Nanodevices Based On Graphene Plasmons And Hexagonal Boron Nitride(h-BN)

Graphene is a two-dimensional material in which the carbon atoms are arranged in a honeycomb lattice,which has,in the past decade,been proposed for use in a wide range of applications from flexible electronics to electrifying inks.By offering the possibilities of significantly improving the existing products and the design of materials and devices with novel functionalities.Compared with the plasmons on noble metal,graphene plasmons(GPs)possesses superior properties,such as tunable carriers concentration and stronger field confinement.The hexagonal boron nitride(h-BN)is a natural vdW crystal with hyperbolic dispersion.The dispersions of hyperbolic phonon-polaritons(HPhPs)modes in h-BN slab depend on the slab thickness and possess very similar properties to graphene plasmons,such as the strong confinement,but with smaller losses.Graphene-h-BN heterostructures have recently attracted great interest due to unusual superiorities,such as the supporting of high quality of graphene and the hybrid plasmons-phonons-polaritons(GPs-HPhP),combining the broadband and tunability of GPs and the low loss of HPhPs.The highlights of this work are summarized as follow:1.The infrared waveguide made of graphene-dielectric hyperbolic metamaterials are first proposed and systematically analyzed with numerical simulations based on both multilayer structures and effective medium approach.The hyperbolic dispersion of metamaterials allows the waveguide to support modes with effective indice as high as 90.When two of the same hyperbolic metamaterials are placed parallelly,they form a slot waveguide structure,which is capable of supporting great local field enhancement and strong power confinement in the slot region.In the slot the electric field amplitude can be enhanced by nearly 20 times in magnitude and over 70%of the modal power is confined.This exceeds the previous slot waveguides based on silicon and conventional hyperbolic metamaterials.2.The non-reciprocal propagating of graphene plasmons and coupling in magneto-optical substrate-dielectric-graphene double layer waveguide structure are first theoretically investigated and numerically verified.The non-reciprocal coupling effect enables us to design a series of non-reciprocal devices based on graphene plasmons,including "plasmons circulator" or "plasmons isolator","magnetically-controlled plasmons logic gates" and "non-reciprocal plasmons resonator".The graphene plasmons isolator possesses an insertion loss as low as 2dB and isolation ratio as high as 42 dB.The enabled direction of isolator can be controlled by the direction of external magnetic field.According to the Boolean Algebra,we design the magnetically-controlled plasmons logic gates,which exhibits OR(AND)logic for one direction of input plasmons,while NAND(NOR)logic for the reversed input direction.The OR(NAND)logic gates could achieve an insertion loss as low as 3 dB and an isolation ratio as large as 13.6 dB.In contrast,the AND(NOR)logic gates performs with an insertion loss of 2.5 dB and a minimum isolation ratio of 17.5 dB.The magnetically tunable plasmons resonator is based on a graphene-coated nanowire made of magneto-optical material.When coupled with a graphene sheet waveguide,the two components form a graphene plasmons filter with non-reciprocal transmittance,which also has potentials to be a plasmons isolator.The isolation ratio could be tuned by the amplitude of magnetic field and the enabled direction can be switched by reversing the magnetic field direction.Under proper structural parameters and magnetic field,an isolation ratio over 25 dB is obtained.3.A graphene-coated tapered nano wire probe is first proposed to obtain strong field enhancement in the infrared regions.Based on the analytical field distributions and characteristic equation of the fundamental waveguide mode,the nanofocusing effect of the probe is investigated via adiabatic approximation.Results have shown that the graphene-coated nanowire probe could achieve a field enhancement factor of 24,which is an order of magnitude larger field enhancement than the metal nanowire probes.Both analytical and numerical methods have revealed the existence of optimal probe structure parameters where the highest field enhancement factor is achieved.A pair of graphene-coated nanowires is also capable of guiding modes.The effective indices,normalized propagation lengths,surface charge distributions and field distributions of the six lowest orders,plasmons modes are presented.Studies have shown that the six lowest orders' modes can be divided into two groups due to the monopole-monopole hybridizations or the dipole-dipole interactions.The field enhancement in the slot region of the graphene-coated nanowire pairs can be as large as 107,which is six orders'magnitude larger than the silver nanowire pairs.The gradient force between the two nanowires can be as high as 20nN·?m-1·mW-1,which is more than 50 times larger than the silver nanowire pairs and the previous slot waveguides.4.Tunable circular polarization analyzers based on graphene-coated spiral shape dielectric gratings or triangle metal nanoantennas are proposed.The graphene-coated dielectric gratings or the nanoantennas perform as plasmons excitation sources by coupling the energy of incident light into graphene plasmons through the guide mode resonances or nanoantennas resonances.Spatially separated solid dot shape(or donut shape)field distribution can be achieved if the geometric phase effect of excitation resource arrays and incident circular polarization possess opposite(or same)chirality.Combined with the strong tunability of graphene plasmons,the operation wavelength of analyzer can be tuned by adjusting the graphene Fermi level without degrading the performance of device.Although the previous analyzer could achieve an extinction ratio over 550,to further enhance the performance of analyzer,the unidirectional excitation of graphene plasmons has been adopted in the design of analyzer.For the arrays of nanoantenna pairs with orthogonal orientations in a spiral shape,the unidirectional graphene plasmons towards the center enhance the solid dot shape field distribution while the ones towards outside further weaken the donut shape field distribution.By utilizing the unidirectional excitation of plasmons,the extinction ratio of analyzer can be improved to over 103.5.A hexagonal boron nitride(h-BN)slot waveguide structure is proposed,which could achieve strong field enhancement and light confinement in slot region and giant gradient force between h-BN slabs.In two-dimensional case,a field enhancement ratio near 60,a power confinement ratio of 80%and a gradient force over-8.5 nN/?m×mW have been achieved,which are much higher than the slot waveguide based on artificial hyperbolic metamaterials.In three-dimensional slot waveguide case,a gradient force of-1.2 nN/mW and a power confinement ratio of 50%have been obtained.The nanofocusing of hybrid GPs-HPhP modes in a tapered graphene-h-BN heterostructure are investigated numerically.Compared with the HPhP modes in h-BN,the hybrid GPs-HPhP in heterostructure exhibit much smaller losses.Moreover,the thickness dependent dispersion of hybrid GPs-HPhP enables the nanofocusing of hybrid GPs-HPhP modes as they propagating along the tapered heterostructure.Compared with tapered h-BN slab,the nanofocusing effect in tapered heterostructure is improved evidently with a maximum enhancement of the amplitude of normalized electric field is over 60.