Nanophotonic structures can be used to dramatically enhance interactions between light and matter. We describe some of
our recent progress in fabricating optical nanostructures suitable for both classical and quantum information processing.
In particular, we present our progress using nanoimprint lithography, a low cost nanoreplication method, to fabricate low
loss photonic crystals.
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.
Nano-engineered devices with potential for trace level detection of chemical or biological species are investigated. The sensor system is a ChemFET device based on micro- and nano-scale silicon wires. The sensor response to changes in pH reveals a significantly higher sensitivity of nano-scale devices compared to micro-scale devices. By immobilizing DNA probe molecules on the silicon wire surface, the ChemFET devices are rendered specific to this DNA sequence. Differential measurements minimize the effects of non-specific binding. At a concentration of CDNA=10μM, two different single stranded 24-base DNA oligonucleotides have been clearly distinguished in the sensor response. DNA hybridization on the silicon wire surface is further corroborated by fluorescence spectroscopy and analysis of characteristic time constants in the sensors response.
The principles behind the chemical field effect sensor are outlined. A block model for the resistance mode of operation is described. Particular attention is paid to the interaction between semiconductor electrostatics, solution electrostatics, and chemical equilibrium at the surface. The site-binding model of the surface potential and the main models of the electrolyte double layer are reviewed. The semiconductor part of the model is generalized to finite channel thickness. Operation is illustrated using pH sensing as an example. The pH sensitivity is analyzed as a function of semiconductor thickness, gate dielectric thickness, ionic strength of the solution, and other factors.