Recent improvements in microparticle synthesis and handling have prompted new research into the engineering and fabrication of single and multilayered microspheres through traditional physical and chemical vapor depositions. At the University of Delaware, we have developed a custom batch coating process utilizing a vibro-fluidized mixing vessel to deposit thin-films onto the surface of microparticle substrates through R.F. magnetron sputtering. This process opens up a number of design possibilities for single and multilayered microsphere technologies that can be used to improve the optical performance of several optical filtering applications. Through the use of custom design and simulation software, we have optimized a number of filter designs and validated these findings through commercial software. Specifically, we have aimed to improve upon the mass extinction performance seen by traditional materials in the long wave infrared spectrum (LWIR, λ=8-12μm). In order to do this, we have run a series of experiments aimed at creating ultra-lightweight metallic hollow-spheres. Aluminum thin-films have been successfully deposited onto a number of substrates including hollow glass microspheres, high density polyethylene microspheres, and polystyrene foam spheres. By depositing the thin-films onto polymer substrates we have been able to remove the solid core after deposition through a thermal decomposition or chemical dissolution process, in an effort to reduce particle mass and improve mass extinction performance of the filter. A quantum cascade laser measurement system has been used to characterize the optical response of these fabricated aluminum hollow-spheres and have largely agreed with the expected simulated results.
As infrared (IR) imaging technologies improve for the commercial market, optical filters complementing this technology are critical to aid in the insertion and benefit of thermal imaging across markets of industry and manufacturing. Thermal imaging, specific to shortwave infrared (SWIR) through longwave infrared (LWIR) provides the means for an observer to collect thermal information from a scene, whether being temperature gradients or spectral signatures of materials. This is beneficial to applications such as chem/bio sensing, where the identification of a chemical species being present or emitted could compromise personnel or the environment. Due to the abundant amount of information within an environment, the difficulty lies within the observer’s ability to extract the information. The use of optical filters paired with thermal imaging provides the means to interrogate a scene by looking at unique infrared signatures. The more efficient the optical filter can either transmit the wavelengths of interest, or suppress other wavelengths increases the finesse of the imaging system. Such optical filters can be fabricated in the form of micro-spheres, which can be dispersed into a scene, where the optical filter’s intimate interaction with the scene can supply information to the observer, specific to material properties and temperature. To this extent, Lumilant has made great progress in the design and fabrication of such micro-sphere optical filters. By engineering the optical filter’s structure, different optical responses can be tuned to their individual application.