Three-dimensional nanofabrication of photonic crystals and polarization splitters and rotators

Abstract

One of the most critical challenges in nanoscale science and engineering is to make functional 3D nanodevices with high-accuracy. While considerable progress has been made in the "bottom-up" approach, the lithographic "top-down" approach remains the only way to encode human engineering effort, and to meet the optimal theoretical designs. Probably the most prominent example of lithographic fabrication is semiconductor manufacturing. However such manufacturing, aside from being extremely expensive, is highly inflexible, virtually excluding any work other than silicon microelectronic devices. Meanwhile, the miniaturization and integration of optical devices can potentially revolutionize the field of optics, with an impact that may prove comparable to the transition of electronics from vacuum tubes to transistors. To achieve high-level functionalities and to meet the stringent tolerance in optical information processing, multilayered structures with both minimum feature sizes and layer-to-layer overlay accuracy down to a few of nanometers are required, thus posing significant challenges in fabrication. Some requirements, such as nanometer-level spatial coherence, are beyond the capability of current semiconductor manufacturing.

(cont.) As part of an effort to develop a low-cost, highly-flexible "nano-machine-shop", this thesis addresses the fabrication of 3D photonic nanostructures. Low-cost planarization and high overlay-accuracy electron-beam lithography were developed to achieve seven functional layers in a new type of 3D Photonic Crystal (PhC). For the first time, designed point defects were introduced in such a 3D crystal, and optical measurement showed excellent agreement with numerical simulations. For a subset of 3D structures, a new fabrication strategy was proposed and implemented, leading to the first demonstration of an on-chip, high-efficiency, and wide-bandwidth polarization splitter/rotator. A new type of lithography, Coherent Diffraction Lithography (CDL) was proposed and demonstrated to be effective in fabricating 3D PhCs with high throughput, high resolution, large area coverage and potentially high overlay accuracy. When combined with tilted x- ray lithography, such fabrication can be extended to general 3D periodic nanostructures. Finally, a few future applications of 3D PhCs, such as those in thermophotovoltaics, miniaturization of femtosecond semiconductor lasers and single-photon sources are proposed.