Electromagnetic Wave Propagation In Graphene-loaded Micro-and Nano-Structures

In the past thirty years,nanophotonics has made huge advances,especially in the development of photonic crystals,plasmonics and metamaterials.Up to date,researchers have found that photonic crystals,metamaterials and subwavelength micro-and nano-structures can efficiently control the excitation and propagation of electromagnetic waves,thus photonic materials and devices with novel functions have been developed which have applications in integrated optics,communication,sensing and imaging.Recently,researches have added active materials in photonic systems to achieve active photonic materials and devices.Since the discovery of graphene in 2004,it is found that this material has linear band structure and much higher mobility than traditional semiconductors which can be used in high-speed electronic devices.Recent studies have proven that graphene is also a promising material in optics providing the platform for active photonics and plasmonics which leads to the advent of graphene photonics and plasmonics.Thus,graphene has provided new materials and notions in the development of novel electromagnetic materials and devices.In this dissertation,we have theoretically and experimentally investigated electromagnetic wave propagation in graphene-loaded micro-and nano-structures.First,we have discovered and theoretically demonstrated that a graphene-loaded metal grating can achieve asymmetric transmission of terahertz waves.We firstly designed the graphene-loaded metal grating under external magnetic fields and secondly calculated the reflection and transmission of transvers-electric(TE)and transvers-magnetic(TM)polarized terahertz waves using scattering matrix.It is found that due to the Hall conductivity of graphene terahertz waves are converted between TE and TM polarization when passing through the system leading to asymmetric transmission.It is also found that such asymmetric transmission can be drastically tuned by external magnetic fields and Fermi level of graphene.Our research may provide a unique approach in the development of active terahertz devicesSecond,we have discovered and theoretically demonstrated that non-reciprocal transmission of terahertz waves can be achieved in graphene-loaded photonics crystals.Due to strong magneto-optic effect of graphene in terahertz frequency region,we have found non-reciprocal photonic band structures in a width frequency range by combining graphene with terahertz photonic crystals.Circularly-polarized terahertz waves show different band structures and transmissions depending on the propagation direction.Adjusting the intensity of magnetic field and Fermi level of graphene,the non-reciprocal transmission of terahertz waves can be drastically tuned.Further,with enough layers of graphene at the interfaces between dielectrics in the photonic crystal,we can achieve unidirectional propagation for circularly-polarized terahertz waves.Our investigations provide a new approach for the development of terahertz devices such as isolators.Third,we have theoretically designed and experimentally demonstrated controlling the dispersion relation of plasmonic waveguide and the propagation of plasmonic waves via graphene.By depositing graphene on a plasmonic waveguide milled with holes,the dispersion relation can be changed and the corresponding resonant dips are red-shifted.Further,we propose a model of graphene for finite difference time domain calculations in the visible range which can fully describe optical properties of graphene.By using this model,we theoretically demonstrated that the dispersion relations of plasmonic waveguides can be tuning by the Fermi level and the nonlinear effects of graphene.Forth,we have proposed and theoretically demonstrated that graphene tube can serve as a plasmonic waveguide in terahertz frequencies.By rolling graphene into a cylindrical tube,we get the so-called graphene tube.Theoretical calculations show that the nonlocal effects of plasmonic waves are pronounced only when the radius of the tube is near tens of nanometers.As for tubes with radius of several micrometers,such nonlocal effects can be neglected and we can treat it as a conductive cylindrical surface.The plasmonic modes have tightly confined electromagnetic waves and relatively long propagation length.By periodically altering the Fermi level along the propagation direction of plasmonic waves,band gaps appear in the dispersion relations within which no modes exist.Thus,we can control the propagation of plasmonic waves in the graphene tubeIn summary,we have theoretically and experimentally investigated electromagnetic wave propagation in graphene-loaded micro-and nano-structures.We have demonstrated asymmetric transmission of terahertz waves in graphene-loaded metal gratings and non-reciprocal transmission of terahertz waves in photonic crystals integrated with graphene which can be tuned by adjusting the intensity of external magnetic field and Fermi level of graphene providing new ways for achieving terahertz isolators.We have experimentally shown that the dispersion relations of plasmonic waveguides can be tuned by graphene which can be predicted by a model we have proposed in the visible range of electromagnetic spectrum.Also,we have proposed graphene tubes as terahertz plasmonic waveguides.These investigations may expand our understanding of the optical properties of graphene and the development of graphene plasmonics which may be useful in active graphene-based terahertz devices and novel opto-electrical materials and devices.