Photolithographic fabrication of periodic nanostructures for photonic applications

Abstract

Artificial micro- and nanostructures have already found numerous applications in various sectors of optics and photonics. Periodic patterns are used as diffraction gratings, photonic crystals, ultrathin polarizers and wave retarders, antireflection coatings, optical filters, plasmonic waveguides, optical antennas and sensors, as well as substrates for surface enhanced fluorescence and Raman spectroscopy (SERS). Surface micro- and nanostructures have also been demonstrated to exhibit superhydro- and superoleophobicity and, for example, the so-called structural colors that do not use any dyes. These properties can lead to fascinating applications, e.g., in self-cleaning eyeglasses and touchscreens, as well as in various types of displays. This thesis describes the development of a set of nanofabrication techniques for manufacturing nanoscale optical components. One of the key ideas was to switch from conventional photolithography based on photoresist to a new type of maskless lithography making use of azobenzene-containing polymers (azo-polymers). This transition fundamentally changes the fabrication process, for example, eliminating wet processing steps, such as photoresist development and stripping. The azo-polymer-based interference lithography developed in this thesis is a fast and simple technique to pattern large-area arrays of perfectly ordered nanofeatures. In addition, the azo-polymers are insensitive to humidity and temperature fluctuations, as well as to stray light. These properties make them an attractive alternative to traditional photoresists. The invented nanofabrication technique was shown to be capable of patterning various materials, such as semiconductors, glass and metals. Using the technique we have fabricated various optical elements, such as plasmonic filters, ultrathin polarizers, all-metal reflective waveplates and substrates for surface enhanced Raman scattering with various degree of complexity. Maskless lithography allows for fast adjustment of the pattern parameters and nearly instant prototyping. The scaling up capability of the technique, meanwhile, opens up the door to industrial applications.