| Graphene,a two-dimensional(2D)form of carbon in which the atoms are arranged in a honeycomb lattice already been shown to possess unique mechanical,electric,magnetic and thermal properties with a multitude of exciting applications that are being vigorously pursued by academia and industry.Motivated by the interesting optical properties of graphene,many graphene-based photonic and optoelectronic applications have been developed recently,such as a graphene waveguide,broadband polarizer,graphene modulator,graphene photodetector and saturable absorber for mode-locked.However,for an undoped monolayer graphene film,the atomically thin carbon layer presents only?2,3%broadband absorption.In order to change the intrinsically weak interaction between light and monolayer graphene,in recent years,graphene plasmons are rapidly emerging as a viable tool for fast electrical manipulation of light.Unlike noble metal plasmons,which suffer from large Ohmic losses and nontunability once the geometry of the structure is fixed,graphene plasmonics have shown many appealing properties,such as extreme confinement,tunability via electrical gating or chemical doping,and low losses resulting from long lifetime with hundreds of optical cycles.In addition,Graphene plasmonics operate at middle and far-infrared ranges,which enables infrared photonic applications(e.g.,infrared photodetection,enhanced infrared absorption,optical communications).Based on these features,it makes graphene plasmonics an attractive alternative to traditional metal plasmonics.In this thesis,we will give detailed studies on the properties of graphene plasmons:their energy dispersion,localization and propagation,plasmon hybridization,lifetimes and damping pathways.By using Graphene plasmonics,we will firstly systematically study the spectral responses of stacked graphene ribbons and demonstrate a practical and efficient implementation of dual-band focusing reflectors based on this configuration.Secondly,a novel electrically tunable EIT-like metamaterial consisting of top-layer gold SRR array and bottom-layer graphene nano-patch array will be explored as well.Based on the study of the graphene-SRR near-field coupling,finally,we will numerically study the mid-infrared CD effect generated by the novel electrically tunable EIT-like metamaterial.The thesis is mainly composed of three sections that are arranged as following:1.We theoretically study the optical properties of graphene metasurfaces consisting of layered graphene ribbons and their potential applications as multifunctional optical devices in the long-wavelength infrared region.By engineering the plasmonic resonance in graphene ribbons with different widths,the phase of reflected light can be tuned over a range of nearly 2?,while the reflectivity is kept relatively high at the target frequencies.Owing to the weak light-graphene interaction in the off-resonance region,independent control of the reflected light can be achieved by stacking multiple layers of graphene ribbons.Since the interlayer coupling between the ribbons is negligible,we have developed a transmissionline-based uncoupled model as a physical interpretation of the stacked metasurface.The modeled results show excellent agreement with numerical simulations.As a proof-of-principle demonstration,we have designed and demonstrated graphene metasurfaces as flat,dual-band focusing reflectors operating at 25 and 16 THz.Our work provides a general design scheme for multiband,multifunctional metasurfaces with various potential applications,including beam steering,optical communication,and information processing.2.We designed and prepared a novel hybrid metamaterial consisting of periodic arrays of graphene nano-patches and gold split-ring resonators has been theoretically proposed to realize an active control of the electromagnetically induced transparency analogue in the mid-infrared regime.A narrow transparency window occurs over a wide absorption band due to the coupling of the high-quality factor mode provided by graphene dipolar resonance and the low-quality factor mode of split-ring resonator magnetic resonance,which is interpreted in terms of the phase change and surface charge distribution.In addition to the obvious dependence of the spectral feature on the geometric parameters of the elements and the surrounding environmental dielectric constant,our proposed metamaterial shows great tunability of the transparency window by tuning the Fermi energy of the graphene nano-patch through electric gating and its electronic mobility without changing the geometric parameters.Furthermore,our proposed metamaterial combines low losses with very large group index associated with the resonance response in the transparency window,showing it suitable for slow light applications and nanophotonic devices for light filter and biosensing.3.We present a new design of chiral metamaterial based on a bi-layer hybrid nanostructure composed of gold split-ring resonators(SRRs)and graphene gratings.Arising from the electromagnetically induced transparency effect of this system due to the destructive interference between the metallic SRR's magnetic resonance and graphene's electric resonance,a strong CD response is obtained.To clarify the underlying physics of the enhanced CD effect,a theoretical analysis based on the coupled-mode theory(CMT)is performed,which fits the numerical simulation very well.The active control of the CD effect is demonstrated through tuning the Fermi energy of graphene by changing the gate voltage,the geometrical parameters and the environmental dielectric constants.In the broad mid-infrared regime of our interest from 6 to 10 ?m,the CD value remarkably maintains greater than 10%.In addition to the strong tunability,the CD signal intensity of our nanostructure is drastically larger than that of the pure graphene-based chiroptical nanostructures reported in the literatures.Our design offers a new strategy to develop the reconfigurable chiral metadevices,which might be potential in application of bio-detection and information processing. |
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