PROCEEDINGS ARTICLE | March 23, 2006
Proc. SPIE. 6151, Emerging Lithographic Technologies X
KEYWORDS: Lithography, Refractive index, Nanoparticles, Polymers, Crystals, Composites, Dielectrics, Photonic crystals, Refraction, Photorefractive polymers
Photonic crystals are structures which exhibit a band gap in the electromagnetic spectrum as a result of dielectric periodicity. These structures present the potential to control electromagnetic waves in a similar manner to the way electrons are controlled by semiconductors. To obtain a photonic band gap in a specific region of the spectrum, there are two important characteristics of the photonic crystal that must be considered. The first is the length scale of the periodicity of the crystal, which governs the frequency range in which the band gap falls. The second is the dielectric contrast between the two media which comprise the crystal, which controls the size of the bang gap. Therefore, to construct a photonic crystal which could be used as an optical device, such as a waveguide or filter, the features should be on the order of optical wavelengths, or nanometers. The dielectric contrast through the visible region should also be large enough to open a band gap. Lithography techniques are ideally suited to pattern such structures. This work focused on the use of step and flash imprint lithography as an ideal patterning technology for two dimensional photonic crystals because of its capability for sub-50 nm patterning. Another attractive aspect of using step and flash imprint lithography is the potential to pattern a functional polymer as the crystal. The feasibility of printing structures needed for photonic crystals using imprint lithography was first demonstrated. Then, a strategy to raise the index of refraction of imprint compatible polymer formulations for large dielectric contrast using metal oxide nanoparticles was investigated. A maximum index of n = 1.65 was achieved, but at the high nanoparticle concentrations needed to reach this value, the formulations would not photocure. At low concentrations, imprints were obtained and uses for the resulting moderate index polymer composites as partial band gap photonic crystals were suggested.
