupling between the bright mode and dark mode,are separately and dynamically modulated by varying the Fermi energy of corresponding graphene finger set without changing the geometrical parameter of the metallic nanostructure.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 polarizationdependent loss.A tunable plasmonic antenna based on metal-graphene nanostructures is proposed numerically at mid-infrared frequencies,which is composed of two identical gold nanorods placed on separated graphene layers.The two coherent point-dipole sources model is utilized to explain the unidirectional side scattering obtained by the plasmonic antenna,whose emissions interfere constructively in one direction and destructively in the opposite direction.The finite-difference time-domain(FDTD)solutions are employed to investigate the resonance modes of the nanorod and calculate the three-dimensional far-field scattering intensity distributions of the plasmonic antenna.The localized surface plasmon resonance induced by each nanorod is tuned by the Fermi energy of the corresponding graphene layer.The symmetry of the plasmonic antenna is broken by applied different voltages on separated graphene layers,resulting in unidirectional side scattering.The electrical control provide a way to modulate the directivity of the plasmonic antenna with significant flexibility,which provide an effective way to manipulate the surface plasmon polaritons in photonic applications,such as integrated optical circuits and optical communications.An optical spatial differentiator based on subwavelength high-contrast gratings(HCG)is proposed and experimentally demonstrated.The spatial differentiation property of the subwavelength HCG is analyzed by calculating its spatial spectral transfer function based on the periodic waveguide theory.By employing the FDTD solutions,the performance of the subwavelength HCG spatial differentiators with different dimensions were investigated numerically.The subwavelength HCG differentiator with thickness at nanoscale was fabricated on the quartz substrate by the electron beam lithography and the Bosch deep silicon etching.Observed under an optical microscope with a CCD camera,the spatial differentiation of the incident field profile was obtained by the subwavelength HCG differentiator in transmission without Fourier lens.By projecting the different images on the subwavelength HCG differentiator,edge detections of these images were obtained in transmission.With the nanoscale HCG structure and simple optical implementation,the proposed optical spatial differentiator provide the prospects for application in optical computing systems and parallel data processing.