Tunable Optical Modulators Based On Graphene/Nanostructure Metasurface

As a type of artificial electromagnetic metamaterial,metasurface has received extensive attention in the field of modern optics.Because of its excellent electromagnetic regulation ability,metasurface based devices have been applied in many aspects,such as perfect absorption,optical imaging,optical polarizing,and so on.However,the physical function of conventional metasurface devices cannot be changed once the structural parameters of the device are fixed,which severely limits its applications in many aspects.As a two-dimensional material,graphene combined with metasurface structure has shown the possibility of solving this problem,and great progress has been made.The Fermi level of Graphene can be tuned by chemical doping and the bias voltage changing,and its electrical conductivity and optical properties can be tuned accordingly in real time.In this thesis,tunable optical modulators based on graphene/nanostructure metasurface,including broadband transmissive optical modulator in the infrared band and broadband optical switches in the communication band,have been investigated.The main contents are as follows:1.A transmissive infrared radiation modulator is proposed by combining silicon based double-layer metal gratings with graphene.The localized surface plasmons of graphene are coupled into silicon nanogratings.Due to the excellent polarization characteristics of double-layer metal gratings and the enhanced confinement of electric field of localized surface plasmons of graphene,a transmission modulation with incident polarization independent can be implemented.The modulator,which has a high modulation depth and a wide modulation range,can strongly reflect TE linearly polarized light,and modulate the transmission of TM linearly polarized light within 7-22 μm band with a modulation depth of greater than 94.57%(12.65 dB),and up to 99.96%(33.77 dB).2.A broadband reflective optical switch,which is independent of incident polarization,is proposed by combining graphene with nano-aluminum cylindrical array.The magnetic plasmon resonance is excited by the metal aluminum cylindrical array,which enhances the absorption ability of graphene and makes the reflected light reduced significantly.Graphene’s dielectric constant changes with the Fermi energy level of graphene,which results in the change of frequency and amplitude of the magnetic resonance of the aluminum cylindrical array.As a result,the function of the optical switch can be realized.Furthermore,the proposed structure can be achieved by combining large area UV exposure with metal coating without the need for ion etching of the metal,which greatly reduces the difficulty of experimental fabrication.Numerical results show that,in the wavelength range of 1400-1700 nm,the reflectivity is close to 0 when the Fermi energy level is near 0 eV or 0.2 eV,which shows strong absorption and can be denoted as "OFF state";the reflectivity is higher than 65%when the Fermi energy level is near 0.6 eV,which shows strong reflection and can be denoted as "ON state".The modulation depth can be higher than 91.36%(10.63 dB)in the whole wavelength range of 1400-1700 nm with a maximum of 99.77%(26.35 dB).3.Experimental fabrication and verification of the broadband optical switch in communication band were conducted.After transferring graphene to a glass substrate,an.array of nano-circular hole was fabricated by using dual-beam and double exposure.A uniform aluminum film was then sputtered and the array of circular nano-holes was completely wrapped by a physical vapor deposition system(PVD).An optical measurement setup is built for the test of the sample performance,and the measurement results proved the relationship between the reflectance and the voltage imposed onto the grapheme at different wavelengths.The experimental maximum modulation depth is,respectively,78.96%(6.77 dB),79.50%(6.88 dB),78.18%(6.61 dB)and 78.90%(6.76 dB)at λ=1510 nm,λ=1550 nm,λ=1575 nm and λ=1623 nm,which validates experimentally the optical switch function in the range of 1510-1630 nm.