Study On Surface Plasmon Chirality Amplification Effect And Optical Devices Based On Metal-Chiral Molecule Hybrid Structures

Chirality is a ubiquitous phenomenon in nature which refers to the property that the mirror image of a structure cannot overlap with itself.The mirror images for a molecule or an object that cannot overlap are called enantiomers.When it comes to chiral drugs,one enantiomer may be used as the drug while the other enantiomer may cause serious side effects or be toxic.Detecting signals and molecules possessing chirality has gained a significant position in the research fields on biopharmaceuticals,food safety,and agricultural chemistry.One of the most commonly used methods for measuring the structure of chiral molecules is by observing the circular dichroism(CD)spectrum due to its high sensitivity to chirality.CD response to natural chiral materials is weak,and the response band is located in the near-ultraviolet range,leading to difficulties in detecting it.To solve this problem,utilizing the characteristics of surface plasmon resonance can provide a new way to enhance chiral signals.Within the near field range,surface plasmon resonance can magnify the incident electromagnetic field up to a hundred times in certain structures.Combining chiral materials with surface plasmon resonance structures can amplify the chirality and CD signals of the materials.Therefore,research based on surface plasmon resonance chiral structures is attracting a lot of attention.In this thesis,we introduce three parts to show the methods of CD singal enhancement:1.Chirality enhancement in metal layer-chiral material-metal layer structure.We design three structure to amplify the chirality of materials.The three structures are silver disk-chiral material-silver disk structure,silver disk array-chiral material-silver substrate structure,and silver hole array-chiral material-silver substrate structure.These three structures turn the incident light into surface plasmon polaritons(SPPs)and amplify the electromagnetic field as hundreds of times as the one of incident in the near field.The amplified fields interact with the chiral materials,which enhances the maximum value of CD spectrum in two orders.Comparing these three structures,we find that CD spectrum can be enhanced in both the strong coupling regime and the weak coupling regime in Ag disk-chiral material-Ag disk structure.The nonradiative SPPs in this structure,which provides the condition of the strong coupling.The strong coupling effect can be easily observed in the absorption spectrum.In the other structures,the enhancement of CD spectra happens in the weak coupling regime.The size of Ag is larger than the one in Ag disk-chiral material-Ag disk structure,and the dissipations in the two structures are larger than the other one.There are only weak coupling effects in the two structures.2.CD enhancement in metal hole array structures and metallic gratings.We fill the holes or the slits with chiral material in silver hole array structures or silver gratings.The hole array structure and the grating are often used for SPPs excitation,and can amplify the incident electromagnetic field in the near field regime which coupling with the chiral materials.CD spectra are enhanced in these structures.Comparing the results with the last section,although the confinement of SPPs in the structures of this section is much weaker than the one of the last section,we find that the propagating SPPs can be also used to enhance CD spectra.A low q factor Fabry-Perot(FP)cavity is formed in the hole in the hole array structure and this FP cavity also amplifies the interaction between SPPs and chiral materials.The enhancement factor of CD spectrum reaches to 75 times.Furthermore,we find that the enhancement factor of CD spectrum in silver grating can reach to 280 time.This mean that the propagating SPPs well amplify CD spectrum.3.Chiral detector based on the Kretschmann structure and hydrogen detector containing perovskite materials.Kretschmann structure,which is renowned for producing surface plasmons,is highly sensitive to even minor variations in the optical response of the medium.This part is separated into two sections.The first section focuses on designing two chiral molecular detectors.In the first type of detector,a known left-handed chiral molecule replaces the prism of the Kretschmann structure,which is then modified with a test chiral molecule on the dielectric.By altering the concentration of the two ends of the molecule,the reflection spectrum is calculated,and the characteristics of the reflection spectrum,including the position and amount of resonance peaks,determines the chirality of the test molecule.The second type of chiral molecular detector determines the Goos-Hanchen(GH)shift of multiple chiral molecules with variations in their resonance positions.This shift difference caused by the different resonance positions of the chiral molecules can discriminate between leftand right-handed TDBC molecules.In the second section,we constructed a hydrogen detector containing perovskite materials that leverage the characteristics of distinguishing chiral molecules.Palladium metal has a considerable hydrogen absorption capacity,causing the refractive index of palladium to differ significantly before and after hydrogen absorption.By modifying the structural parameters of the detector,we identified the maximum difference between the reflection spectra of palladium before and after hydrogen absorption,serving as the groundwork for future experiments.