Study Of The Fabrication Techniques And Chiroptical Responses Of Metal Micro/Nanostructures

Chiral plasmonic systems have been extensively studied recently for their extraordinary functionalities in the field of modern science and technology.Researchers designed artificial metal nanostructure,metamaterials,which can enhance the weak chiroptical responses of the naturally occurring chiral materials.Chiral nanowires have played an important role in the miniaturization of the modern optoelectronic devices and biosensors.These metamaterials exhibit some intriguing phenomena such as circular dichroism(CD),optical rotatory dispertion(OAD),and asymmetric transmission(AT).CD is the absorption or transmission difference of left-handed circularly polarized and righthanded circularly polarized light,which can be used to indicate the intensity of chirality.CD spectroscopy can be utilized in biomedical detection,secondary and tertiary structure of proteins and DNA,and solar energy harvesting etc.To fulfill the required necessary conditions for the aforementioned potential applications,fabrication of artificial plasmonic metamaterials with strong optical chirality is essential.Different approaches are made to achieve chiral nanostructures with high CD responses.In this dissertation,we proposed novel chiral plasmonic nanostructures with easy and concise fabrication,generating strong CD spectra in the visible and near-IR regions that could be useful in modern micro/nanophotonic devices and biosensors.The main contents of this dissertation are as follow:(1)Chiral plasmonics nanostructures(CPNs)of a silver nanorod coupled with a silver nanosurface,were proposed.Glancing angle deposition(GLAD)method was used to fabricate CPNs.The fabricated CPNs generated a strong circular dichroism(CD)signal under circularly polarized light illumination.Three obvious resonant modes are observed in the visible and near infrared regions.Due to the strong coupling of the nanorod and the nanosurface,a high peak was observed at approximately 600 nm.An increasing trend of the CD intensity and a redshift were confirmed when the area of the nanosurface was increased.With an active control of the vapor deposition angle(glancing angle of the substrate)in the GLAD method,the width of the nanosurface can be increased which intern causes an increase in the area of the nanosurface.The generated CD could be tuned easily by changing the area of the nanosurface.The models of the right-handed-CPN are numerically simulated in COMSOL Multiphysics software.The simulation and experimental results are greatly matched,and the trend of increasing CD with a redshift is also observed in the simulation results.This study not only proposes a concise and highly scalable technique for fabricating CPNs but also contributes to a better understanding of the physical mechanism of CD generation.(2)A bilayer Ag metastructure is proposed,in which a chiral L-shaped nanostructure at the bottom layer is coupled with an achiral nanorod acquiring different positions in the top layer with respect to the long and/or short arm of the chiral L-shaped nanostructure at the bottom layer.The metastructure generates a giant circular dichroism(CD)signal resulting from the strong coupling of the multipolar and dipolar resonant modes on the two layers,in the visible and near-infrared regions.With changing the position of the achiral nanorod,an unusual reversal of the CD spectra is observed,along with a fourfold increase in CD intensity in the short wavelength range due to the multipolar resonant modes.The position of the achiral nanorod is tailored by the azimuthal angle of the substrate during the fabrication of the metastructure using the oblique angle deposition method(OAD).The numerical simulations are in good agreement with the experimental results since all the resonant modes in the experiment are reproduced in the simulation results.This study provides insights into the variation of the coupling strength between a chiral L-shaped nanostructure and an achiral nanorod.The results can be useful in designing chiral-achiral composite nano-antennas for sensing devices.(3)Chiral metallic films are demonstrated that are normally deposited onto fan-shaped chiral templates fabricated on the top of pre-patterned polystyrene nanobeads monolayers,resulting in fan-shaped chiral nanostructures(FCNs).Oblique angle deposition(OAD)technique is utilized to experimentally fabricate the proposed FCNs.The fabricated FCNs generate strong CD signals under normal excitation of circularly polarized light.The generated CD signals exhibit three resonant modes in the visible and near-IR regions.The generated CD can achieve intensities as large as thirteen times of the initial values.The designed FCNs offer enormous tuning capability of the generated CD signals by changing the thickness of the metal film.The effect of changing the bead size of the monolayers is also investigated.An increasing trend was observed in the CD intensity with an obvious red shift of the resonant peaks.These results could be helpful in understanding and designing novel chiral film structures for sensing applications.(4)Two-string chiral nanowire is designed by different chiral L-shaped components arranged in an alternating manner.The nanowire arrays are theoretically investigated using the radio frequency module of COMSOL Multiphysics.Large CD spectra are generated by the nanowire,having three strong resonant modes in the visible and near-IR regions.With changing the structural parameters of different parts of the nanowire,an increase is observed in the intensity of the CD spectra along with the redshift of the peaks positions.The proposed structure is very simple to design,can be easily fabricated by the combination of electron beam lithography and physical vapor deposition techniques.The nanowire arrays offer a broad window for tuning the intensity and wavelength position of every single resonant mode of the generated CD spectra.Such novel chiral nanostructures could broaden applications in the fields of bio-sensing,medicine,and modern micro/nano-optoelectronics devices.