Enhanced optical transmission through ridge nanoapertures for near-field applications.

摘要

It is of great importance to manipulate light in a small spatial scale in order to fulfill the continuous miniaturization of electronic, optical and optoelectronic devices. A subwavelength hole is often used to achieve the optical resolution beyond the diffraction limit. However, a small hole suffers the low light transmission due to the waveguide cutoff effect. In this thesis, a new type of nanoapertures in metal films, i.e., ridge nanoapertures in H and bowtie shapes, is proposed, and their unique optical properties of concentrating light into a nanometer-sized spot combined with enhanced optical transmission are studied.; Finite difference time domain numerical computations and waveguide cutoff analyses are conducted to understand the transmission mechanism through ridge nanoapertures. The TE10 waveguide propagation mode confined in the nanometer-sized gap between the ridges enables the unique optical transmission properties of ridge nanoapertures. Surface plasmon excitation of ridge nanoapertures in noble metals further enhances the transmission but destroys the collimated optical near-field from the H-shaped ridge nanoapertures. However, the resonant excitation of localized surface plasmon in a bowtie nanoaperture with sharp tips can be utilized to achieve super confined light spot with strongly enhanced local electrical field. Optimization guidelines for the design of ridge nanoapertures are also provided.; A near-field scanning optical microscope (NSOM) is developed from a commercial atomic force microscope and FIB-micromachined cantilever aperture probes are used to achieve high optical resolution as small as 60 nm. The optical near-field from ridge nanoapertures fabricated in various metal thin films was characterized using the home-built NSOM system. Nanoscale light spots with transmission enhancement of orders of magnitude higher than that of regular nanoapertures were achieved by these ridge nanoapertures. Far-field transmission measurements were conducted and the resonant transmission through bowtie nanoapertures for a tailored application was demonstrated.; The ability of high transmission ridge nanoapertures to confine light to a nanometer-sized spot potentially has myriad near-field applications: notably in ultrahigh density data storage, nanolithography and nanopatterning, nanoimaging, and biochemical sensing.