This paper introduces approaches that combine micro/nanomolding, or nanoimprinting, techniques with proximity optical phase mask lithographic methods to form three dimensional (3D) nanostructures in thick, transparent layers of photopolymers. The results demonstrate three strategies of this type, where molded relief structures in these photopolymers represent (i) fine (<1 μm) features that serve as the phase masks for their own exposure, (ii) coarse features (>1 μm) that are used with phase masks to provide access to large structure dimensions, and (iii) fine structures that are used together phase masks to achieve large, multilevel phase modulations. Several examples are provided, together with optical modeling of the fabrication process and the transmission properties of certain of the fabricated structures. Lastly, these approaches provide capabilities in 3D fabrication that complement those of other techniques, with potential applications in photonics, microfluidics, drug delivery and other areas.
The LIGA microfabrication technique offers a unique method for fabricating 3-dimensional photonic lattices based on the Iowa State "logpile" structure. These structures represent the [111] orientation of the [100] logpile structures previously demonstrated by Sandia National Laboratories, The novelty to this approach is the single step process that does not require any alignment. The mask and substrate are fixed to one another and exposed twice from different angles using a synchrotron light source. The first exposure patterns the resist at an angle of 45 degrees normal to the substrate with a rotation of 8 degrees. The second exposure requires a 180 degree rotation about the normal of the mask and substrate. The resulting pattern is a vertically oriented logpile pattern that is rotated slightly off axis. The exposed PMMA is developed in a single step to produce an inverse lattice structure. This mold is filled with electroplated gold and stripped away to create a usable gold photonic crystal. Tilted logpiles demonstrate band characteristics very similar to those observed from [100] logpiles. Reflectivity tests show a band edge around 5 μm and compare well with numerical simulations.
Nano photonic materials are synthetically manufactured crystals at the nano scale with the target of creating a microstructure with a special electro-magnetic periodicity. Such nano photonic materials have the ability to control light propagation and thus are capable of creating photonic bandgaps in the frequency domain. We propose using nano photonic crystals as sensors to detect microdamage in composite materials. We demonstrate using a simulation model that a nano photonic sensor attached to a composite bar experiences a significant change in its bandgap profile when damage is induced in the composite bar. The model predicts the frequency response of the nano photonic sensor using the transfer matrix method. A damage metric to evaluate the change in the frequency response is developed. Successful developments of nano photonic sensors allow damage identification at scales not attainable using current sensing technologies.
Light propagating through a subwavelength aperture can be dramatically increased by etching a grating in the metal around the hole. Moreover, light that would typically broadly diverge when passing through an unpatterned subwavelength hole can be directed into a narrow beam by utilizing a specific pattern around the aperture. While the increased transmission and narrowed angular emission appear to defy far-field diffraction theory, they are consistent with a fortuitous plasmon/photon coupling. In addition, the coupling between photons and surface plasmons affects the emissivity of a surface comprised of such structures. These properties are useful across several strategic areas of interest to Sandia. A controllable emission spectrum could benefit satellite and military application areas. Photolithography and near-field microscopy are natural applications for a system that controls light beyond the diffraction limit in a manner that is easily parallelizable. Over the one year of this LDRD, we have built or modified the numerical tools necessary to model such structures. These numerical codes and the knowledge base for using them appropriately will be available in the future for modeling work on surface plasmons or other optical modeling at Sandia. Using these tools, we have designed and optimized structures for various transmission or emission properties. We demonstrate the ability to design a metallic skin with an emissivity peak at a pre-determined wavelength in the spectrum. We optimize structures for maximum light transmission and show transmitted beams that beat the far-field diffraction limit.