| dc.description.abstract | This thesis investigates the optics of micro and nano scaled structures, called microoptics and nanooptics. Microoptics and nanooptics continue to advance and diversify due to rising demands for miniaturization, cost reduction, functional integration, and increased performance in optical and photonic systems. Micro- and nano-optics are playing increasing roles in a wide range of applications, including sensors, communications, biomedical, data storage, and other consumer-driven and technology-driven areas. This thesis focuses on the microoptical and nanooptical devices which can be used in optical pickup head system. The main content of this thesis can be separated into two parts: A part of this dissertation presents the realization of a miniaturizing optical head using microoptical and micromachining techniques. Another part of this dissertation presents the near-field properties of a nano aperture which has high potential for optical storage applications.
By using the microoptical technology, we have developed a novel stacked silicon-based microoptical system, which is optical-on-axis and transmissible in both visible and infrared ranges. By using this new microoptical system technique, we fabricated a miniaturized optical pickup head module. This optical pickup head consisted of a 650-nm laser diode, a 45o silicon reflector, a grating, a holographic optical element, and some aspherical Fresnel lenses. These optical phase elements fabricated on a SiNx membrane were suspended on Si chips. Each element was then stacked by chip bonding. The integrated optical head had an area of 10x10x5 mm3. The total weight was about 1.25g. The optical performance was successfully characterized. The tracking servo signal pattern on optical disc was measured and a focal spot with an FWHM diameter of 3.1?m was obtained, while the diffraction-limited spot size was 0.7?m. The optical phase elements are made on free-standing SiNx membranes which provide versatile optical functions, such as focusing, splitting, and so on. Since the fabrication process is based on silicon micromachining technology, the optical element is easily integrated with other active and passive devices on a silicon substrate.
As mentioned, the other part of this dissertation presents the near-field properties of a nano aperture. The spatial resolution of a small aperture is not limited by diffraction but generally determined by the aperture size, beyond the diffraction limit can be achieved using a nano aperture. Unfortunately, a conventional small aperture has a devastating problem of extremely low transmission. For a small circular aperture, calculations and experiments show that power throughput decays as the fourth power of the aperture size. This low transmission problem greatly hinders the application of a nano aperture for solving significant problems.
It has been shown that a large transmission enhancement can be obtained when a nano-scaled slit is surrounded by periodic trenches on the entrance plane of a metallic film. However, until now, the transmission through the nanostructure-surrounded nano-scaled slit is still too low for practical applications. In this thesis, two new methods for further transmission enhancement are proposed: 1. Metallic bumps are used for recycling the surface waves; 2. Tapered substrate is used for propagating constant matching and efficiently exciting surface waves.
In this thesis, a very large transmission, 20%, of light through a nano metallic slit bordered by both nano trenches and bumps has been demonstrated theoretically. To the best of our knowledge, this is the first time that a bump structure is proposed for transmission enhancement. The trenches bordering the nano slit are used to excite free-space light into surface waves, while the bumps bordering the trenches are used to confine surface wave leakage. Over 50% of the escaping surface waves can be reclaimed by using a pair of bumps with a reflectivity larger than 99%. As a result, the transmission of a trench-surrounded slit bordered by a pair of bumps can be enhanced 1.5 fold. Furthermore, a nano slit on a tapered metallic substrate was also investigated. By using a 45o tapered structure rather than a traditional metallic plate, a 6-fold transmission enhancement could be achieved, due to the asymmetrical excitation of surface waves and the matching of propagation constants between the surface waves and slit waveguide. | en_US |