Nanophotonics for dark materials, filters, and optical magnetism

Man, Mengren. Ph.D., Purdue University, August 2016. Nanophotonics for Dark Materials, Filters, and Optical Magnetism. Major Professor: Kevin J. Webb. Research on nanophotonic structures for three application areas is described, a near perfect optical absorber based on a graphene/dielectric stack, an ultraviolet bandpass filter formed with an aluminum/dielectric stack, and structures exhibiting homogenizable magnetic properties at infrared frequencies. The graphene stack can be treated as a effective, homogenized medium that can be designed to reflect little light and absorb an astoundingly high amount per unit thickness, making it an ideal dark material and providing a new avenue for photonic devices based on twodimensional materials. Another material stack arrangement with thin layers of metal and insulator forms a multi-cavity filter that can effectively act as an ultraviolet filter without the usual sensitivity of the incident angle of the light. This is important in sensing applications where the visible part of the spectrum is to be removed, allowing detection of ultraviolet signals. Finally, achieving a magnetic material that functions at optical frequencies would be of enormous scientific and technological impact, including for imaging, sensing and optical storage applications. The challenge has been to find a guiding principle and a suitable arrangement of constituent materials. A lattice of dielectric spheres is shown to provide a legitimately homogenized material with a magnetic response. This should pave the way for experimental studies. More specifically, a graphene stack is designed, fabricated and characterized. The structure shows strong absorption of light. Spectroscopic ellipsometry is used to obtain the complex sheet conductivity of graphene. Further modeling results establish the graphene stack as the darkest optical material, with lower reflectivity and higher per-unit-length absorption than alternative light-absorbing materials.