Design Of Dynamic Reconfigurable Plasmon Optical Devices Based On Liquid Crystals

In recent years,due to its unique characteristics,plasmons have received wide attention from the scientific field.The surface plasmon resonance is a mixed excited state formed by the interaction between the freely oscillating charge of the metal surface and the incident electromagnetic field.Under this interaction action,the collective oscillation of free charges and electromagnetic waves produces many novel and unique properties.Recently,great progress has been made in the study of surface plasmons.In the cross-infiltration with other fields,surface plasmons has also appeared in many new research branches.The applications of plasmonics in super-resolution imaging,surface Raman enhancement,sensors and optical stealth have been reported.Many other potential applications are constantly being explored.An important issue about the design of plasmonic devices is that the plasmon micro/nano structure is a passive device with the fixed optical properties.For the same input,the output can't be changed,which greatly limits their potential applications.If you need to change the output,you need to make another similar device,which requires extra time and cost.These problems have spawned a new field of research—Reconfigurable plasmonics.This field enables reconfigurable functions after device fabrication by means of an active medium that responds to certain excitation.Many active controllable medias have been used to construct active plasmonics devices,including semi-conductors,graphene,molecular machines,elastomeric polymers,etc.Among them,due to the characteristics of birefringence,low threshold and multi-drive methods,liquid crystal(LC)stands out from these materials.The birefringence region of the LC spans from visible light to the microwave band.And the orientation of the LC can be easily controlled by many methods such as temperature,electric field,light wave,sound wave,etc.As an all-organic substance,LC can be processed and synthesized by chemical methods.LC is also compatible with almost all important photoelectric materials.These characteristics make LC a good candidate for the development of tunable plasmonics.Combining LC with surface plasmonics,we were able to design dynamically tunable plasmonics materials and devices with superior performance.The main contents and results of the dissertation are as follows:(I)A liquid-crystal-based tunable plasmon waveguide filter is proposed in theory,and its filtering characteristics are numerically studied.The filter consists of a Metal-Insulator-Metal(MIM)waveguide structure with nanocavities.Filling the nanocavity with birefringent liquid crystal,we can change the effective refractive index of the nanocavity by controlling the arrangement of LC molecules,hence,the filter can be tuned.The tunable filtering characteristics are further analyzed by time domain Coupled Mode Theory(CMT)and Finite-Difference Time-Domain(FDTD)method.Without considering the internal loss of the nanocavity and the waveguide,the simulation data shows that the resonant wavelength increases linearly with the increase of the refractive index of the nanocavity.And the coupling efficiency exceeds 65%.The simulation results and calculations are in great agreement.This tunable filter has many important potential applications in high density integrated circuits.(2)Two reconfigurable chiral metasurface absorbers are theoretically proposed,and their chiral optical properties are numerically studied.The chiral metasurface absorbers are all MIM structures,which can strongly absorb a circularly polarized light of a spin state and reflect the opposite spin state light to form a strong circular dichroism(CD).In the two chiral metasurface structures,one kind uses the intrinsic chiral mechanism,which utilizes birefringent liquid crystal as the insulating layer,and changes the spin state of the incident light by controlling the orientation of the LC molecules,thereby,changing the direction and size of the CD.The simulation results show that the sign of the CD will be reversed with the liquid crystal molecules orientation rearranged from 0°to 90°.The other kind is by changing the extrinsic chiral mechanism of the incident light to adjust the magnitude and sign of the CD.Our proposed chiral metasurface absorbers are simple and compact,which are conducive to integration.Reconfigurable chiral absorbers have broad application prospects in the fields of biodetection/sensing,polarization imaging and optical communication,etc.