Designs And Applications Of Bowtie Aperture Optical Antenna

As an extension of microwave/radio frequency antenna, optical antennas attract considerable attentions for its ability to manipulate optical field at subwavelength scale, which can be applied to areas such as near-field lithography, near-field microscopy, nanomanipulation and nonlinear optics. Among kinds of optical antennas, bowtie aperture antenna has received special attention for extraordinary transmission and near-field enhancement and confinement. Accordingly, our work presents a novel backside milling method to fabricate bowtie apertures with sub-16 nm gap. Near-field localization is apparently improved. Then we applied this method to mask fabrication for near-field scanning lithography, and achieved 16 nm lithography resolution. After that, we demonstrated a novel bridge-connect ing bowtie aperture antenna to produce co-enhancing and -confining electric and magnetic field. Finally, we connect multiple bowtie aperture to generate Fano resonance and realize nanoscale color sorting. Our work has potential applications in optical antenna fabrication, high-resolution low-cost nanofabrication and optical metamaterials.Main research contents of this thesis are listed below:1. We studied the optical properties of bowtie aperture antennas and the underlying physical mechanics. Gap size on the exit plane is proposed as the critical parameter for near-field localization. Then we analyze the nonvertical sidewall in focused ion beam milling and its influence on gap size broadening. Based on this, we report the backside milling fabrication method to deal with the nonvertical sidewall, and fabricate bowtie apertures with sub-16 nm gap size. By finite-difference time-domain (FDTD) method and scattering scanning near-field optical microscopy (s-SNOM), we measured its near-field distribution numerically and experimentally, and compared with the bowtie aperture fabricated using front side milling. Near-field localization is significantly improved. After that, we combined the proximal method and the backside milling method to fabricate bowtie apertures with sub-5 nm gap.2. We applied the backside milling method to mask fabrication for near-field scanning lithography and fabricated bowtie apertures with sub-15 nm gap. Utilizing modulation transfer function (MTF), we calculated and compared the achievable resolution of bowtie apertures fabricated using the backside milling and the front side milling method. A near-field scanning lithography system is established with a passive flexure stage for contact control between the mask and the resist-coated substrate. We explored experimentally the lithography resolution of the backside milling aperture and the front side milling aperture, and also the influence of gap size on lithography resolution. Experimental results are then compared with the numerical results. By optimization of exposure parameters and scanning speed,16 nm resolution is achieved.3. We presented a novel plasmonic device, bridge-connecting bowtie aperture antenna for producing co-enhanced and -confined electric and magnetic field, which are explained based on the optical circuit theory and the virtual current loop theory. The dependence of the electric and magnetic field amplitude on the three dimensional size of the metal bridge and the film thickness is studied to further reveal the underlying physical mechanism. After optimization of its structural parameters, the maximum electric and magnetic field enhanced is respectively 585 and 170 above the bridge. The two values are 5200 and 1439 below the bridge. As to the field localization in x and y direction, they are 46 nm and 64 nm for the electric component, and are 32 nm and 34 nm for the magnetic component. The backside milling method is used to fabricate this kind of antenna with tunable structural parameters. Experimental scheme is proposed to measure the near-field electric and magnetic field distribution.4. We demonstrated a novel muhi-bowtie aperture antenna for nanoscale color sorting. Under vertical incidence, a superradiant broadband dipole and a subradiant narrow band multipole mode are excited and interfere with each other, generating Fano resonance. By finite-difference time domain calculation and scattering-type scanning near-field optical microscopy measurement, near-field amplitude and phase distribution switch significantly across the Fano resonance, resulting from modulated interference situation. Comprehensive analysis of the amplitude and phase information reveals the underlying mechanism of the Fano resonance. Based on the near-field pattern switching, the multi-bowtie aperture antenna can be used for nanoscale color sorting. Incident photons with different wavelength are filtered through different channels. The color sorting properties can be judged by the parameter filtering ratio.The innovations of this thesis are listed below:1. The backside milling method is proposed to fabrication bowtie aperture antenna with sub-16 nm gap. Scattering-type scanning near-field optical microscopy is utilized to characterize the near-field confinement. Applying the backside milling method to mask fabrication for near-field scanning lithography and combining the flexure stage technique, we can achieve sub-16 nm resolution near-field lithography.2. A novel bridge-connecting bowtie aperture antenna is presented to produce co-enhancing and -confining electric and magnetic field, which is explained by the optical circuit theory and the virtual current loop theory. The influence of bridge dimensions and film thickness on field intensity is discussed and the structural parameters are optimized.3. A novel multibowtie aperture antenna is demonstrated with Fano resonance for nanoscale color sorting. Scattering-type scanning near-field optical microscopy with multiple wavelengths are applied to plot its near-field distribution both in amplitude and phase. Its underlying physical mechanism is revealed.