Study Of Structural Coloration And Molecular Sensor Devices Based On Resonant Antennas

For a long time,the manipulation of electromagnetic waves at optical frequencies mainly relies on lenses,prisms,optical fibers,and various diffractive elements,etc.These conventional optical elements are bulky and often used as separate elements,which present a limitation in terms of system integration.Antennas with sub-wavelength size can realize energy coupling and conversion of electromagnetic waves from boundless free-space to a localized subwavelength space.They also enable effective control over optical field in terms of amplitude,phase and polarization,etc.Currently,antenna theory and technology have relative mature in microwave region,and have been widely used in the control and application of microwave.The development of antennas in optical region opens a space for exploring interaction between light with artificial materials and for developing new optical devices.Using Fabry-Perot dielectric antenna and metal microstrip antenna structures,novel structural coloration devices and molecular sensors are designed and fabricated in this thesis,which enriches device functionality of optical antennas and shed lights on related physical mechanisms.The main research results of this thesis include:1.Based on selective mode absorption mechanism in Fabry-Perot dielectric antenna,a high-purity structural coloration device is demonstrated without lithographic processes.Structural coloration method avoids critical dependence on materials themselves,which is an advantage over traditional chemical coloration method.Structural coloration is thus promising for developing pollution-free coloration technique with the advantages of resistance to high temperature and mechanic cratch.In this thesis,we propose a wavelength selective absorption and spectral shaping method by placing an ultrathin absorbing film atthe antinode of Fabry-Perot resonance mode.Taking structural red color as an example,a four-layer structure consisted of Al/SU-8/Ge/SU-8 is designed,in which the absorbing material Ge is at the antinode of the 454 nm resonance mode.The experimental results show that the Fabry-Perot dielectric antenna has a broadband near perfect absorption between 400 nm and 600nm wavelength range.It is bright red with a purity of up to 76%,which is 16%higher than the literature film structures.This measured color purityis comparable to those of nanostructured plasmonic structures bassed on localized surface plasmon resonances.2.Based on field enhancement effect in Fabry-Perot dielectric antenna,a molecular fingerprint sensor made of thin films without patterning fabrication is developed.Most organic molecules have their characteristic absorptions in infrared region as originated from their vibration and rotation modes of chemical bonds.Surface enhanced infrared spectroscopy?SEIRA?is currently the most common method for detecting optical molecular fingerprints.This method is based on the local plasmon resonances in nano metal particles or metal antennas,whose fabrication requires complex lithographic and etching processes.In this thesis,a molecular fingerprint sensor made of thin film structure is designed and fabricated.The structure is composed of a single-layer ZnSe film on a metal substrate.The thickness of the ZnSe film is designed to be one-fourth of the molecular characteristic absorption wavelengthThe resonance mode characteristics and surface electric field properties of the film dielectric cavity for different polarizations and different angles are revealed,which suggests that the s-polarization resonance mode has a surface electric field enhancement factor of 2 at large angles,and a high quality factor,well suitable for molecular fingerprint sensing.Using self-assembled octadecanethiol?ODT?monolayer molecule as an example target,its CH₂ and CH₃ stretching vibration modes are measured.At s polarization and 75°angle,the measured response signal of feature absorptionis as high as 8.54%,comparable to the values reported in metal nanostructures.This high response signal originates from the energy coupling and conversion mechanism between the optical resonance mode of the dielectric antenna and the molecular chemical bond vibration.3.Based on both local and propagating surface plasmon modes in mid-infrared spectral region,a refractive index sensor made of metagrating is demonstrated with monolayer sensitivity.As originated from the wavelength-dependent properties,surface plasmons are generally considered to have strong field confinement in visible and near-infrared.As wavelength increases up to the mid-infrared region,the field confinement becomes weakened,and thus mid-infrared light is is rarely used for the molecular sensing outside molecular fingerprint region.The thesis examines the possibility of molecular refractive index sensing using mid-infrared plasmons.Both theory and experiments have proved that the mid-infrared surface plasmon has the ability of sensing monolayer molecules.The metal microstrip antenna is designed to be a three-layer structure made of gold/ZnSe/Cu microstrip array.Its localized surface plasmon mode and propagating surface plasmon mode are excited in the wavelength range of 3?m to 9?m.In response to a monolayer of octadecanethiol?ODT?molecules,which are attached to the surface of antenna via self-assembling process,both local and propagating surface plasmon modes exhibit clear redshifts in wavelength.The local surface plasmon modes have larger shifts than the propagating surface plasmon modes due to their stronger field confinement.This demonstrated metal microstrip antenna sensor operates in the spectral range outside the infrared molecular fingerprint region,and thus does not require frequency matching between the molecular fingerprint and the plasmonic mode.Therefore,it is insensitive to small changes in the resonant frequency of the plasmonic mode and more tolerant to sample feature size variations.