Molecular Sensing Properties Of Asymmetric Optical Antennas

Antenna is an electromagnetic device with sub-wavelength structure,which is well-suitable for energy coupling between free space and highly localized space.In microwave region,operating frequency of antenna is in the atmospherical transparency window,and microwave antenna is widely used in wireless communication.For the infrared and optical frequencies,metals exhibits plasmonic which creates many new effects and applications of optical antennas,among which molecular sensing has attracted much interest.The antenna-based molecular sensing can generally divided into two type,namely refractive index sensing and fingerprint absorption sensing.The refractive index sensing is based on frequency shift of antenna resonance caused by the presence of molecules,which thus doesn't offer information on the chemical bonds.In contrast,fingerprint absorption sensing is based on the enhanced vibrational or rotational absorption of molecules via antenna resonance,which is desirable for identifying molecular species.Traditional optical antennas used in molecular sensing mostly operate at a single resonance frequency,which is unsuitable for detect multiple molecular fingerprints at different frequencies or multiple molecules in parallel.Meanwhile,the antenna structures are also complicated which requires challenging nano-fabrications.In this thesis,we design and fabricate asymmetric rectangular patch antennas for molecular sensing.The demonstrated antenna is relatively simpler in structure and fabrication.Our main research results are summarized as below:(1)Based on the theory of microstrip transmission line and localized surface plasmon an asymmetric rectangular infrared patch antenna was designed and simulated.FDTD simulations show that the antenna exhibits two different resonances at 735 and991 cm^-1 for two orthogonal polarizations.These anisotropic resonant responses are suitable for sensing molecular fingerprint absorptions at multiple spectral bands.(2)The rectangular patch antennas were fabricated and their molecular sensing properties were characterized.Infrared spectroscopy measurements reveal two feature absorptions at 752cm^-1 and 843cm^-1 with signal intensities of 1.5%and 1.25%,respectively,which correspond to the in-plane vibration modes of C-C-O bond and CH₂bond in PMMA.We also analyzed a comparison sample without the antenna structure,in which the molecular fingerprint absorptions are nearly un-resolvable.This comparison verifies the resonance-enhanced molecular absorption within the antennas.(3)The effects of antenna sizes and analyte molecules on antenna resonance were also analyzed.Our theoretical and experimental results indicate that the increase of antenna length(or width)leads to a dramatic redshift for the resonance mode with polarization parallel to the length(or width),but only slight redshift for the resonance mode with orthogonal polarization.In addition,we also examined the influence of PMMA analyte molecules.The adding of PMMA layer cause obvious redshift of the antenna resonance modes.These redshifts of resonance frequencies are explained with effective refractive index theory.