Enhanced Circular Dichroism Using Superchiral Hotspot In Micro/Nano Structures

Due to the potential damage in the human body,chiral molecules are widly studied in the field of structural biology,chemistry and toxicology.The non-invasive and high-efficiency detection characteristics of circular dichroism spectroscopy make it beheaded in a wide variety of chiral identification and detection methods.This optical method depends on the circular dichroism of chiral molecules’ response.However,CD signal of chiral molecules is usually very weak due to some factors such as the mismatch between the optical wavelength and the scale of chiral molecules,which proposes a great challenge to the measurement process.In recent years,the maturity of nanoscale processing technology provides a new research perspective for improving CD signal.Various novel micro/nanostructure designs emerge in an endless stream.At present,signal enhancing strategies based on metal structures,dielectric structures and metal/dielectric hybrid structures have been developed.These remarkable progresses are of great significance in single-molecule detection and high-sensitivity chiral sensing.The interaction between optical field and micro/nanostructure will transform the electromagnetic field distribution,and is expected to realize superchiral field in the region of interest.Furthermore,the generated superchiral field is verified to be beneficial for CD signal enhancement and has become the mainstream method of chiral sensing.In this paper,a silver bull’s eye structure and achiral dielectric metasurface consisting of an array of silicon nanocylinder dimers are designed respectively.The physical mechanism and the broad application prospects of superchiral hotspot in improving CD signal are explored.The application prospect,the specific research content is as follows:1.The electromagnetic cross-polarization properties of chiral molecules were introduced,and the propagation properties of optical fields in multilayer media were deduced and discussed.Based on commercial finite-element electromagnetic solver,the relevant details of simulation model establishment were illustrated,and the reflection and transmission characteristics of chiral materials were simulated.2.The enhanced mechanism of CD signal based on the metallic bull’s eye structure was theoretically studied.Firstly,the calculation and analysis methods of CD signal were introduced,which provided a powerful tool for studying the microscopic origin of the CD signal.Secondly,a bull’s eye structure etched on the silver film was designed.A superchiral hotspot with controllable chirality was generated above the annular groove,and the optical chirality density was enhanced by 20 times.Finally,chiral molecules were introduced into the superchiral localized field,and the simulation results showed that the existance of hotspot was able to enhance the CD signal of chiral molecules by 17 times,in which the inherent CD signal occupied the main contribution.3.The enhanced mechanism of CD signal based on achiral dielectric metasurface was theoretically investigated.Firstly,the scattering multipole expansion theory was introduced,which provided a powerful method for the calculation of electromagnetic multipoles in the scattering field of micro/nanostructures.Secondly,the silicon nanocylinder dimer was designed.Due to the excitation of magnetic dipole mode,a superchiral hotspot with an optical chirality density enhancement of 8 times could be generated in the dimer gap under the excitation of offaxis linearly polarized light.Finally,a metasurface structure of periodic dimer was designed on the basis of the above dimer structure,the optical chirality density enhancement could be further improved to 180 times.When the chiral molecules were attached in the hotspot region,the CD signal was enhanced by a factor of 120.Unlike metal structures,the introduction of silicon material is capable of minimizing the influence of background noise.