Theoretical Study On Boundary States Of Plasmonic Crystals

Surface plasmon polaritons are collective oscillations of free electrons and optical wave that can propagate along interface between metal and dielectric.Because of their unique advantages of bandwidth,speed and small size,surface plasmons have great potential in high-speed photoelectric interconnection and the next generation integrated miniaturized optical circuit.Among these building blocks based on surface plasmon polaritons,plasmonic boundary states can trap and enhance the propagating plasmon polaritons,which has potential applications in fluorescence enhancement,biosensors and optical nonlinearity.In order to improve the anti-interference ability and tuning ability,we have proposed plasmonic boundary states in periodic metal-dielectric-metal structure and graphene plasmonic waveguides with alternate chemical potentials.Considering the similarity of physical mechanism,we classify the waveguide system with strong Bragg reflection into generalized plasmonic crystal,to design and optimize the properties of boundary state by exploring the energy band structure of crystal.Our main research results are as follows:(1)We have constructed two periodic waveguides with different structural parameters of core layers.Similar to the method in photonic crystal,and analyzed the Zak phase of different energy bands in detail,and found that two periodic waveguide structures have different topological properties at the third energy band,as a result,there is a kind of topologically protected boundary state between two connected semi-infinite plasmon crystals in this wavelength range.For the symmetric system with fixed Zak phase,we find that the symmetric plasmonic unit cell can be regarded as homogenous layer with effective phase delay and equivalent characterized admittance.Combining the concept with the transmission matrix method,one can optimize the geometric parameters of plasmonic structure quickly.In the process of exciting the boundary state,more than 99% of the incident plasmon energy are compressed into the boundary state,and we found that the Full Width at Half Maxima(FWHM)of the absorption peak is only 24.1 nm.Moreover,we find the boundary state of the plasmonic crystal are against the structural disturbances.Our research is of great significance to design anti-interference narrow-band absorbers based on thepropagating plasmons.(2)We have constructed plasmonic boundary states in the periodically modulated Fermi level graphene waveguide system.Unlike the case in the metallic plasmonic boundary state,the graphene plasmonic crystal boundary must provide an additional boundary matching layer to compensate the abnormal reflection phase at the abrupt graphene edge.In order to obtain the width of the matching layer,we have extended the Wiener-Hopf method to optimize the structure of the system.We found that more than 97% of the incident energy can be localized into the boundary state by only two graphene plasmonic unit cells.Our proposed device is only one hundred nanometers in length,which is about two orders of magnitude smaller than wavelength of the free light.However,in our system,the propagating graphene plasmons are significantly enhanced,and their electromagnetic field are raised one order of magnitude.In further,we found that this field enhancement effect plays an important role in the sensitivity of biochemical sensing.Our research is of great significance for designing graphene plasmons based devices.